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¥,) ay Oa ie wi

tak JOURNAL

OF

EXPERIMENTAL ZOOLOGY

EDITED BY

WILLIAM K. BROOKS Johns Hopkins University WILLIAM E. CASTLE Harvard University EDWIN G. CONKLIN University of Pennsylvania CHARLES B. DAVENPORT Carnegie Institution ROSS G. HARRISON Yale University HERBERT S. JENNINGS Johns Hopkins University

FRANK R. LILLIE University of Chicago JACQUES LOEB University of California THOMAS H. MORGAN Columbia University GEORGE H. PARKER Harvard University CHARLES O. WHITMAN University of Chicago EDMUND B. WILSON Columbia University

ROSS G. HARRISON, Managing Editor 2 HILLHOUSE AVENUE, NEW HAVEN, CONN.

VOLUME V

PUBLISHED QUARTERLY BY THE WISTAR INSTITUTE OF ANATOMY AND BIOLOGY 36th STREET AND WOODLAND AVENUE PHILADELPHIA, PA,

CONTENTS

No. 1—November, 1907

Davin Day WHITNEY

Determination of Sex in Hydatina senta

ArTHUR B. Lams A New Explanation of the Mechanics of Mitosis. With Two Figures... .

HERBERT EUGENE WALTER The Reactions of Planarians to Light. With Fourteen Figures... .

No. 2—December, 1907

HERBERT EUGENE WALTER

The Reactions of Planarians to Light. With Fourteen Figures (con- chided) pete Sy lsterie ce wo eta MEMES ora CR eae eee

Mary ISABELLE STEELE

Regeneration in Compound Eyes of Crustacea. With Sixteen Plates and De Mane WOR endian mace edcae bdeauoheb hor sorgoesse onan

H. V. WiLson

On Some Phenomena of Coalescence and Regeneration in Sponges.

ANS NI SOME MN ateS a hace boo manbeu ss sco un bob aw ou oaU pmaOal dick

Hans PrzIpRAM Equilibrium of Animal Form. With) Templo reson rerescaheteeretieeeteha (ter

CHARLES ZELENY

The Effect of Degree of Injury, Successive Injury and Functional Activity upon Regeneration in the Scyphomedusan, Cassiopea xamachana.

\WitchvBourphiipuresssteilt qesicteece tty sa te cletmetstteret teeters Oot acy ett ALEXANDER PETRUNKEVITCH Studies in Adaptation. I. The Sense of Sight in Spiders. With Six INANE, Foc pone ee Ree ah: SE Sk

245

259

No. 3—March, 1908 Gruman A. Drew

The Physiology of the Nervous System of the Razor-Shell Clam (Ensis directus, Cons)) sWathi@neiPlate see cen: enecace.o = ote rer 311

FLORENCE PEEBLES

The Influence of Grafting on the Polarity of Tubularia. With Twenty- Chay writs choeoss Guid bod O An ou 6 DMbore iD ROME ad oso. vac 32

N. M. STEvENS

A Study of the Germ Cells of Certain Diptera, with Reference to the Heterochromosomes and the Phenomena of Synapsis. With Four

Pl atesi hire cei uakeee iS cphane ence BRS eea: ADE he kee Sus 9, ed Sie ee Re ere 359

Ratpu S. LILiie Momentary Elevation of Temperature as a Means of Producing Artificial Parthenogenesis in Starfish Eggs and the Condition of its Action....... 375

Tuos. H. Monrcomery, JR. The Sex Ratio and Cocooning Habit of an Aranead and the Genesis of Sex Rationse With aewoll i punes mec canioe aii «6 Meee a Ae 429

No. 4—June, 1908 N. M. STEVENS

The Chromosomes in Diabrotica vittata, Diabrotica soror and Diabrotica 12-punctata. A Contribution to the Literature on Heterochromosomes and Sex Determination. With Three Plates...................-. 453

Victor. E. EMMEL

The Experimenal Control of Asymmetry at Different Stages in the Devel- opmentofithe lobsters.) ome eee Erte one 471

C. M. CuiLtp

Physiological Basis of Form-Regulation. With One Figure .......... 485 H. H. Newman The Process of Heredity as Exhibited by the Development of Fundulus Hybrids. With Five Plates and Sixteen Figures in the Text ..... 503

C. C. GUTHRIE

Further Results of Transplantation of Ovaries in Chickens. With Three Bigures acct raer siete rapt esi cterers reels, ee ieiateaer eer eS eee 563

H. S. JENNINGS Heredity, Variation and Evolution in Protozoa. With Twenty-two

Figures SiS ce hectare ease eerieiee tee ann. Oe eee 577

DETERMINATION OF SEX IN HYDATINA SENTA

BY

DAVID DAY WHITNEY

Mie oIntro duction: cram c yet eeieveieree vein etctetede ol 1eCotelsfoisislatavers:= nvessioLole/eale/ouclerarearfourtaletere: tae e/e}selel I TE Material and methods: «<..2 2.0 cccceec cence creases deeceneensce cee eeseeuseecteee 3) III Influence of temperature 4 1 Maupas’ experiments......-.00-es cece cece ete cee e rere eee t eee sence r een ers ces 4

2 Author’s experiments.........6 02-6 e secre cere eee e teen eee ener teen e eee ees 5

a Temperature 20° to 22°C. .... 1... e cece eee eee eet e eee t tn t eens 5

b Temperature 25° to 29°C... 1. eee eee eee eee tee teen ete e ene ees 8

c Temperature 14° to 15°C... 2. eee eee eee teen eter e teen eens 9

IV The relative number of eggs which a male-laying female and a female-laying female produc: 10 I Temperature 20° to 22°C... 21. cece cece tenet eet eee e cent ence eens II

2 Temperature 24° to 29° C.... 2.222 see eee erent ee tence ete cnt e erect ees II

V_ Early production of male-laying females in a family of daughter-females...............--. 13 Villm lin men celo Mt Oo samc isrovarcitereisveleilalalelsvol-porelaye ofosshaxeseps¥elcVerebesepsltaretceseleieds laidtsiel-(o[olaloriaretiere 15 1 Temperature 20° to 22°C... 2... ee eee ee ee eee ete teen entree cent es eees 16

2 Temperature 14° to 15° C.... 6.00 eee eee eee cent eee eee ee tenet ee eee ees 18

3 Temperature 25° to 26° C.... 1... cece eee eter e eect eee erence tees 18

aVI Male and female'strains........0..20.0ccecccs cer ecce cece cece etsccestesteneceerersce 19

VIII Production of fertilized eggs... ... 0... ce cece cece ec eee cere eet t eee eet e teen eee ene 23 YR S we ary ate ea Yay set saket ole vole aisieke orelalsatcla = ehtetaret=te)sfoin in) tove:wielalete\oL-setehegesonel se eisieieielape=;nheyaks 25

I INTRODUCTION

On account of the supposed influence of external factors in determining sex in Hydatina senta, this rotifer has attracted much interest in recent years. As is well known Hydatina produces three kinds of eggs, viz: (1) parthenogenetic eggs which develop into females; (2) smaller parthenogenetic eggs which develop into males; and (3) fertilized eggs which develop into females. Each female produces only one of these three kinds of eggs. ‘Thus three types of females may be distinguished, viz: (1) females which produce females parthenogenetically, or female-laying females, 2 2; (2) females which produce males parthenogeneti- cally, or male-laying females, @ 9; and (3) the sexual females that lay fertilized eggs.

Tur Journat or ExreriMENTAL ZOOLOGY, VOL. V, NO. f.

2 David Day Whitney

Both female-laying females and male-laying females can be impregnated by males, but on the former, impregnation is sup- posed to have no effect. If the male-laying females are impreg- nated by the male in the first few hours after they leave the egg, such females produce fertilized eggs instead of parthenogenetic male eggs, thus showing that male-laying females can develop into sexual females that lay fertilized eggs.

The female-laying female can produce a family of daughter- females, some of which may lay female eggs and others may lay male eggs.

With the view of finding out the ratio in which these two classes of daughter-females are produced under various conditions I have carried out the experiments to be described.

Maupas found that a temperature of 26° to 28° C. would pro- duce as high as 95 per cent of male-laying females while a tem- perature of about 14°C. would produce as low as 5 per cent of male-laying females.

Nussbaum, on the contrary, came to the conclusion that nutri- tion and not temperature is the sex controlling factor. He found that by starving the young females for the first few hours after they emerge from the egg they would produce a high percentage of males, but if they were fed at the time they leave the egg they pro- duce a high percentage of females.

Punnett has carried out a few experiments along the lines laid down by Maupas and Nussbaum and finds that neither tempera- ture nor nutrition is influential in determining the sex. He finds, on the contrary, that there are definite “‘sex strains.”’ Some strains produce 40 to 50 per cent of males, others produce a very low percentage, 2 to 5 per cent, while others produce no males at all, although reared through as many as seventy-two generations.

The greater part of the work of the present paper was planned and begun in the spring of 1906, under the direction of Prof. T. H. Morgan, before the results of Punnett were published. Not knowing how to obtain proper food cultures the rotifers all died in July and the continuation of the experiments was deferred until October, 1906.

Determination of Sex in Hydatina senta 3

Il MATERIAL AND METHODS

In the latter part of April, 1906, Hydatina senta was discovered in great numbers in a small pool on the Palisades of New Jersey near Grantwood. The pool was fed by a little stream or ditch which carried away the drainage from several cottages. “The ditch was an extremely favorable place for the growth of Euglena viridis which collected in large patches on the sides and bottom. Immense numbers of Euglena floated down into the pool at the end of the ditch and served as food for the rotifers which abounded there in countless thousands. Sometimes as many as 150 to 250 individuals could be drawn up by a pipette in a few cc. of water.

About May 15 the pool dried up completely. The ditch still contained water but no rotifers were found in it after May 20. At this time there were innumerable larve of insects in the ditch and perhaps they exterminated the rotifers by feeding upon them.

In all experiments each individual female was isolated in a square or round watch glass which contained about 5 cc. of water and fed with Euglena, other protozoa and bacteria.

In order to obtain the Euglena and other protozoa a culture of horse manure and water (one to two ounces to a quart) was made, inoculated with Euglena and allowed to stand for two to three weeks at room temperature. At the end of this time the green coating of alg, Euglena, etc., could be removed from the sides of the glass jar and served as an excellent food for the rotifers.

Great care was taken to keep these food cultures uncontami- nated by rotifers. All watch glasses were placed in hot water after each experiment in order to destroy all eggs which adhered to the sides, thus preventing contamination of the following experi- ments by eggs of the preceding ones.

The experiments at temperature of 24° to 29° C. were con- ducted in an incubator. ‘Those at a temperature of 20° to 22° C. were conducted on the laboratory tables at room temperature, while those at a temperature of 14° to 15° C. were carried on in an ice chest.

These rotifers are exceedingly hardy and can be very easily kept in the laboratory throughout the year. In Mayof 1906 a Euglena culture was prepared in a glass jar containing 2000 cc. of water

4 David Day Whitney

and a few rotifers put into it. The jar was covered so as to pre- vent evaporation of the water. Rotifers have lived in it to this time, April, 1907, although no more food material has ever been added. It is absolutely necessary that the surface of the water be free from a scum for the rotifers will die within a few hours if it is present. It is safer, in order to keep the surface free, to tie the horse manure in a muslin cloth and place it in a well covered jar nearly filled with water.

III INFLUENCE OF TEMPERATURE

I Maupas’ Ex periments

The experiments of Maupas were so briefly described that it is very difhcult to understand clearly how he obtained his results.

Nussbaum and Punnett are inclined to believe he determined that a female was a male-laying or a female-laying individual by the size of the eggs that she produced. Small eggs being assumed always to give rise to males while larger eggs give rise to females. Nussbaum has measured a series of both male and female eggs and found that in some instances the two kinds of eggs over-lap in size. ‘Thus he points out an error through which Maupas’ results might have been obtained.

Isolating and counting the eggs of this rotifer would be exceed- ingly tedious and require almost constant attention. As the sexes can be readily distinguished at any period and as it requires only 36 to 48 hours for a female to mature and produce eggs it seems to me extremely probable that Maupas must have allowed some at least of the eggs to hatch before recording his results.

As his experiments are so few and briefly described it may be well to present them here in order that they may be compared with and interpreted by my own. Experiment I. Lot A, tempera- ture 26° to 28°C. Five female-laying female sisters produced 104 eggs; 97 per cent developed into male-laying females. Lot B, temperature 14°C. Five other female-laying females, which were sisters of lot A, produced 260 eggs; 5 per cent developed into male-laying females.

Determination of Sex in Hydatina senta 5

Experiment II, temperature 14° C. Five female-laying females, kept from the time of hatching at this temperature, produced 110 eges; 24 per cent developed into male-laying females.

The same five female-laying females were then placed at a tem- perature of 26° to 28° C. and produced 81 per cent male-laying females.

Experiment III, temperature 14° C. Six female-laying females which had been kept at this temperature from the time of hatch- ing produced 34 eggs, of which 12 per cent developed into male- laying females.

The same six female-laying females were then placed at a tem- perature of 26° to 28° C. and allowed to produce 44 eggs, of which 95 per cent developed into male-laying females. These six females were alternately placed at 14° C. and 28° C. several times and always gave a high percentage of male-laying females at the higher temperature.

2 Author's Experiments a ‘Temperature 20° to 22°C.

Experiment I, October 24, 1906. A female-laying female was isolated from a jar which was stocked with rotifers collected Octo- ber 2, from the same pool in which the animals were found in the preceding spring.

This strain was carried through twelve generations and the percentage of male-laying females determined. Each female was supplied with an abundance of food from the time of hatching, isolated in a separate watch-glass, and kept upon the laboratory table at room-temperature.

Table I gives the ratio of the mother individuals producing male and female offspring in the 3264 daughter-females of 95 female-laying females in the twelve generations.

This experiment was made in order to obtain the percentage of male-laying females produced at room temperature of 20° to 22° C., in order to be able to have some standard percentage of male-laying females with which to compare the results of the experiments conducted at lower and higher temperatures.

6 David Day Whitney

TABLE I Record of the production of male-laying and female-laying females among the 3264 daughter- females of 95 female-laying mothers.

Temperature 20° to 22° C.

No. | Eggs | Offspring | Per | | No. Eges | Offspring | Per Gen. | 22 ale [> ae || cent -||Gen--) 299 oc | po ercireeceneenn ECCOE | mother fe | oe yc Wh ears: mother laid (sional) Louse eine) SS | eta | Se ape = see | | | | I I ar | ets Vere) | 48a 3 32 8 | 24 25 | | 4 15 3 12 20 I I 28. || 0: |) 28 ° | Ce Ne 223 Sie ess 18+ } 2 | 53 | 8 | 45 | ase | 6 48: 9) rte! Rage eae | 3 27) hn: 23.«| «14+ | 7 25 o | 25 ro) | | | 8.938 | 15) 23 tao It I 440 cay | a7 38+ | 9 17 La be 23+ | 2 | 20.0) ara 325) 20h 10 48 15 3300 Stats | | | II 48 | 16 33+ IV Te al ees I so | i+ | 12 43 4 39 | 9+ 2 | 47 15 32 | Bit 13 44 | 10 34. |) 722-5 30 N52 elo Mh iea6 tao 14 49 | +14 35. | 28+ lisa asoonie 71/8 28 ulezo | Sx¢/ 1 Wasa ees 27 40 | 16 16 | 6 10 37+ My I 47 Our 47 ot 17 45 | 4 41 8+ 2 44 2 42 Piaoe Ml 18 50 7 43 14 3 41 40 2+ | 19 | cpa il) A 28 12+ 4 45 ° 45 | | 20 21) |" 0 21 ° 21) 0) 2) See 17 19+ NAD pag ae Mie 12 20 27+ 2220 2 18 10 i) ee 45 4 41 8+ 230+) «925 ° 25 ° Saas o | 36 | © | | eae illsa7 | go. | 14+ ar 49 8 40 16+ 5 | 48 22 26 «| 45+ } | | | XII I 38 3 35 7+ VIE]; 1 30 15 15 | ‘50 2 45 9 36 | 20 | 2 35 I 34 2+ 3 48 4 44 8+ 3 | 27 ° 27 ° 4 31 5 26 16+ 4 24 ° 24 ° 5 shee (aie) 26 | 33+ 5 19 5 14 | 26+ | 6 31 10 21 32+ | latZ 54 To 440) 18+ Vill I 41 16 25 39+ es 45 6 39 13+ Panes 17 I 16 | s+ | 9 35 II 24 31+ 3 47 18 29 38+ | 10 43 5 B85 u) Erte 4 26 ° ye | xr 38 3 35 iia 5 38 | ° 38 ° 12 27 9 18 33+ 13 42 16 | 26 |- 38+ Ix I 24 | I 23 4+ 14 34 ei Og Bae 15 83 15 | 28 | 34+ x I 49 | 13 36 0 | 26+ | 16 37 6 | 31 16 axe 45 10 35 224+ | | 47 31 26 16+

Determination of Sex in Hydatina senta

TABLE I—Continued

Temperature 20° to 22° C.

Now Eeeeet| a Obspaugs |) be Noo ees ||) Otspung, || Se Gen. Se a TERE. || |i ea | ass Gen. 2h) heat Ise — mother | au oon] Lo) {0} ae mother Gs ae? 29 XII 18 26 7 19 | 22+ || XII | 30 16 5 II 19 33s S| 3r 9 oe A 20 4 | 3 41 6+ 32 18 8 | 10 21 20 ° 20 Qi | 33 20 6 14 Bam Aleetsc i ltee 36 | 20 | 34 47 14 33 | | } 31 ut ZO E350) 35 x9) 4 15 24 28 10 i | 35+ 36 26 8 18 25 15 3 2 [20 | 37 II ° II 26 34 I 33 2+ 38 37 II 26 27 15 ° 152) Ce sl 3934 19 15 28 14 3 II 20 40 25 6 19 29 6 I 5 | 16+ | 41 35 4 31

| \ 1

cent

31+ Dict

44+

3°

The nature of the sex-producing power of the daughter-females of each individual mother is given separately in order to show that the ratio between the daughter-females producing male and female offspring varies with different mother-individuals, and also varies as much in the daughter females of sister-mother-individuals.

Summary of each generation in Table I and also the final summary of all the generations taken

29

mother

TABLE II together. Temperature 20° to 21° C. Offspri Per Werceonl EE eesa | les aR: Sb \inoeher laid 29 29 Sad Gen. Tye 31 15 16 | 48+ | VIZ |

I 3 108 12 96 i+ | VOI Ii 2 go 31 59 34+ ee x

IV 4 185 39 146 21+ XI

Vv 4 177 3 174 I+ xi

VAS WG 208 45 163 21+

Eggs Offspring Per aid Ee | ae cent Sa a? _ | a al 135 21) 114 | 15+ | | 169 35 | 134 | 20+ 24 I |} 23 4+ 819 176 643 -| 21+ 49 8 41 16+ 1269 268 | roor | 21+ 3264 654 | 2610 | 204

8 David Day Whitney

Table II gives the summary of each generation and the final summary of the twelve generations.

In this experiment 95 mother-individuals produced 3264 daugh- ter-females of which 20+ per cent were male-laying females.

It will also be noted that the percentage of male-laying females varied in the different generations from I + per cent to 48+ per cent regardless of the number of isolations in each generation.

b Temperature 25° to 29° C.

Experiment II, October 26. Two female-laying females were

TABLE III

Record of the production of male-laying and female-laying females among the 208 daughter-females of 26 female-laying mothers.

Temperature 25° to 26° C. Temperature 26° to 29° C. | No. Leges| Ofspring| Per No. Eggs Offspring | Per Gen.| 99 | ale __| cent Gen.| 29 laid | — | cent | mother | ] 82/29} oe | mother J 2)9 9 | oS? | | | | | | | StrainIV) I) 1 | 17| o| 17] © Strain TT IX | I | Werey [Naz | °

| | 2 || go" | ral ° |

I I (Ts ae Ly | we (or | | | | | | xX 1 |e 31, ONS StrainIII| I I 17 corals 4 | o | 2 Sa sol) 5 ° | {<3} 1h Srh3|) an Fo) IE100 Ul I 13} 4] 9] jot+ AL || <t\feo: | ex ° 5 8| 2 | 6 | 24+ peel I 16 | 2/14] 12+ 6 6) t] 5 | 16+ | oh |W 2 | 6| 24+ Vil | I 16 | Mal 12+ 8 oat foie} ° | | 2 | 10 | 3 7 30 9 | 5 2 | 3) 4° 3 TE | (6:5 Gi Sat Tor | 143) 225i) 450 | 4 25 | 13 | 12 | 52+ are Nie CNAs aac : 725 |/91\) 05203 ae 13 i at I Ce 0c | 1 | 10 3| 7] 30 | ef | © | 100 | 26/208 46 |162 | 22+

isolated from the same stock jar in a similar manner as in Experi- ment I, and placed in an incubator. ‘Two generations from one

Determination of Sex in Hydatina senta 9

individual, strain [V, and six generations from the other individual, strain II],were recorded. Six generations were kept at a tempera- ture of 25° to 26° C. and two generations at 26° to 29° C.

Table III gives the results of the experiment obtained at this higher temperature. The 26 female-laying mothers from eight generations, and from two strains, produced 208 daughter-females of which 22 + per cent were male-laying females. ‘This percent- age is practically the same as that obtained at room temperature.

@ Wemperature 14° to 15°C.

Experiment III, October 8. A female-laying female was isolated from stock jar as in Experiment I, and placed in an ice chest. A record of only a few of her offspring was kept and is shown in

Table IV.

TABLE IV

Record of the production of male-laying and female-laying females among the 167 daughter-females of 7 female-laying mothers. Temperature 14° to 15° C.

No. of |

eo | Eggs ; Ofspring ; Per cent mother | laid | Fed oo | 99 Fe I 48 47 11 36 23+ 2 so «| «28 4 24 14+ 3 30 20 3 17 15 4 | 22 9 2 7 22-+- 5 41 19 5 14 26+ 6 | 36 15 ° 15 ° 7 | 51 2 ro) | 19 34+ 7 | 278 167 35 132 20+

Out of 167 daughter-females from 7 different mothers 20+ per cent were male-laying females. “Uhe mother-individuals were reared at this low temperature as well as the daughter-females.

The percentage of male-laying females is about the same as that obtained at temperatures 20° to 22° C. and 25° to 29° C.

The foregoing results agree with those obtained by Nussbaum and Punnett but seem contrary to Maupas’ results.

10 David Day Whitney

IV THE RELATIVE NUMBER OF EGGS WHICH A MALE-LAYING FEMALE AND A FEMALE-LAYING FEMALE PRODUCE

It seems evident from Maupas’ account of his own experiments that he did not isolate each female-laying mother and each one of her daughter-females but kept the female-laying mothers to- gether in one dish and their daughter-females together in another dish.

If it is assumed that Maupas made no mistake in determining the sex character of the eggs before they hatched, or even that he allowed all eggs to hatch before he recorded their sex character, his results can be easily explained.

TABLE V

The number of eggs laid by each of 13 sisters, of which 6 were male-laying and 7 were female-laying, showing that the average number of eggs laid by each of the two kinds of females is very nearly the same.

Temperature 20° to 22° C.

I 46 I 47 I 50 I 38 I 47 Li 37 I 46 I 43 I 41 I 38 I 31 I 45 I 38

6 261 434 7 268 408 -

He gives no results of experiments conducted at a tempera- ture midway between 14° and 28° C. but only results obtained at these two extremes. The results that were obtained at 14° C. may very likely be identical with those that could have been obtained at a room temperature around 20° C,

Maupas recognized the fact that male-laying females produce eggs faster than female-laying females but makes no mention of the number of eggs that each kind of female may produce at dif- ferent temperatures. He seems to assume that they always pro-

Determination of Sex in Hydatina senta II

duce about an equal number, 40 to 50 each, but the following experiments will show the error of this assumption.

I Temperature 20° to 22° C.

Experiment IV, November 5. Of 13 sister individuals kept at room temperature and with the same amount of food 6 pro- duced male eggs and 7 produced female eggs. ‘The average num- ber of eggs produced by each female was nearly the same. ‘The results are shown in Table V.

2 Temperature 24° to 29° C.

Experiment V, November 9. ‘Three lots of sister-individuals from three different mother-individuals were kept in an incubator

TABLE VI Record of the number of eggs laid by 11 sisters, of which 6 were male-laying and 5 were female-laying, showing that the average number of eggs produced by the male-laying females is about two times as great as the average number produced by the female-laying females.

Temperature 24° to 25° C.

II sisters

|

7 A =) route) 9 a? | 2 2 |

oO z 2

| Av.

Mother Eggs we Mother | Eggs | if

= —- —_ ls = I 37 I 16 u 35 1 17 I 26 I 16 I 38 I II I 26 I 10

I 22 e 184 305 se a ze 14

and the number and sex character of the eggs that each produced was very carefully noted. The results are shown in Tables VI, VII and VIII. The records of the individuals in Tables VI and VII were taken at the same temperature of 24° to 25° C. while those of Table VIII were taken at a higher temperature of 26° to.29° C.

These tables show a decided change in the ratio between the number of eggs produced by a male-laying female and a female- laying female. As the temperature is raised the female-laying

12 David Day Whitney

TABLE VII

Record of the number of eggs laid by 21 sisters, of which 9 were male-laying and 12 were female- laying, showing the average number of eggs produced by the male-laying females is about two times as great as the average number produced by the female-laying females.

Temperature 24° to 25°C.

21 sisters rr Peles se = oe oe | 29 g Mother | Eggs aN | Mother Eggs Ny loa fe ee I 25 «| I 6 I 39 | ee 14 I 34 I 16 I 37 I II I 28 | | I 10 I 25] | I 12 I 29 | st 12 I 23504 | I 9 I 22. 4 | I 17 \fesjeat 14 | | I 16 A Pee se | a | F ae ; 9 261 294 lh 33 158 13 TABLE VUI

Record of the number of eggs laid by 14 sisters, of which 2 were male-laying and 12 were female-lay- ing, showing that the average number of eggs produced by the male-laying females is nearly four times as great as the average number produced by the female-laying females.

Temperature 26° to 29° C.

14 sisters a5 Se a ey oun) a | ; | eye} ie) Mother | Eggs ox Mother | Eggs ENG | = = — | | I 23 | I 3 | ee | 1 5 | | I I | I 8 I 6 I 8 : 3 | a 5 | 1 4 | I 4 | : Dial Al od ees oe feces 2 | 42 [he oan | 12 66 Ci 53

Determination of Sex in Hydatina senta 13

females produce fewer and fewer eggs while the decrease in the number of eggs produced by male-laying females is not as great. Table LX shows a rough approximation of the ratio in which the male and female eggs are produced at these different tempera- tures. TABLE IX

The approximate ratios in which the males and females are produced at different temperatures, as seen in Tables V-VIII.

40

1 | x | Temp. 20°to22°C. Table V 2 1 Temp.24°to25°C. Table VI-VII 4 1 Temp. 26°to 29°C. Table VIII

From the foregoing experiments and tables it is evident that temperature has nothing to do directly with determining sex in Hydatina senta but indirectly it determines the number of each sex produced by regulating the number of eggs that each kind of female lays. At a temperature of 20° to 22° C. the male-laying and female-laying females lay about the same number of eggs each, but at a higher temperature of 26° to 29° C. the male-laying females lay about four times as many as the female-laying females.

V EARLY PRODUCTION OF MALE-LAYING FEMALES IN A FAMILY OF DAUGHTER-FEMALES

None of the previous workers with Hydatina senta have iso- lated the eggs of a female-laying female in the order in which they were produced to determine whether there is any tendency for the earlier laid eggs to produce more male-laying females than the later laid eggs.

In my experiments in which all the daughter-females of each individual mother were carefully isolated and the sex character of their immediate offspring was recorded it is clearly shown that the male-laying daughter females appear among the earlier ones in the family rather than among the later ones.

In Diagram 1 the plotted line indicates the production of male- laying females among the 472 daughter-females of eleven mother-

14 David Day Whitney

individuals, each one of which produced 40 to 44 (average 421%

eggs. ‘The eggs were allowed to hatch in the dish with each mother and the young daughter-females isolated soon after hatch- ing. Their different sizes would indicate their relative ages and thus the approximate order in which the eggs were produced. The young daughter-females were isolated in lots from 1 to 8. This manner of isolation is subject to some error but on the whole gives a fairly good approximation of the truth.

Pe Pee [ea] CEE PEEEEHE a ae y N HH fs ran Reece poet

ducing male-laying females

Number and location of eggs pro-

NUMBER AND ORDER OF FEMALE EGGS PRODUCED

Diagram 1 Record of the egg production of 11 female-laying females from Table I, showing in which part of the egg laying period the male-laying females were produced. Each female laid 40 to 44 (average 42+) eggs. Of the 472 daughter-females 20+ per cent were male-laying females.

Nearly all of the male-laying females were produced among the first 28 eggs laid. Only two male-laying females were produced from the twenty-ninth to the forty-second laid eggs. Of the daughter-females 20 + per cent were male-laying females.

Diagram 2. ‘This is to show the same point as Diagram 1.

oping into male-laying females

Number and location of eggs deve!-

NUMEER AND ORDER OF FEMALE EGGS PRODUCED

Diagram 2 Record of the egg production of 12 female-laying females from Table I, showing in which part of the egg laying period the male-laying females were produced. Each female laid 35 to 39 (average 36+) eggs. Of the 441 daughter-females 16+ per cent were male-laying.

Determination of Sex in Hydatina senta 15

Twelve other female-laying females, each of which laid 35 to 39 (average 36+) eggs, produced all their male-laying daughter- females among the first twenty-four eggs laid. This is a clearer case than Diagram 1, because there are no scattering male-laying females among the later produced eggs. Of the daughter-females 16+ per cent were male-laying females.

In these two diagrams the mother-individuals were not specially selected but the record of all mothers, in Table I, producing 40 to 44 daughter-females, is shown in one diagram and the record of all mothers, in Table I, producing 35 to 39 daughter-females is shown in the other diagram. The numbers 35 to 39 and 40 to 44 were chosen because they seemed more likely to be the normal than a higher one.

These results, together with those obtained at different tem- peratures throw a great deal of light upon Maupas’ results. In -his experiments the highest percentages of males was always obtained from mothers which developed from early laid eggs.

In Table I it is seen that an individual mother may produce O to 40+ per cent of daughter male-laying females. This fact must also be taken into account when explaining Maupas’ few experiments. ”

VI INFLUENCE OF FOOD

Nussbaum supported Maupas’ conclusions that external factors can change the sex ratio in Hydatina but explains this change as being due to poor nutrition of the females and not due directly to the influence of temperature. At the higher temperature the processes of metabolism are taking place so rapidly that the ani- mals cannot eat food and assimilate it fast enough to prevent their tissues from being in a semi-starved condition. Nussbaum’s experiments seemed to show evidence that young females which are starved for several hours as soon as they leave the egg produce a higher percentage of males than those that are fed from the moment they hatch.

In many of his experiments he kept many individual females together and did not follow the history of each individual sepa- rately.

16 David Day Whitney

Punnett has pointed out that all the starved females of Nuss- baum’s experiments did not produce males, which invalidates his general conclusions.

Punnett has isolated female eggsof a “pure female strain,” and after they hatched starved the young females from 2 to 20 hours but no males ever appeared, although the young females were starved for several consecutive generations.

I have followed the history of many females which have been starved for several hours immediately after hatching at a tempera- ture ranging from 14° to 29° C. and have found no trace of evidence that a higher percentage of male-laying females is pro-

duced.

I Temperature 20° to 22°C.

Experiment I. Sixty-two eggs were selected at random from the sets of eggs produced by four female-laying females. They

TABLE X TABLE XI Record of the sex character of the eggs laid Record of the sex character of the eggs laid by 27 sisters, 15 of which were without food for by 45 sisters, 11 of which were without food for the first 6 to 26 hours after hatching and 12 the first 21 to 26 hours after hatching and 34 were abundantly supplied with food from the were abundantly supplied with food from the moment they hatched. moment they hatched. ~ Temperature 20 to 22° C. Temperature 20 to 22° C.

ee a | hours - % 2 : II 21-26 | 2 ; aS 2 13 fed rol 5 a i 21 fed | 9 I 12 2 a La eS I 16 Q 2 21 g I 23 rol I 2 fof 2 fed rol

were placed in “Great Bear” spring water, such as is sold in New

York City for drinking purposes, and allowed to hatch. After

Determination of Sex in H ydatina senta 17

hatching each daughter-female was kept in this water without food from 6 to 71 hours. Of tog daughter-females from the same mother as the above,

62 were well supplied with food from the moment they hatched. Tables X to XIII give the detailed results and Table XIV gives the summary. The mother-individuals of Tables X and XII

TABLE XII TABLE XIII

Record of the sex character of the eggs laid enoud Cees Sea of Rea 5 we 3 by 46 sisters, 5 of which were without food for

by 53 sisters, 31 of which were without food for Fee Beek:

the first 11 to 59 hours after hatching and 22 Gre Te So 0) 7 Ce : ey Aa ng One 2

eereepundantly; supplied wwithfoad) orl the were abundantly supplied with food from the

mom hey hatched. moment they hatched. oment they hatche

Temperature 20 to 22° C. Temperature 20° to 22° C.

| Character of eggs Character of eggs

Sister- | Starved from Sister- | Starved from

were sisters. The 11 sisters-individuals of Table XI were starved in filtered boiled spring water placed in sterilized test tubes with cotton stoppers.

The percentage of male-laying females among all the starved daughter-females is slightly lower than that of those which were

fed.

18 David Day Whitney

TABLE XIV

Summary of Tables X to XTIT, showing the percentage of male-laying females that occurred among the females which were without food for the first 6 to 71 hours after hatching, and also the percentage of male-laying females which occurred among the females that were abundantly supplied with food from

the moment they hatched. Temperatnre 20° to 22° C,

Starved Character of eggs |

Indi- | from produced Per cent viduals! time of | @ hatching fou 4 hours 62 | 6-71 12 50 19+ 1og | fed 27 82 | 24+ 171 | 39 132 | 22+

2 Temperature 14° to 15° C.

Experiment II, October 29. “Two female-laying females were reared at this temperature and their daughter-females isolated. Thirty-five daughter-females were without food from 11 to 64 hours after they left the egg, and 49 were abundantly supplied with food as soon as they hatched.

The detailed results are shown in Tables XV and XVI while Table XVII gives the summary.

The difference between the percentage of males produced by those starved and those fed is not very great and probably means nothing.

3 Temperature 25° to 26° C.

Experiments III, October 31. “Twenty-six female eggs from several individuals were produced at this temperature and as soon as they hatched the young females were starved from I to 13 hours. 19+ per cent of these starved females produced male eggs.

Tables XVIII and XIX give the detailed history and Table XX

gives the summary.

Determination of Sex in Hydatina senta 19

These three experiments, including Tables X to XX, clearly dem- onstrate that food has no influence in determining whether a female shall produce male or female offspring.

TABLE XV TABLE XVI Record of the sex character of the eggs laid Record of the sex character of the eggs laid by 49 sisters, 19 of which were without food for by 35 sisters, 16 of which were without food for the first 11 to 41 hours after hatching and 30 the first 20 to 64 hours after hatching and 19 weie abundantly supplied with food from the were abundantly supplied with food from the moment they hatched. moment they hatched. Temperature 14° to 15° C. Temperature 14° to 15° C. Sister- | Starved from Sa CeCNICEES Sister- | Starved from eeoerec cau Ee individ- | time of hatch- __Produced individ- | time of hatch- |__ Produced uals ing 3 ro) uals ing 3 9 | | hours hours | 4 II g 2 20 9 I 20 te) I 21 cr || 2 | 20 fot 2 21 co] 3 21 2 2 23 | @ I 23 9 I 46 fol 5 25 g I 46 } I 25 fol I 48 I 27 9 1 53 | I 41 ge I 53 lees, 4 fed rot I 58 2 26 fed 9 I 61 of v — ae ce I 61 g I 64 fot 5 fed fot 14 fed g

VII MALE AND FEMALE STRAINS

Punnett says: “My experiments have led me to the conclusion that among the rotifers I used were certainly three different types of thelytokous (female-laying) females, viz:

A Females producing a high percentage of arrenotokous (male-laying) females.

B Females producing a low percentage of arrenotokous females.

C Purely thelytokous females producing no arrenotokous females”’ (p. 226).

20 David Day Whitney

TABLE XVII

Summary of Tables XV to XVI, showing the percentage of male-laying femal s that occurred among the females which were without food for the first 11 to 64 hours after hatching, and also the percentage of male-laying females which occurred among the females that were abun- dantly supplied with food from the moment they hatched.

Temperature 14° to 15° C.

Starved Character of eggs |

i- | Per aac | from produced

vidu- | time of sae | oe

=

als hatching a | 2 Ge

35 | 11-64 8 27 22+

49 fed 9 40 18 +

84 H7 |||| 167 20 +

TABLE XIX

Record of the sex character of the eggs laid by 19 individuals females which were without food for the first 1 to 13 hours after hatching.

Temperature 25° to 26° C.

| | Character of eggs

re Starved from Individ- time of hatch- produc’) = uals : ing rol fo} hours I I | 2 2 2 | Q I 3 | ice 3 3 % I 4 fos I | 5 | of 2 5 2 2 6 | 2 I 8 fof I 3 10 | & 2 13 | 2

TABLE XVIII

Record of the sex character of the eggs laid by 15 sisters, 7 of which were without food for the first 7 to 13 hours after hatching and 8 were abundantly supplied with food from the moment they hatched.

Temperature 25° to 26° C.

| Starved from | Character of eggs

suis m- | time of hatch- produced dividuals : ing fot 9 hours | 2 7 | 2 2 10 ee : 10 a | 2 13 | 2 I fed rol | 7 fed ° TABLE XX

Summary of Tables XVIII to XIX, show- ing the percentage of male-laying females that occurred among the 26 females that were with- out food for the first 1 to 13 hours after hatch- ing.

Temperature 25° to 26° C.

Determination of Sex in H ydatina senta ZT

Punnett realized that these conclusions were based on rather scanty data. His data can be shown to be entirely insufhcient. His type 4 is based upon only one experiment which extended through 23 generations and included 109 individuals. 42 + per cent of these 109 individuals were male-laying females.

Type C is based upon much more evidence, but it is not sufh- cient to warrant a decisive conclusion.

In October, 1906, I started a strain or pedigree culture which extended through 62 generations including 167 mother female- laying females and 3959 daughter-females. ‘This strain was kept at room temperature of 20° to 22° C. Its history is recorded in Tables I and XXI.

Table I. Out of 3264 daughter-females from g5 mother-indi- viduals which extended through 12 generations 20+ per cent were male-laying females.

Table XXI. In r5 generations, XIII to XXVIII, including 76 daughter-females from 15 mother-individuals only 9 + per cent were male-laying females.

In 17 generations, XXIX to XLV, including 208 daughter- females from 17 mother-individuals no male-laying females ap- peared. Ingeneration XLVI the first 327 daughter-females from 18 mother-individuals yielded 48+ per cent of male-laying females.

The next 11 generations XLVII to LVII including 58 daughter- females from 11 mother-individuals gave 29 + per cent male- laying females.

These results show that a strain producing a higher percentage of male-laying females can develop into a strain yielding a much lower percentage, or even into a strain yielding no male-laying females at all. Furthermore, the apparently pure female-laying female strain can develop into one which will give a very high percentage of male-laying females.

Thus the three strains or types of Punnett can be found in one strain and each is capable of giving rise to the other types accord- ing as the data is scanty or extensive.

The high percentage of male-laying females in generation XLVI can be readily explained by the results shown in DE seme I and 2 which clearly demonstrate that the male-laying eles are pro-

22 David Day Whitney

duced earlier in a set of eggs than are the majority of the female- laying females. In this generation the 18 mothers produced only an average of 18 + eggs each, because the experiment was discontinued at this point.

TABLE XXI

Continuation of the strain of which the beginning is recorded in the 12 generations of Table I.

ay Daugh- 29 | Daugh- Gen. maopkee |. ter 2 ae 2 9 Gen. Mother | _ ter 9 32 99

isolated | isolated XIII I 6 I 5 XXXVUI Tae hy 37, ° 37 XIV I Gye ar 5 XXXIX I | 6 ° 6 RV) ia Parla as x! Mere a6 o | 26 XVI I 6 ° 6 XLI Ta 25 ° 25 XVII I 2 ° 2 XLII Ti] 14 ° 14 XVIII I I ° I XLill rife 78 ° 18 xIX I 5 ° 5 XLIV I 17 ° 17 xx I 6 | ° 6 XLV I 18 ° 18 XxI I 6 ° 6 XLVI 18 327 160 167 XXII I 6 | 3 3 XLVII I I ° I xxmm| 1 Pant ee al XLVI] 1 6 al 2 XXIV I 6 ° 6 XLIX I 6 3 3 XXV I 6 ° 6 L Teal 6 2 4 XXVI I 6 I 5 LI I 6 ° 6 XXVII I I ° I LIL I 6 ° 6 XXVIII I 2 | 1 I LUI I | 6 ° 6 XXTX I Se le 30 5 LIV I 6 5 I XXX I 6 ° 6 LV 1) 5 2 2} XXXI I 4 ° 4 LVI I | 6 5 XXXII I I ° I LVI I 4 ° 4 XXXII I 5 ° 5 LVI 3 6 ° 6 XXXIV 1 Gia sl xe 6 LIX 1 6 ° 6 xXXxXXV I 6 ° 6 LV 2 6 ° 6 XXXVI I 6 ° 6 LXI 6 ° 6 XXXVII I 6 | o 6 LXII 2 4 I 3

How a seemingly pure female-laying female strain is obtained when only a few individuals are isolated from each generation while a parallel strain does not yield the same results is not yet clear. It may be due to some trick of selection in isolating the young females of each generation.

Determination of Sex in Hydatina senta 23

If a large number, 45+ per cent, of the daughter-females of one mother produce males it does not necessarily follow that a high or a low percentage of the daughter-females of the next gen- eration will produce males. Nor does it seem to be true that if the daughter-females of one generation produce all female off- spring that the daughter-females of the following generation will do so. ‘Table XXII shows the history of both classes of daughter- females in five generations.

TABLE XXII

Record of all the female-laying females that were isolated in five consecutive generations, showing that there is no constant relationship between the percentage of male-laying females that are produced by a mother and the percentage of male-laying females that are produced by the daughter-female in the next generation.

Off- Moth- Off- Off- Moth Off- Gen. Sisters spring er spring Gen. | Sisters spring | er | spring a2 | 99) 99 | ae |ee|ee I I AV 5 15 15 II 16 ] 36 I 34 ic ° 27 Ut 4 ° 45 | ° 24 2 42 | § 14 I 42 i) | 47 it IV 5 12 20 4 41 ° 36 7 40 22 | 26

VIII THE PRODUCTION OF FERTILIZED EGGS

The winter or fertilized egg is supposed to be the male partheno- genetic egg which has been fertilized. This produces a female. The egg has much more yolk material and a thicker shell than the male parthenogenetic egg. A female produces from twelve to twenty fertilized eggs, while a male-laying female produces from forty to fifty parthenogenetic eggs. In order to obtain fertilized eggs males must copulate with very young male-laying females.

24 ; David Day Whitney

In a few experiments, comprising several hundred females which had copulated with males when very young, Maupas found that the percentage of females producing fertilized eggs was the same as the percentage of male-laying females from several hundred fe- males that never had copulated with males. He concluded that the fertilized egg is the male parthenogenetic egg which has been fertilized.

I have repeated his experiments on a smaller scale and the same results were obtained. Furthermore, the producers of fertilized eggs appeared among the early laid eggs of the mother-individuals.

Diagram 3 shows the occurrence of the layers of fertilized eggs among their sister-individuals from five mothers which _pro- duced 125 eggs.

Number and location of eggs developing into layers of fertilized eggs

NUMBER AND ORDER OF FEMALE EGGS PRODUCED

Diagram 3 Record of the egg production of five female-laying females showing in which part of the egg-laying period the layers of fertilized or ‘‘winter eggs” were produced. Each female laid 21 to 27 (average 25) eggs. Of the 125 daughter-females 36 + per cent laid fertilized eggs.

Many males, fifteen to twenty, were constantly kept in the dishes with each of the five mothers, so that as each daughter- female emerged from the egg there were many males present.

Only one male-laying female appeared among the 125 daughter- females. ‘This experiment was conducted in the fourteenth gen- eration of the strain in Tables I and XXIJ, soon after the isolation of the large numbers in generation XII which gave 21 + per cent of male-laying females.

Of the 125 daughter-females 36 + per cent produced fertilized eggs. This percentage is high because the mother-females pro- duced, on an average, only twenty-five eggs each, but if they had produced forty eggs each the percentage would have fallen to

Determination of Sex in Hydatina Senta 25

about twenty-three, provided that there had been no more mothers of fertilized eggs to have been produced. “Vhe Diagram 3 shows only one occurring between the eighteenth and twenty-seventh egg.

The number and order of occurrence of the mothers of fertilized eggs together with the number and occurrence of the layers of male eggs in parallel sets of daughter-females seem to indicate that the Sayer of male eggs and the layers of fertilized or winter eggs are identical at one stage of their life.

In another species of rotifer, Asplancha, Lauterborn has observed winter eggs and male embryos in the same individual. Among the Daphnia, Issakowitsch has found that the same female may produce winter eggs and male eggs.

Therefore it is not unreasonable to suppose that the immature male-laying female of Hydatina senta is capable of developing into a layer of fertilized eggs or a layer of male eggs, according to the impregnation or lack of impregnation by the males

IX SUMMARY

1 Temperature has no influence in determining the sex of Hydatina senta.

2 About 22 per cent of the females at any temperature from 14° to 29° C. are male-laying.

3 A male-laying female produces eggs faster than a female- laying female and at a temperature of 25° to 29° C. a male-lay- ing female produces more eggs throughout her lifetime than a female-laying female.

4 The male-laying females occur in the early part of a family of daughter-females.

5 Starving the young females for the first few hours after they hatch does not cause them to produce a higher percentage of male eggs.

6 ‘There are no strains that constantly produce a high or a low percentage of male-laying females.

7 The “pure female-laying female strain” can give rise to the normal percentage, 22 +, of male-laying females.

26 David Day Whitney

8 The male-laying female may produce fertilized or winter eggs, provided that she has been impregnated by a male at the proper time.

Zoological Laboratory Columbia University

May 1, 1907

BIBLIOGRAPHY

Hupson, C. T., anpD Goss, P. H., °89—The Rotifera. IssakowrrscH, A.—Geschlechtsbestimmende Ursachen bei den Daphniden. Biol. Centralb., xxv, 1905. LautTersorn.—Ueber die zyklische Fortpflanzung limnetischer Rotatorien. Biol. Centralb., xviii, 1898. LeussenN.—Contnbution a |’étude du developpement et de la maturation des ceufs chez Hydatina senta. 4a cellule, xiv, 1808. Maupas, M.—Sur la multiplication et la fécondation de |’Hydatina senta, Ehr. C. R. AcuSe: ‘Paris; cxi, 1890. Sur la fécondation de |’Hydatina senta, Ehr. C. R. Ac. Sc. Paris, exi, 1890. Sur le déterminisme de la sexualité chez |’Hydatina senta, Ehr. C. R. Ac. Sc. Paris, cxiii, 1891. Nussspaum, M.—Die Entstehung des Geschlechts bei Hydatina senta. Archiv fiir mikroskopische Anatomie, xlix, 1897. Punnett, R. C.—Sex-determination in Hydatina, with some Remarks on Par- thenogenesis. Proceedings of the Royal Society, B. vol. 78, 1906.

From the Havemeyer Chemical Laboratory, New York University, New York

A NEW EXPLANATION OF THE MECHANICS OF MITOSIS

BY

ARTHUR B. LAMB

Wirth Two Ficures

The almost universal recurrence of essentially the same regular arrangement of the chromatin substance in dividing cells indicates emphatically that the same very definitely acting force or complex of forces is operative in them all. Numerous suggestions have been made as to what this omnipresent force may be, but none of them have been able to meet the many requirements of the prob- lem. Moreover, our real knowledge of the whole matter is so scanty that any explanation seems at present a little premature. I am, nevertheless, going to offer still another explanation of this phenomenon; first, because it may prove suggestive to others and may prompt fresh observation, and second, because it calls attention to a phenomenon which deserves the consideration of cytologists, whether it has any application to the present case or not.

The marked polarity which mitotic figures exhibit, best de- scribed by saying that they resemble the configuration assumed by iron filings between unlike magnetic poles, together with the move- ments which the chromatin substances execute about one another oblige us to believe that this unknown force is of a polar nature, that is, acts outward from a center and exerts its influence at a distance. ‘The only objection to this conclusion is that a crossing of astral rays has been observed. ‘The lines of force in the field of any polar force cannot, however, cross, and consequently the astral rays which would be assumed to follow these lines of force also cannot, or should not, cross. This crossing, though certainly real, is not, apparently, the prevailing condition, and it can be

THe Journar or Experimenta Zo6.ocy, VOL. v, No. I.

28 Arthur B. Lamb

explained on the assumption of an intermittent or non-synchronous activity of the centers, as Reinke’ has shown.’

Assuming, then, the existence of some polar force exerting its action at a distance, we are confronted with two possible alterna- tives regarding the sign of this action. “That is, we may imagine either that the centrosomes attract, or that they repel each other. Wilson® has urged that the astral centers represent centers of trac- tion, caused, perhaps, by a volume change at those places. This is in entire agreement with the configuration assumed by the astral rays and the spindle fibers. They simulate the magnetic field between opposite poles, as pointed out above. But this view is quite at variance with the actual movements of the centrosomes. They move apart, even at a stage when astral rays are well devel- oped and hence seem to repel each other and not attract as they ought if they represent opposite poles. Lilliet adopts the other alternative, as did Meves.° He considers the astral centers to repel each other. In this way he explains the movements of the centrosomes satisfactorily enough, but is confronted by the difh- culty of accounting for the configuration of the astral rays. Lillie assumes that electric charges located on the centrosomes are the particular forces which produce the repulsion. He would explain the unexpected configuration of the fibers and the astral rays by the rather dubious assumption of a localized positive inter-astral area which superposes its effect on the purely repellent action of the astral centers. Looking at the matter more closely we see that for every unit of negative electricity on the chromatin sub- stance there should be a corresponding unit of positive electric-

1 Reinke, Fr.: Arch. f. Entwicklungsmech., ix, 1900.

*Rhumbler: Ibid., iii, iv and v, 1896, 1897 and 1899, has suggested a non-polar force to explain the astral rays independently progressing rays of crystallization out of a supersaturated solution. While avoiding the difficulty of crossed fibers this explanation encounters the still more formidable one of accounting for the universal occurrence of curved fibers.

8 Tbid., xiii, p. 354-395, 1901. See also his book, The Cell in Development and Inheritance, 3d Ed. The Macmillan Company, New York. It is a pleasure to express my thanks for a most profitable discussion of this whole question with Professor Wilson, who, it seems, had already considered the pos- sibility of a hydrodynamic explanation.

4 Amer. Jour. Physiol., xv, 46-84, 1905.

5 Ergebn. d. Anat. u. Entwick., vii, viii. Merkel u. Bonnet.

The Mechanics of Mitosis 29

ity in or on the surrounding aqueous solution. Moreover, since this aqueous solution contains inorganic, ionized salts it must be a conductor of electricity, and the positive charge must be distrib- uted over the whole solution. Any localized positively charged area in the electrolyte, except for the supposed “double layer” around each charged particle seems, consequently, unlikely. Lillie has encountered a similar difficulty in accounting for the configuration of the chromosomes. ‘They ought not only to be repelled from the astral centers but also to migrate toward the boundary of the equatorial plate. This latter thing they do not do, and Lillie is therefore again obliged to make the assumption of a localized positive inter-astral region.

There is, however, another force which might well come into play here, which so far as I know has not been mentioned in this connection before, and which involves none of the objections urged against the electrostatic explanation. I refer to the mutual repulsions and attractions, exerted by bodies pulsating or oscil- lating in a fluid medium. We owe our knowledge of this branch of Eydradyaamics chiefly to the two Byerknes, father and son.* They have shown that bodies pulsating or oscillating synchro- nously in.a liquid attract or repel one another cenendinae on whether they pulsate or oscillate in the same or opposite phase. Further- more, these bodies set up lines of flow in the liquid, real hydro- dynamic lines of force, which simulate exactly the lines of force in magnetic or electric fields. The sign of this force is, however, in general, just the reverse of that in electric or magnetic fields. Bodies pulsating synchronously and in opposite phase repel each other, although the form of the jie old the y produce 1s 1dentical with amerocenmnlibe magnetic poles which attract each other. Simi- larly, two spheres oscillating synchronously and in the same phase repel each other, elu they too produce a field like that between opposite magnetic poles. The following experimentally derived figures, taken from Byerknes’ text-book, illustrate this identity of form and reversal of sign in the electric and hydrodyna- mic energies:

6 See Hydrodynamische Fernkrafte, v. Bjerknes; 2 vols. Leipzig, J. A. Barth, 1902.

30 Arthur B. Lamb

wt

es

a AR) XX PATA AWA | 4 ee

Fig. 1 Lines of force between unlike magnetic poles. REPuLston.

\ i ! pe - ° : t ! \ Sel f ae nee ‘ ; ha / = / y = \ . \ ee Sy \ \ ee yi a Ny 22 Ss a a ea a Se ==, ba aa a = ee! = See —_ SO St ee ad 7 Zi Dae ae Ne ee es N eee sf NR <7) iK , S Games {'\ NX Se, \ 7 ah .\. Se fon tite ‘ 0 : ‘\ \ Lae / \ \ / y \ ‘ i ee C t \ \ ! ean . Z, f '

Fig. 2 Lines of flow between oppositely pulsating bodies. Atrraction.

It is this exact reversal of sign of hydrodynamic action at a dis- tance, as compared with electric and magnetic actions, which makes this force peculiarly applicable to the case of mitotic figures, obviating many objections which beset the previous explanations, and particularly the fact that with previously considered polar forces if one made an assumption which would explain the form of the field, the motion of the centrosomes appeared contra dictory

The Mechanics of Mitosis 31

and vice versa. If, however, we assume that the centrosomes are pulsating synchronously and in opposite phases, or oscillating synchronously and in the same phase, we obtain the desired repul- sion, and at the same time we get mitotic figures corresponding to the configuration of the lines of magnetic force between opposite poles. That is, we get a configuration of spindle fibers and astral rays precisely like the actual ones.

The cases of tri- and multi-polar spindles, so difficult to explain on electrostatic grounds present much less difficulty here. If each centrosome were oscillating along a path radial to the common nuclear center and in the same phase, mutual repulsion, combined with the proper configuration of the astral rays, would be obtained.

The movements and configuration of the chromosomes are also better explained on hydrodynamic grounds than by previous assumptions. It is not even necessary to assume that they execute any independent oscillatory or pulsating motions. Byerknes has shown that bodies suspended within the field of force of oscillating or pulsating bodies are attracted or repelled depending on whether they are lighter or heavier than the surrounding medium. ‘This attraction or repulsion is due to oscillations induced in the sus- pended bodies by the permanently oscillating or pulsating bodies. The chromosomes, if heavier than water, or the cell fluid in which they are suspended, would be repelled from each centrosome and would come to occupy a position midway between them in the equatorial plate. Moreover, they would not move outward to the boundary of the equatorial plate. ‘Their induced oscillations, though repelling them from each centrosome, would attract them toward each other, and this action would tend to keep them in the observed axial position. If the chromosomes should become lighter than the cell liquid, the repulsion from the centrosomes would change to an attraction. ‘This immediately suggests that it may be simply a change in specific gravity of the chromosomes which causes them to diverge, after splitting, back toward the cen- trosomes.

[t is now of interest to inquire whether this hydrodynamic action at a distance could possibly be strong enough to account for the actual movements of the centrosomes. It is, of course, almost

32 Arthur B. Lamb

impossible to decide this by calculation, in the present state of our knowledge, but if we assume that the centrosomes are smooth, hard spheres and that the cell fluid is homogeneous and as mobile as water, it is not difficult to calculate how vigorously they must oscillate or pulsate in order that they shall move apart with the observed velocity.’ Taking the radius as 0.0002 cm., the dis- tance apart as 0.003 cm., the time required for this maximum separation of the centrosomes as fifteen minutes; if the amplitude of oscillation equaled two diameters, the frequency required would be 2000 oscillations per second; if the amplitude were eight diame- ters, the required frequency would be 100 oscillations per second. With similar dimensions, if the centrosome pulsated so that its greatest volume were three times its least volume, a frequency of some 130 pulsations per second would be required.®

These frequencies are greater than one would expect. They do not, however, involve any great linear velocities, for the dimen- sions of the particles are very small. ‘Thus the frequency of 2000 oscillations to the second only means a linear velocity of the centro-

7 The formula of Stokes F=6zpur (Brit. Assoc. Report, p. 445, 1887) applying to the motion of spheres though viscous media was used to determine the force needed to give the centrosomes the observed velocity. To find the needful fre-

quency of oscillation this was equated to the expression

6 a : 47d!

derived by Bjerknes for the attraction or repulsion between oscillating spheres.

Similarly, to find the needful frequency of pulsation it was equated to the expression

a a)

8/9 — (a°— 1)? p?

also based on a formula derived by Bjerknes. In all of these expressions r represents the radius, d the distance between the centrosomes, /t the coefficient of viscosity of the medium (water), p the frequency, S the “action moment,” v the velocity, and a the ratio between the mean and the maximum radius of the pulsating sphere.

8It might also be pointed out here that similar calculations on the hypothesis of electrostatic action show that, if the capacity of the centrosomes is simply that of conducting spheres in an isolating medium, a potential difference of nearly two volts would be required; if the capacity is that of spheres surrounded by a ‘Helmholtz double layer,” a potential difference of only a few thousandths of a volt would be

necessary.

The Mechanics of Mitosts 33

some of 2 cm. per second. It is, of course, almost impossible to say what effect a viscous, heterogeneous field would have upon the calculation, so we are obliged to leave the quantitative side of the question as quite unsettled.

Besides the oscillatory currents produced by pulsating and oscil- lating bodies, Bjerknes has shown the existence of steady currents in the fluid medium toward and away from the centers of motion. Similar currents have been observed in dividing cells, particularly centripetal currents between the astral centers. It does not, how- ever, seem wise to treat this or similarly less pronounced phe- nomena in our present state of ignorance.

The assumption of a pulsating centrosome or centrosphere is by no means an impossible one. The assumption of an oscillating centrosome is not even improbable. The assumption of syn- chronous pulsations or oscillations involves no mysterious synchro- nizing mechanism. Random oscillations or pulsations would cer- tainly tend to become synchronous by mutual interaction, while after the closed spindle fibers had formed, whatever their nature may be, any other rate of oscillation would be very improbable.

The fact that such oscillation or pulsation have not been de- scribed is not conclusive.

Our knowledge of the subject is based almost wholly on dead material, and moreover the oscillations and pulsations may be very rapid and small.

In conclusion, I would again like to emphasize that the above is nothing but an ad hoc constructed hypothesis and intrinsically therefore only of hypothetical value. If, however, it calls atten- tion to a little known phenomenon or stimulates fresh observation it will have served its purpose.

CONTRIBUTIONS FROM THE ZOOLOGICAL LABORATORY OF THE MUSEUM OF COMPARATIVE ZOOLOGY AT HARVARD COLLEGE. E. L. MARK, Director. No. 193

a

THE REACTIONS OF PLANARIANS ITO LIGHT

BY

HERBERT EUGENE WALTER:

Witu Fourteen Ficures

Mp te Od CEO ML tejasetlcrassy sieve e/ave os stase aves ste aie soe a1els ate etre areloye avatecera leis evainiereinnsraicaete o eeioreele 37

UG MELISCOLIGAL faetercyatsfetctereroieialeislele eictal-isyaio nreiercte HopduacpaduanducooapoubooD Adasen ousbabop 38

SVT Gee Milter l mere etetey terisiozestortaresetovecoxeiais )caayoictetere cle st cteiete sisiersss/ayesctersrereicieieiarelnere erates sie eteteieee 45

Va Criteriaitormedsuring bel avion ves deratsei= stelaeistars steisiclorsietel cereale rater ae 47

WA © Dsery cl tloriswreretoyesetes staves tetetet rater cie/crsie ese vereiste lvls) s]aisve: «vial crstatactote rents orate cleaver nee cheer 49

Hime E Told) stl Sycooodo o GUD RUE Aa Dao no Reon aUraeacoaed se dopendoceodernadaacassad 49

. LX. AUDEN iO dteh Cela Sapo hbenaeroTgnbenElsiua GoctnceonucdeonocbTd 66500000 50 Rate OfllocOmOtlOnns fatarsiniesctctors oscle) oferctarereyereasieraect=tale ae tereie rarest 50

. ARG bhy355 dap oe on DpH eR pAarAUAehEEnEddutechododacucnootoapeds SI | (ChEpTTSO Ichi sone opodbentandonpaavadpeooduMdcdaasavconDes 51 SUMMA reine lars sisinisia\cisicls oy=taiere oie'aYelaye\ ic /aasysiejaicretratvstcictetetetbersieteicle sr

Biss Non=directive light -jsr<\ays/ats ters e)ars(ti 2istsyaistsracei nel te/eicterniclerels\emmiale sereieeie elec 52

Gam AD Paratusara str cecisseleitectciersteleiswiois a cleletsemtanteinet perenne ears 52

De Results Serer rstate;oss,aforesajotelasyalotclaters ayars 2ts1saje eictaieysleisiotsie ste/stanietersteteie aceite 56

IRafelohlocomotioDy «ssh tsisiate/siets vieltersssierars Yeeieceineier eos oeiee eee 56

Alin sth eae ho ceonnotno BouaeRqoEnDooEsmErndanusscoodadsodncuac 58

GChanpeahcourse. poate steletererelals.erais Actors atersthslefeetane eter ere ee eee 59

60

61

62

63

Beeps ACE srornistassisielaie elctersTasa(eletedate:nialetai Taya os aise oes tiers nrateleraetaectioilerste 64

SUM Maryietatersiancelaterictaists ofeletateetaitearcta cistercierercie etsvete lettereietacsteereterate 71

PRRENOLOCAKIS ARB defector ttcracier ryt cteteve or SS Se Sat ate a ee Tae in eee 72

Avemuconstantidirective lighteacs tists da stetaetditerisicliis ecto seletem ceiseten tetas 72

Orientation seer iceciocctietorsmtere oti eislercareieiereis revels reenter eeicrornieracntiets 72

IRACG OF LOCOMOLION sjeysielcscjerofsiscorsisielore sfercisinicietelcistann eieieteistersinletele ete orale 75

Ghanpeanicharacteroficourse seins ivi-rsicieis sin arsercisierarsiorsieteleeicierersiereve 78

Prcetiracviohorienta tons sre iejeleister-infeci= ce acieie cisistalcieisiciciarsie’s sleistelerevareie 719

IDR ORANG Hioht-adpotde send sodanbarenbasdcnontusaodoedeouua 80 Durationiobiactivityatectuisnviesiccisene einicnis cietsven elatcieciace ice sm ace 81

= Mimeixequired tolleave a mmitcircle. --.icisj-islcidiels cc slalsieis cis ervicecsntys 82 Manner of coming to rest 83

SSN Ea Goh St gonbeko Apopoud oh doen aoanAneUe COUN oneNo ene sea ne 86

Journar or Exrerimenrar ZooLocy, vor. v, No. I.

Herbert Eugene Walter

B; In.changing: directive laghes <i... 1<t-)-)-1=/o)z)atels/=1=1=/=1al= stele = sl =\oic/etelelotola/eeraiete teeters 87 (Ghanges imlintensitye:jetssce! a= o\aye.c rer tereicssis iotetels sinislessfelonateteteis1<eteyeistere 87 GhanpesimdirectiOninse aoe remiers ce selec s eisia eee n/heioie eters 88 Ritinlol iy sap enna AED Aan DG IuobaupaenandoraaragdaosouAnoDaccda 89

C inycombinationwwith'other responses: <1... .e.0.)nis ins sleleieisinicisie'e.aislsisizisietaisfeieels 89 (Esai Fs shedhoasdaor coded as muMaAadbosbn bocobcunonoDonNAsDOSNUGd go PHI SMOtaxis eecicciai eer eras crohenateletaietapetcte ctataleiste atersiatel= atateraes aceeterete gz Goniotaxissrn..-. «licence Nels e slsesicisier/aee setae see eee eee eile 94 Chemotaxis es vous cctasickes tele eior acetals semaine stratermtelet aces 94 SUMMA YG rors arctasstolecs Grek opatalarevainioleiers tistencte ave otesstoloatet atetete @teteyose 96

gy lRinds/of behavior: joer ace ceiseniee is ae aioe aie. cioisioe sisiseidke See ei stole otete ers sepieeis 97

A (Genencjand:specifie behavior jateressvcreoicretet sry -\alossloles ola lose = ei ele/alevetetaye ateradsfeleiat 98 Percenta re obmegatrveresssistetsiete sta (oy: che utalotelate shetelsieveletateletates tetsteneiets 98 Character of the course in directive light..............0+eeeee0ee 99 Duration of movements (ris sis:aicictetayere l= afa'= ie (ele c's/n)sis/ayeraters =tetslols/oreiare 105 De greeiOl wandering states cic ssotaias<tesetersieisiete aletsiele! sto)<\s ojeleterepateieretetiata 106 Rate of locomonons 4 2.7 << cersierss vives once ¢ vis he vivieue sre mieisieue ieee 107 Tuimexequired;tolleave:aimniticurcles <ste:aiaresets areal= siacecereieleyaclae elem 107 The effects of fatigue....... Rasen TadpeorniadacmianaaicGecnttads 108 Responses to chanpesmintensitycncclaan siete eels sieleholeite ieeereieetel ere 109 Manner ofl coming tonrestisysisjsc/stsrepac se cise ain ctere oratahetetefenstevaystelsteerate 110 Bint ty cor Sen on non EBL on qOscao CeO EEOcabHoInohbeacundoc 110

IB Individwallbehaviorgencyetascissleterie selon <saieeicis sls oslo siet-toisiers reer enter III ‘Rate omisuccessive sd ayss..c dois + sic151s susie, ef- os) arorerenaieto sovatersraiatetetstotniate 112 Relative value of individual behavior.............-+0+seeeeeeeeees 113 Avcave: planariam' <7. cli-tsyatt a/atels niotels c/a stole ete nese ieee eet oars 114 Rilke s Sanco A HOnHOOUS HAC OnronneEnOdD oo Uanomasn dodoncoonss 116

(Pages 117 to 162 are printed in vel. v, no. 2)

4. Basisiofibehaviorss2cescicons-seete ees ane aves aint ee Cine CeaieT ee eee rete 117 A” Morphological’ basisiofibelnawior.-/crcc,ct01</=:e:cjes!/apa)s'a(e =! ntela(o/o\oaielstelsteievala}eetesrataye 117 a), General! formof the bOdy ss sic. s:esseis:thate-<:t1n's1a/e'el<ie'ace siete eareistersteratsitatelenciele 118

be (Photoreceptorss sewiqseectcsecice/icle ois elienryteleieteisya staretstetterctee cre rene 122 SUMUMAY ses aterctercloyaje ceislelaeie ss elie neste eeimetteietelererere sentir 127

B Physiolopicalibasis of behaviors tater. slois/ateieiale vl eyetstetereteyoieteletais vet stersfofetatstere ateiete 128 a Classification of: physiological’states/.c%.. wei vieleloaus aiele ese rlerersieie rete ster 129

b Changes in physiological states induced by light..............+.+s-0005 130

Effect of different intensities i.e: ejere:ctetois: store laietn tots oretetstestet teat etateteeen 130

Effect of'excessivellight: mimesis el slekeielalseiieveloeierileeieie eitater (eters 131

Effect of sudden change in light conditions.............++s+eeeeeee 131

Effect of continued exposure to light............-6 0.0 eee ee seen eee 131

Effect of previous ex posure:to)d ark: jelsteiers\stele ator ster te siatatsteteteperetsr ictal 132

ELCs iqgnappnnodacuacncnuoudan dscancu Joona osaecesteoee 133

G@: Psycholopicalibasisiof behaviors. eve peyre/eseletetatstejesstar ste interac eter etniens oe felotereratofonste 134 a, How,muchican'planarians)see' tise eicje siete! =rasetat=\ sietele)s(aiein e\efeleterers s7els\alele|</s 134

b: “Areiplanarsans/ablejtojlearm ? rarelateetersjereteters tet leterslete ele iets/aratelet=te\e\ele late? 135

The Reactions of Planarians to Light 27

Vili General: Conclusions fare <ls/slats(olayar/afols\elarer/=\elsta\elelatolaIs\ele\e7e dbopondonuecvonooacoRaass ae 138 He PD IFeckOn OLMteL sity jo/ate ys <lnysieteictoi<isl te = 1m eleialarelase]sictelse EES as cbatete ohessi seaponstetsvaae es creieya se 138

Age Historicalusetcmstacpicaste stich ererietec cient viachavsicts eferecitie eerie aie cetarssretes 138

B; Conclusions;with reference to;planariansicc. cic cee sc sence meine weog asincice vee 140

a The distinction between direction and intensity..............--..+2455 140

b “Lhe modifying, influenceiofidirettOn sis. ccicis:= «0 ce wie ee ee rieie cide eee 141

c Instances of behavior due to intensity alone ...............+--+---+00 142

d Theimoditying effect of other factors<.. 5. (-..-..6ce:cree nial @ sieinieisisie anise 144

Wuininih 97 sone vaccndades Some mag tnaUNoh 4 one ssocsDomsesenoon 145

PB aictllriveliGegim@relaa}y hint aohconoshoe codon apNBesResaaamasnopacnpocaDesboodpo 146 Siininkla Honshoshossouppocptuponvedeadtuparnouscacscoconocotc 153

€) YE eit caoo fo cosciponnoddadobenongon dob ocpnunaunocpbos house deeHoGnuCEos 153 Suing gusnp esac ana oo too sbunpEbenDo cH sonmpood CoD doc dao! 155

Wat Tyo) Hoya doh Mabe onaenon bao odparodoesssvenootodnonud BODO SNGOBRAD EAD nOSROCaoUs 155

I InrrRopucTION

Light is one of the physical factors which influence the behavior of organisms. ‘The great majority of living things are normally subjected to regular periodic changes in the amount of light to which they are exposed during the alternation of day and night. In addition to these constant periodic changes, there are innu- merable irregular gradations in both the intensity and the character of the light naturally acting upon any organism. An agent of such wide range and almost universal influence as light ought, therefore, when properly analyzed, to prove of material service in interpreting the behavior of animals and plants. The depend- ence upon light of animals provided with organs of sight, is self evident. ‘The direct bearing, too, of light upon chlorophyllaceous plants in the manufacture of their food substance, is plain. But how far light plays a direct part in the life of non-photosynthetic plants and of animals which cannot “see,”’ is less clear.

Although possessing eyes, it is very probable that planarians are unable to see in the sense of distinguishing shapes, and it is questionable how far they can distinguish between even large regions of different light intensity.

The object of the following paper is to examine the relation of light to animal behavior as applied to certain planarians.

38 Herbert Eugene Walter

Il Hisroricar

Our knowledge of planarians, as of most other animals, has passed through certain historical phases, during which emphasis has been laid first upon taxonomy and anatomy and latterly upon embryology and zoogeography. The results of these various forms of investigation are highly important since they make the foundation for all future work upon this group of animals. They have, however, only an indirect interest in the present connection and do not, therefore, require review.

Perhaps the most modern advance in our knowledge of pla- narians is represented by the school which treats of them as living objects whose individual behavior is to be intimately correlated both with their structure and environment. ‘The most noteworthy contribution from this standpoint has been made by Pearl (03), who has analyzed in considerable detail the reactions of fresh- water planarians (notably Planaria maculata, Planaria doroto- cephala and Dendroccelum lacteum) to various stimuli. He has not, however, discussed the effects of light except incidentally.

The earliest reference to the relation of planarians to light is by Dalyell (’14). In his interesting volume on planarians a great number of keen observations upon the general habits and struc- ture of planarians are made, which have since been confirmed, together with certain statements which have not fared as well with the advance of scientific knowledge.

He makes the statement (’14, p- 9) that “most planariz court the light indeed;' but P. flexilis rather inclines to shun it, less, we may conjecture, from being warned of its presence by the specks or eyes, than from some disagreeable sensation produced on the body.” Again, referring to P. felina (’14, p. 46), ‘“ This planaria, like the rest of its genus, is powerfully excited to motion by the presence of light. If a number be confined in a glass vessel, the whole assemble in a quiescent state, on the side next the light It is a little surprising that Dalyell should have received the impres- sion that the majority of planarians “court the light,” since he clearly points out the nocturnal habits of these worms. He

1 The italics are mine.

The Reactions of Planarians to Light 39

doubts whether the eyes are of service in finding food and says of worms under aquarium conditions (14, p. 107), “If remaining aconsiderable time unchanged, the planariz decrease more rapidly, they become languid, scarcely moving either by the influence of the light or heat, and at last adhere entirely to the side of the contain- ing vessel, where they perish.”

Dugés (’28) observed that when light 1s concentrated by means of a lens upon either Dendroccelum (?) or Planaria, movement results which is most pronounced when directed toward the ante- rior end of the worm. He tested the effects of direct sunlight and of diffuse daylight as well as of candle-light, and concluded that the response increases with the intensity of the light. The non- dioptric character of the eyes he has described remarkably well for one working so long before the days of the microtome, and his conclusion, already suggested by Dalyell and later confirmed by Kennel (88), and others, that the eyes play no part in the finding of food, is noteworthy. He also notes that planarians seek the dark.

Dalyell (’53, p. 99), in a later volume says, “On April 29 I pro- cured a fine specimen of Planaria cornuta, which spawned soon afterward. The spawn had been breaking up for two or three days preceding May 24, when multitudes of extremely minute yellow specks were seen swimming in the water. Their motion was sufficiently active, without being very quick; it was pursued in all directions and the spawn being contained in a small cylin- drical jar, the specks crowded to the sides next the light whereon numbers remained almost stationary.’’ Again (’53, p. 104), “When withdrawn from the dark the young Planariz rose in great numbers toward the surface of the water, congregating on the sides next the light.”” It is extremely doubtful whether the organ- isms here described were really young planarians. It is more likely that they were the young of some other aquatic animal. Dalyell correctly describes Planaria lactea (Dendroccelum lac- teum?) as being nocturnal. He observed that numbers of this species, beginning activity in the evening, rose on the sides of the jar, although many had descended again by morning.

More recently attention has been specifically directed to the

40 Herbert Eugene Walter

light relations of planarians in various papers by Loeb, whose important contribution in 18go, ‘Der Heliotropismus der Thiere und seine Uebereinstimmung mit dem Heliotropismus der Pflan- zen,” paved the way in general for all work on this subject. He found (’93b, p. tor) that Planaria torva is not “‘heliotropic”’ in the strict sense, but rather “unterschiedsempfindlich,”’ that is to say, it did not always move away from thesource of the light in the direction of the rays and remain as far removed as possible, but moved about more or less at random, coming torest in some area of lessened light intensity.

In a later paper Loeb (’94, p. 255) states that when planarians are suddenly brought into the light they begin to move, an increase in light intensity leading to activity and conversely a decrease, to rest. [he grounds for this conclusion are not made clear. He further confirms the view that planarians are active at night, com- ing to rest in situations of lowered light intensity in daytime.

In further experiments P. torva when decapitated was found to react to light precisely as normal worms do with the difference that the reaction required more time. Thysanozo6n brochii, a polyclad, on the other hand, lost its power to respond to light when

“the eyes and brain were amputated, from which Loeb draws the conclusion that animals which are closely related morphologically may exhibit wide physiological differences.

Hesse (’97), in his classic study of the anatomy of the turbel- larian eye, mentions some experiments and observations on the behavior of planarians and in addition makes valuable contribu- tions to the morphological basis of light reactions. He observed that planarians become active at twilight and he also experimented upon decapitated worms, apparently without being aware of the previous work by Loeb, with whose results his own agreed. He found, as Loeb did, that worms deprived of eyes finally come to rest in areas of lesser intensity much as do normal worms but after a longer time. Hesse found, too, that Dendroccelum lacteum came to rest in the dark 119 times out of a possible 120, whereas with Euplanaria (Planaria) gonocephala the same result was effected in only 88 out of 120 times, notwithstanding the fact that the latter has more highly developed eyes than Dendroccelum.

The Reactions of Planartans to Light AI

This led him to state (’97, p. 552) that “die Starke der Reaction auf Lichtwirkung nicht der Starke der Lichtwahrnehmung ent- spricht,”” and he ascribes this difference in behavior to a differ- ence in the ‘Gefihlston”’ of the two species. Amongother observa- tions described by Hesse, the two following are of importance in this connection, namely, that a sudden introduction of light caused an almost immediate turning away on the part of the worm, and, that worms with eyes could not be made to remain in the light when escape was possible. In his opinion this apparent perception on the part of the worm is due not to the animal’s ability to distinguish light but rather to unpleasant chemical reac- tions set up within the organism as the result of light stimulation. And, lastly, Hesse showed that the general position of the eyes of a planarian, together with the arrangement of their sensory portion, partly enclosed as it is within pigment cups, affords a device whereby the worm can be oriented to light. By means of this simple apparatus it receives a localized stimulus, which enables it to distinguish the direction from which the light comes. If light, striking the eye of a worm, fell-upon sense cells which were unscreened in any way by pigment, there would result a general stimulation without localization of the stimulus and consequently orientation could not be effected.

Parker and Burnett (00) sought, by quantitative methods and with more accuracy than Loeb or Hesse, to establish the part played by the eyes in light responses of planarians. They came to the same general conclusion as these authors since they found that Planaria gonocephala without eyes reacts to light essentially as normal animals do, except that the reaction time is somewhat longer. They also showed that worms when pointed toward the source of light travel at a slower rate than when headed in the opposite direction. With regard to the mechanism of the light , response they say (’00, p. 383): ‘“‘We have seen nothing in our experiments that supports the opinion suggested by Hesse (p. 551) that reactions such as we have described are due to the direct influence of light on the internal parts of the planarians, and we are more inclined to the view that these reactions are initiated by the effect of light on the integument of the animal, 7. e., are

42 Herbert Eugene Walter

due to what Graber (’83, p. 229) has called a ‘dermatropic’ function.”

Bardeen (’ora, p. 13), speaking of Planaria maculata, states that “susceptibility to light is apt to become lost if worms are kept in captivity,’ and he notes the fact, already brought out by Chichkoff (92), that pigment becomes reduced in sunlight. Hesse had previously emphasized the point that the pigment of the eye of any organism has in itself primarily nothing whatever to do with light perception. Bardeen further found that small pieces of planarians capable of locomotion will respond to light in the same way as uninjured animals, and he notes (’o1a, p. 13), that the worms seem “to move about more by night than by day.” In a later paper (’otb) he speaks of the fact that when a dish containing planarians is brought into light the worms are commonly roused to activity, although how far such activity is due to light and how far to mechanical disturbances he does not make clear.

Lillie (’o1), experimenting upon the regeneration of Dendro- coelum lacteum, discovered that posterior headless parts fail to give the typical reaction to light and are incapable of regeneration. He draws the conclusion (’or, p. 132) that “any symmetrical piece of Dendroccelum capable of regeneration tends to come to rest in the shaded part of the dish precisely like a normal individual” and that parts incapable of regenerating ‘‘also become incapable, after a day or two, of performing the usual reactions to light.” These results on Dendroccelum, it will be seen, are similar to those Loeb obtained in experiments upon the polyclad Thysanozoon.

Curtis (’02) reports from laboratory observations 42 cases of fission in Planaria maculata, of which number 39 occurred between Io p.m. and6a.m. He adds, however (’02, p. 524), that “this did not seem due to the amount of light to which the animals were subjected during the day, for some of the dishes were so shaded that there was practically no light, day or night, except when they were being examined, and the division was the same in these as in others which were exposed to full daylight.”” A case of division in Bipalium also occurring by night is described by Lehnert (91).

In a contribution to the geographical distribution of Planaria

The Reactions of Planarians to Light 43

gonocephala, P. cornuta and P. alpina,*? Voigt (’o4) incidentally refers to the manner in which these animals come to rest in the darkest part of a dish. He afhirms that when an aquarium is sud- denly lighted at night only those that are hungry, :. ¢., those with comparatively empty digestive tracts, are found in motion, and he notes that in certain conditions worms may remain quiescent for weeks. The statement made earlier by Duges, that the eyes of planarians play no part in finding food, Voigt confirms. ‘These organs he explains are an aid in distinguishing differences in light intensity as well as the direction from which light comes but are entirely incapable, owing to the simplicity of their structure, of discerning the form of objects. In his opinion worms crawl into hollow stems and similar sheltered places to escape light rather than for warmth, as Wilhelm: ('04) suggests. Neither author, apparently, considers the possible part played by thigmotaxis under such circumstances. Of the delicacy with which worms react to light Voigt says (’04, p. 173): “Die Empfindlichkeit der Planariden gegen plotzliche Belichtung tritt so scharf hervor, das sie fiir den Unterricht eines der anschaulichsten Beispiele zur Demonstration der Lichtflucht bei niederen Tiere darbieten.” Notwithstanding this high degree of sensitiveness to light, he finds that the worms when seeking their food leave the shade and come out even into direct sunlight. And, finally, concerning the bearing which light has on the problem of distribution, he concludes (’04, p- 175): “Auf die Verbreitung im Allgemeinen hat die Belichtung der Bache wenig Einfluss, da sich in der Regel genug dunkele Schlupfwinkel finden, in denen sich die Tiere verbergen konnen.”’

Darwin (’44, p. 242) observed that /and planarians, “especially Planaria tasmania, had an immediate apprehension and _ dislike of light, which they showed by crawling, when the lid of the box was taken off, to the under side of pieces of rotten wood,” and in his enumeration of the places where various species of land pla- narians were found, their avoidance of light is plainly shown.

A note by Leidy (’58) refers to finding Rhynchodemus sylvati- cus crawling about on fences frequently at night, but rarely by day.

Pp. alpina = P. torva according to Borelli (’93).

44 Herbert Eugene Walter

Moseley (’74, p. 111) states that “land planarians are probably all of them nocturnal in habit.” Speaking of the Ceylon land planarians in particular he says: “They are found in dark places, such as under large fallen leaves, and in confnement they coil themselves up away from light.’’ He mentions also the fact that Planaria torva and Dendroccelum lacteum choose the dark side of the vessel in which they are contained.

As has already been mentioned, Lehnert (’91) found Bipalium kewense undergoing fission in the dark. Both Bipalium and Geodesmus, hee says, seek continually to hide in shadowy places avoiding even diffuse daylight. Concerning the degree of light perception possessed by planarians, he offers the opinion (’9I, p. 326) that ““Bipalium scheint mit seinem Augen die Umrisse von Gegenstanden in Lichte wahrnehmen zu kénnen.”

Hogg (’97) notes that Bipalium is nocturnal in habit, remaining pieae during the day.

Only Fiseiel references to the polyclads are found bearing upon the question of light reactions, as for example this sentence, which occurs in Lang’s exhaustive monograph (’84, p. 641), “Die meisten Arten scheuen das directe Sonnenlicht.”’ “The behavior of Thysanozoon with reference to light has already been mentioned

(Loeb, ’94).

Concerning the light reactions of the rhabdoceeles, especially cer- tain green fone 4 in Shih the green cells are probably symbiotuc, a considerable literature may be found. ‘The principal papers relating to these forms are as follows: On Convoluta schultzii, by Geddes. (79), Barthélémy (84) and Delage (86); on Convo- luta roscoffensis, by Haberlandt | COL), Bohn: Coza, (o3b, Ose); Gamble and Keeble (’03) and Fiihner (’06). Vortex viridis and Mesostomum viridatum (?) are discussed by Schultze (’51), von Graff (’84) and Sekera (’03). A résumé of these papers is, however, out of place here, since the presence of green cells in the organisms involves an entirely different problem from that which is under consideration.

The foregoing historical sketch furnishes the basis of the follow- ing general summary of facts which have thus been established with more or less certainty regarding the reactions of planarians to light.

The Reactions of Planarians to Light 45

1 Planarians are nocturnal, seeking the dark when exposed to light. 2 The eyes are useless in finding food. 3. The anterior end of the body is the part most responsive to light 4 Decapitated worms act normally except for a slower reac- tion time. 5 Onientation to light depends largely upon the character of the pigment cups of the eyes. 6 The relative energy of the response is dependent upon the intensity of the light. 7 Pigment is reduced in sunlight. 8 Pieces of worms which are large enough to move or regener- ate react to light. g Fission may occur more readily in the dark. 10 Different species respond differently to light. 11 Light reactions diminish during “captivity.” 12 Planariansare “unterschiedsempfindlich” instead of “helio- tropic.”

III Marertar

The species principally used in the following investigations were Planaria maculata Leidy; Planaria gonocephala Dugés; Phagocata gracilis Leidy; Dendroccelum lacteum Oersted; and Bdelloura candida Guard, all of which are inhabitants of fresh water except Bdelloura, a salt-water species, found living semi- parasitically on the horseshoe crab (Limulus polyphemus). Some observations also were made upon a cave planarian, that as yet has not been identified but which may belong to the genus Phagocata. This interesting worm was kindly placed at my spose oy Dr: A. M. Banta.

At any season of the year an ample supply of fresh material was easily obtained except in midwinter, when it was necessary to cut through the ice and dredge up from the bottom water-weeds to which the worms cling.

The source of supply for Planaria gonocephala was a small. pond to the west of Fresh Pond in Cambridge, Mass., while Pla-

46 Herbert Eugene Walter

naria maculata, Dendroccelum and Phagocata were chiefly obtained from a pond at Falmouth, Mass., where they are especially abun- dant. ‘Twice, through the kindness of Professor Parker, aquaria were generously stocked with Dendroccelum, from a spring on Mount Monadnock, N.H. Bdellourawas obtained from Wood’s Hole, Mass., during the summer from freshly caught horseshoe crabs and, later in the year, from specimens kept in captivity.

The setting-up of balanced aquaria in which planarians would thrive did not prove to be a difficult matter. The following method, based largely upon suggestions by Wilhelmi (’04), was used. Jars were filled to the depth of two or three inches with cinders, dirt and dead leaves, over which was spread an equally deep layer of clean sand. Clear water was then poured into the remaining space and the whole allowed to settle, after which afew such plants as Anacharis or Myriophyllum, with whatever micro- scopic life might adhere to them, were added, together with a hand- ful of large pebbles to diversify the bottom. ‘The jars were kept covered from dust in a cool place and occasionally a crushed snail was dropped into each one to supply the worms with food.

Planarians require pure water. Whenever for any reason the water in which they are kept becomes foul they will desert their places of concealment and crawl up the sides of the jar, while water that has been standing in lead or iron pipes quickly causes them to disintegrate. Rainwater or water taken directly from some natural source, gives better results than that which has been conveyed through pipes. Naturally the least chemical disturbance takes place when the worms are kept in water dipped up at the time and place of their capture.

Planarians will live without being fed for over three months when isolated in jars containing nothing except pure water, but meanwhile they decrease regularly in size. It seems to be impos- sible to “starve” them in the sense in which higher animals may be forced to die from lack of food leaving behind a dead body. These worms instead simply consume their own substance almost to the vanishing point.

During a part of the summer of 1905 observations and experi- ments were carried on at the laboratory of the U. S. Fisheries

The Reactions of Planarians to Light 47

Bureau at Wood’s Hole, Mass., and I wish here to express my thanks to the director, Dr. F. B. Sumner, as well as to others in authority there, for their uniform courtesies. The bulk of the investigation, however, was made at Harvard University. I am deeply indebted to Professor Mark for the privilege of having a place in his laboratory and particularly to Prof. G. H. Parker, under whose immediate direction the work was done and whose daily counsels and generous suggestions were indispensable.

IV Crirerta ror Measurinc BEHAVIOR

Both the form and the structure of an animal set a limit to the character and degree of its movements, which no combination of stimuli, external or internal, can force it to overstep. In estimat- ing the influence of light upon planarians, therefore, it is necessary to know not only the normal behavior of the worms but also the possible range of their reactions under any circumstances. For example, the ordinary gliding locomotion of planarians is accom- plished by means of cilia beating in a mucus track and augmented by muscular contraction. It is physically impossible for this sort of locomotion, even under the most favorable conditions, to exceed a certain rate. By the use of excessive stimuli, however, a worm may be forced to abandon this accustomed gliding for a somewhat faster method of progression known as “crawling”’ or “humping,” in which the muscles are used more than the cilia. But when this is done the limit of possible rate of locomotion has been reached, at least for fresh water planarians, which cannot be urged to abandon entirely contact with some support and to swim freely in water, although the marine form, Bdelloura, does have this addition to its repertory of behavior.

The following observations may illustrate more specifically what is meant by range of behavior. Planaria maculata, when gliding on the bottom of a dish, was lightly touched on the anterior end with a hair mounted on a glass rod. During one hundred trials of this kind eight different responses resulted, which may be indi- cated as follows:

48 Herbert Eugene Walter

Times

1) | Contracted wand turned) asi Garerctetrs:sccxsieteyeraressictesest1oia/ sissies ets ofeiars stat siayarets aistelatet atest steyetateteneterete 32 2 Contracted, lifted up the anterior end, and turned aside........-....0+eeeceeeeeeceeccece 27 3 Contracted, lifted up the anterior end and went straight forward ...........-.......+se005 17 4 Contracted momentarily and then went straight ahead .............020.020seeceeeeeees 5 5» Didmot,contractibutiturmed asides a. cccreerae sclas ace -acigad meine eee CPE ere tEee 2 6 Did not contract but lifted up the anterior end and turned aside..........-.0.00eseeeeeeee 7 7 Did not contract but lifted up the anterior end and went straight forward.................+ 9 8 Did not contract but went straight ahead...... IPR ACO NOD Onan dopeTctn cacdacoacdad I

DO tal sicrerayats store state senceloepeteite re tetogen Me oteis lel stoi sveie/o\s0ore co, 312 agsibia, nia sia) el ofer ticle Meiers tear eee meteteore 100

Animals which, like planarians, present a limited range of be- havior are, therefore, more favorable subjects for experimentation than higher forms whose structural complexity increases their possi- ble responses, making in consequence the analysis of cause and effect in their activities more difficult. It is evidently desirable, then, to have as many different ways for measuring behavior as possible, in order not to state these responses loosely from general impressions but in quantitative terms. ‘The principal criteria of planarian reactions to light used in this study, follow:

1 Rate of Locomotion. Since the entire range of possible rates of locomotion depends upon the structure of the worm and is not very great, slight differences become significant.

2. Amountand Character of T urning, that is, whether persistent or irregular, decided or vague, clockwise or contra-clockwise.

3. Change of Course. A change in the character, but not neces- sarily in the direction, of the course is referred to here. “Circus movements,” for example, would not be included under this head- ing because the curving path in such cases, although constantly changing in direction, does not change in character. “Tangents to a circle, however, as well as angular and abrupt deviations from a straight line may properly be regarded as changes of course.

4. Interval of Response. Vhe apparent effect of light is not immediate in all cases, therefore, the time elapsing between the application of the stimulus and the response to it is a valuable measure of reaction.

5 Degree of Wandering. Ina sense the degree of wandering shown by a worm is a measure of its indifference to the stimuli acting upon it. It must be noted, however, that apparent indif-

The Reactions of Planarians to Light 49

ference may sometimes be due to a balance of opposing stimuli, in which case wandering or aimlessness is not a true measure of the effect of any single stimulus.

6 Orientation. his is a measure of behavior with reference to the source of the light. It is expressed by the degree of posi- tiveness or negativeness which the worm exhibits.

7 Duration of Movement. The time it takes a worm to tire out when subjected to certain stimuli or, in other words, a measure of fatigue.

8 Effect of Repetition. A measure of response is here referred to which may be expressed quantitatively in units of time or quali- tatively in manner of behavior.

g Wigwag Movements. These are waving movements of the anterior end of the planarian, which appear to be a definite attempt on the part of the worm to become adjusted to the stimuli acting upon it.

10 The Time Required to Leave a Unit Circle. This is a rather unsatisfactory criterion because it may indicate in some cases a combination of several conditions as, for instance, latency of response, rate of locomotion and degree of wandering.

11 Manner of Coming to Rest. Included under this heading are such points as the position assumed, the locality selected, and the abruptness of the act.

Naturally some of the foregoing measures of behavior will be seen to have more application and value than others in the follow- ing study.

V OBSERVATIONS I PHOTOKINESIS

The term photokinesis was introduced by Engelmann (’83) to denote the activities which are induced solely by the intensity of light when the directive or orienting factor has been eliminated.

In this section will be considered, (A) the behavior of planarians in the absence of light; (B) their behavior in different intensities of non-directive light, and (C) the effect of abrupt changes, both in time and space, in the intensity of non-directive light.

50 Herbert Eugene Walter

A Behavior in Dark

Darkness may be called the zero point in the scale of light inten- sities. ‘That light is not essential to the activity of planarians is shown by their performances in its absence, as is demonstrated by the following facts.

Rate of Locomotion. ‘The average rate of ten individuals of Planaria gonocephala was found to be 0.50 mm. per second in the dark while the same ten worms, subjected to a light from above of 38 c.m.,? with all the other conditions unchanged, averaged 0.82 mm. per second.

Again, ten worms of the same species were allowed to travel in the dark ten minutes in one set of experiments and six minutes in another, when their average rates were found to be 0.42 and 0.57 mm. per second, respectively.

The method devised for obtaining the above records, previously used in experiments upon fresh water snails (Walter, ’06), although tedious was comparatively accurate. A clean glass plate was submerged in a dish of water and the latter placed in a light-proof receptacle. A single worm was then allowed to travel on this glass for a unit of time, after which the plate was removed and “developed” bk pouring over it powdered carmine shaken up in water. A sufficier.t number of the insoluble carmine particles adhered to the mucus-track left on the glass by the gliding worm to make it possible to wipe dry the reverse side of the plate and to trace thereon in ink the exact course taken by the worm. ‘This permanent ink line was then measured by means of a map meas- urer such as is in common use for measuring sinuous lines.

A series of experiments, to be described more in detail later (Table III, p. 57), forms a basis of comparison with the foregoing records in the dark, and further shows that there is an increase in the rate of locomotion in the light.

Ten worms, subjected to various intensities of light projected from above and ranging from less than one to several hundred candle meters, showed rates which in all cases were greater than the rate traveled in the dark.

$ The abbreviation c.m. is used to denote candle meters.

The Reactions of Planarians to Light 51

Turning. That planarians do more turning in the dark than they do in various intensities of non-directive light is apparent from the following table of percentages.

TABLE I Per cent of LS of Planaria gonocephala in the dark and in various intensities uf use ——— = Light in candle meters...........-..+ ° .. AI Ir | 39 | i 78 | eel 155 | 217 431 |Av. of all (dark) | | | intensities Percent off turmin pete -ater(eieistesinteeciet= | 87 76 | 66 | 69 | 81 | 67 | 75 | 77 | 65 72 . | Per cent of straight paths............ | 13 24 | 34 | 31 | 19 | 33 | 25 | 23) 35 28 Number of observations............. 71 79 | 67 | 85 | 57 | 62 | 67 | 57 8

Furthermore, out of a total of 40 cases of turnings made by dif- ferent individuals of Planaria gonocephala in the dark 23 were clockwise and 23 contra-clockwise. This perfect balance in be- havior did not recur when the same worms performed turning evolutions in the light.

Change of Course. As to what constitutes “definite” and what “indefinite”? changes of path, an S-shaped course is to be regarded as an indefinite efenless wandering, whereas angles in a snare: path or tangents in a curving path are classed as deanite responses because they are what would normally occur if some directive stim- ulus were interposed. It was found that P. gonocephala made indefinite changes in its course more frequently in the dark than in any series of light intensities to which it was subjected for an equal length of time. On the other hand definite changes occurred oftener in the light, although the factor of directive light had been excluded.

Table II summarizes 350 recouas on 10 different worms with the results reduced to percentages.

It will be seen that the percent of S-shaped (‘‘indefinite”’) paths in the dark decidedly eclipses that which was made in any inten- sity of light, while the per cent of angular and tangential paths (“‘definite’’) laid in the dark is exceeded in every instance by that made in any intensity of light with one exception, viz: II ¢.m., which, however, is not sufficient to change the average result.

Summary. Planarians move about in the dark but at a slower rate than in non-directive light whatever the intensity. They

52 Herbert Eugene Walter

turn more in the dark than in the light, going clockwise or contra- clockwise with equal readiness. Finally, they make more indef- nite changes in their paths in the dark, but fewer definite changes than in the light.

TABLE II

Percentage of definite and indefinite changes in the character of the course in dark and in light of different

intensities

Details of the several intensities employed

| Average |

|

Light in candle meters................-- o | for allin- Jo.94) 11 | 37 | 78 | 126) 155] 217] 431 |(dark)) tensities

Definite changes (angular or tangential

changes) percents ccoeecasssasanasn| 18 28.5 30°] 15) | 27 |) 32: |i 35;) 21 || 32" |'40 Indefinite changes (S-shaped paths), per) |

CONE serretays ee etic este tte stete pein esets 47 | 23 21 | 30 | 23 | 20 | 26 | 34 | 20 | 99 No change in character of course, percent} 35 | 48.5 | 49 | 54 | 50] 48 | 39 | 45 | 48 | 51

Number of observations......../.:..... 34 316 49 | 35 | 48 | 37 | 31 | 48 | 35 | 33

B Non-Drrective Light

a Apparatus

To test the effect of purely non-directive light, it is of course necessary to eliminate the possible influence of directive light. This may be done by projecting the light upon the moving worms in such a way that they are unable to go either toward or away from the source of the light. Whatever effect is obtained under such circumstances must be ascribed to the non-directive power of light.

The elimination of the directive influence of light can be accom- plished by means of various devices. (1) The light may be made to fall vertically from above upon a horizontal field; (2) it may be reflected vertically from below so as to pass through a transparent held at right angles to the plane of the field; (3) methods 1 and 2 may be combined. ‘The apparatus finally used in the majority of experiments with non-directive light, was based upon the method first mentioned.

The Reactions of Planarians to Light 53

Fig. 1 shows a diagrammatic vertical section of this apparatus. The light (4), an incandescent electric lamp, was mounted in a black sheet-iron hood (B) to prevent the escape of any lateral light. This hood was suspended from the ceiling of the dark room where the experiments were carried on and was arranged so that it could be easily raised or lowered, thus changing the height and consequently the intensity of the light with reference to any fixed point below. In the hood, beneath the light, was supported

Fig.1 4, light; B, walls of hood; C, heat screen; DD, diaphragm; E, roof of hood; F, plate-glass floor of aquarium; G, paraffine wall of aquarium; HH, diaphragm to cut off light reflections from paraffine wall; J, wall of reflector box; ¥, open side of reflector box; K, mirror; L, walls of tunnel; MM, black draperies; N, table.

a flat-bottomed, clear-glass dish (C) containing distilled water to a depth of about three centimeters. The heat screenthus obtained effectually filtered out the heat rays, allowing only the light rays to pass through. A few inches under the heat screen was inserted a diaphragm (D), painted black, the purpose of which was to aid in cutting out side reflections besides allowing only a central col- umn of light to escape below. A black sheet-iron roof (£) con-

54 Herbert Eugene Walter

fined the upward rays to reflections within the hood itself, at the same time permitting the escape of heated air. Ona table directly under the suspended light lay a horizontal sheet of plate glass (F,) affixed to the upper surface of which was a circular ring (G) made of a mixture of parafine and lampblack. There was thus formed a circular water-tight aquarium twenty centimeters in diameter and two centimeters deep, in which the worms could be observed. On the top of this circular ring rested a black dia- phragm (/7), the aperture of which was sufficiently small to exclude any side reflections which might come from the black paraffine wall.

The aquarium, it must be explained, did not rest directly on the table but was mounted as the cover of a box (J), the interior of which had been rendered largely free from reflecting surfaces by the use of black camera-paint. One side of the box was removed and, facing the opening thus made, a mirror (K) was placed at an inclination of 45° with the horizon. The end of a square tun- nel (L), ten feet long and made of black cloth stretched upon a framework of wood, fitted close up to this opening. Suspended from the lower edge of the hood and surrounding the aquarium were adjustable black draperies (/) designed to shut out possible side light and at the same time to allow a hole for the eye of the observer. It will be seen that all light reaching the aquarium comes from the lamp above by passing through the heat screen.

After illuminating the field of observation the light passes through the glass floor of the aquarium and is reflected by the mirror into the black tunnel. Most of the light is absorbed in the tunnel, only an insignificant minimum being reflected back to the aquarium floor. Otherwise complications in the character and intensity of the light might arise.

By moving the hood (B) up and down and by using lamps of different candle powers a variety of intensities was obtained. The lamps used were tested by means of a Lummer-Brodhun pho- tometer, the loss by reflection from the surface of the water both at the heat screen and at the aquarium being reckoned out in deter- mining the different intensities employed.

By simple observation, data for such criteria of behavior as

The Reactions of Planarians to Light 55

amount of turning, changes in course, degree of wandering, inter- val of response and manner of coming to rest, could be obtained in this apparatus with approximate correctness. To determine the rate of locomotion, however, required a device which would measure accurately the distance traveled in a unit of time. The

Fig.2 ABCF, pantograph; C, fixed point; D, paraffine wall of aquarium; £, plate glass bottom of aquarium; F, place where the arm 4 is grasped by the operator. A style is located at end of arm A, in contact with under side of aquariumfloor. J, style at end of tracing arm B, in contact with smoked paper; , beginning of a course traced on the smoked paper; K, drawing board for attachment of smoked paper; L, sheet of smoked paper fastened to drawing board; M, actual course of the worm.

method already mentioned of measuring rate from mucus-tracks developed by means of powdered carmine, proved too tedious and uncertain except for the worm’s maneuvers in the dark, when it seemed the only available way.

56 Herbert Eugene Walter

To avoid the inconveniences of this method an attachment was devised for directly duplicating the path of a worm by means of a style traveling over a sheet of smoked paper. The records thus traced were made permanent by immersing the smoked sheets in a weak solution of resin in alcohol and allowing them to dry, after which the paths could be accurately measured and the rates com- puted.

The arrangement of this attachment, as seen from above, is shown in Fig. 2. The diaphragm (Fig. 1, H) has been removed for the sake of clearness. At the tip of arm 4F a style directed upward comes in contact with the under surface of the aquarium bottom (Fig. 1, F), while at the tip of arm B a similar style that is pointed downward traces a line on the sheet of smoked paper L at the left. After a little practice it was not difficult to keep the style of arm 4 directly under the posterior end of a gliding worm, thus duplicating its movements with considerable accuracy. The expiration of any time interval can be indicated on the smoked paper record by a crosswise scratch in the path.

Arm 4 was rendered as non-reflecting as possible by black cam- era paint as well as by being made triangular in cross section with the apex of the triangle upward. ‘Thus whatever rays struck it from above were mostly either absorbed or reflected in a horizon- tal direction, so that they did not reach the worm under experiment.

b- Results

Rate of Locomotion. Planaria gonocephala moves somewhat more quickly in non-directive light than it does in dark. Ten apparently normal and representative worms were selected and isolated in individual aquaria. They were kept in the dim light of the dark room in water of the same temperature as that of the experimental aquarium in which they were observed. At the end of thirty-four days of experimentation these worms showed prac- tically the same average rates under the same intensities of light as they did at first. By alternating the individuals these trials were so made that fatigue effects had little part in the results, while the succession of light intensities was varied in such a way

The Reactions of Planarians to Light 57

that cumulative effects and the influence of previous exposures were largely avoided. The results obtained in 259 trials are condensed in Table III.

TABLE III Rate of locomotion in millimeters per second of Planaria gonocephala in various intensities of non-directive light | ie | a ‘ | =p | Candlejmeterssaacacie sere see | ° | 0.94 | II | 39 | 78 | 126 | 155 217 | 431 Average mm. per sec....... 0.57 | 0.66 | 0.69 | 0.75 | 0.64 | 0.66 | 0.69 | 0.70 | 0.63 Number of records ............ | 30 «|. 28 30 | 29 | 27 || 301) 030 2 28

The mechanical stimulus resulting from the removal of the worms, by means of a camel-hair brush, from their individual aquaria to the observation aquarium was practically the same in all cases as were al] the other external stimuli except light. The difference in the rate of locomotion appearing in these averages is, therefore, clearly due to differences in the light intensity em- ployed.

It will be seen also that rate does not increase progressively with intensity. ‘The series of rates and intensities under Table III, if plotted in a frequency curve would give two modes, one at 39 and the other at 217 candle meters, with a slight depression be- tween the two. Still, as has been already pointed out, any inten- sity of light gives a faster rate than no light at all.

The slowest average rate was made under the highest intensity of light employed. Certain facts to be brought forward later favor the opinion that this was not an accidental result.

Under continuous exposure to one intensity of light the rate of locomotion decreases. ‘The worms seem to “run down” gradually, so that at the end of ten minutes their rate is only about half that during the first minute. Data illustrating this point are given in Table IV.

The rate of locomotion depends not so much upon the intensity of light as upon other factors which tend to produce individual behavior upon the part of each particular worm. Stated in another way, there is greater variation between different individuals in the average rate of their locomotion under all intensities than there

58 Herbert Eugene Walter

is in the average rate of all individuals collectively under different intensities. [he data for this latter point based upon the average rate of ten worms (259 observations) under different intensities has already been given in Table III (p. 57). The extremes in rate there shown are 0.57 mm. per sec. at zero intensity and 0.75

TABLE IV

Average rate of locomotion of Planaria gonocephala in successive minutes of exposure to 39 c.m. of non- directive light

Number of minute.......) Ist | 2d | 3d | 4th sth | 6th | 7th 8th | gth | roth | 11th | 12th | | | |

| | | | | |

| 4 > |b 48 0] Siena aes ee 3 29 | .25 | 29

No. of records averaged. . 17 | 15 | 12 | 7| 5 2

Rate in mm. per second..) .63 | .625) .565) .55 | -53 | -55 | -375] -39 | -39

mm. per sec. at 39 c.m. intensity, which makes a range of 0.18 mm. per sec. When the same data are rearranged to show the average rate for each individual for all intensities, as in Table V, the extremes are 0.49 mm. per sec. and 0.83 mm. per sec. with a range of 0.34 mm. per sec.

In fact the individual behavior of these ten worms, despite their apparent similarity, was sufficiently distinct to allow each one to be thereby identified.

Turning. Attention has already been called to the fact that there is less turning in light of various intensities than in the dark. A return to Table I will make plain that there fails to be any

TABLE V

Average rate of locomotion for each of ten worms (Planaria gonocephala) based on trials with non-directive

light of various intensities

Identification number of worm........ | I 2 3 4 5 6 7 8 9 10

pressed in mm. persec.............|0.79 |0-57 0.68 (0.64 0.83 |0.70 |o.72 0.58 [0-49 |0.62 ! = ae Seas

definite correlation between the degree of intensity of the light and the amount of turning, although the least turning occurs under the highest intensity. This latter point, however, rests upon a very slight difference and may not be significant. It 1s neverthe- less worth mentioning, since it is in line with the effect of the

os

The Reactions of Planarians to Light 59

highest intensity upon rate, as well as with certain other evidence to be discussed later.

The small excess of clockwise over contra-clockwise turnings is not explainable upon the ground of varying intensities of light. A distribution of the cases under the several intensities of light (Table VI) makes it plain that this peculiarity is due rather to individual causes than to light intensities. Indeed it would be difhcult to conceive theoretically how varying intensities of non- directive light could influence a worm in such a way as to affect the direction in which it turns. ‘The natural expectation accord- ing to chance would be an equal number of turnings in either direction. ‘The excess of clockwise turns seems, therefore, un- doubtedly due to internal causes which render certain worms more liable to go one way than another. In fact, when the records were arranged according to individual behavior it was found that of the ten worms seven averaged a majority of clock- wise turns while only three fell in the contra-clockwise column.

TABLE VI Character of turning of Planaria gonocephala in non-directive light of various intensities = 7 = ea =n “Sea se aiihea a = | an . Light in candle meters...............| © | 0.94] II 39 | 78 126 | 155 | 217 | 431 |Total Clockwise turns........... deemuaayie 2g bas. Wier mae | 17) x7) |22 | 24 | 22 | 203 Contra-clockwise turns............--| 23 23) ||) 37; 20 | 17 14 | 166

18 17 | 17

occur in the light than in the dark, but fewer “indefinite” changes. This point requires no further exposition as its corollary has already been given.

The behavior of the worm in this respect seems to be more closely correlated with the highest intensity (431 c.m.) than with any other. In the highest intensity employed there are indicated (Table II, p. 52) 40 per cent of definite changes, which is con- siderably in excess of the percentage of such changes made in any other intensity. Onthe other hand indefinite, or S-shaped, changes constitute only g per cent of all records taken at the highest intensity, which is less than half the number of indefinite paths made in any other intensity.

60 Herbert Eugene Walter

While the extremes of the series of definite changes indicate a general rise in the percentage of their occurrence with an increase of intensity, and while in the same way the extremes of the series of indefinite changes suggest in general a decrease of frequency with the increase of intensity, it can hardly be maintained that the character of the changes in course is definitely correlated in the majority of cases with changes in intensity.

Degree of Wandering. Wandering is not closely correlated with the intensities of light. In Table VII, which deals with the percentage of straight paths made by P. gonocephala under dif- ferent intensities of non-directive light, this fact is expressed nega- tively, since it is held that a straight path is a good indication of the absence of aimlessness or wandering and may thus serve as a negative measure of such behavior.

TABLE VII

Percentage of straight paths made by P. gonocephala in the dark and also in non-directive light of different

intensities

Light in candle meters... . Hogeoanmnsite o |0.94| 11 | 39

In this respect again the behavior of the worms under the high- est intensity 1s more pronounced than under any other intensity since the greatest number of straight paths were laid at an inten- sity of 431 c.m.

Interval of Response. There seems to be some evidence that the interval of time elapsing between the reception of a light stim- ulus on the part of a worm and its consequent response, may be quite considerable. ‘Three facts were established that may sup- port this conclusion.

First, when two-minute records were made under various inten- sities, it was found that the worms averaged a faster rate during the second minute of exposure to the light than during the first, in spite of the facts that the mechanical stimulus due to placing the worm in the light machine had a more quickening influence during the first minute and that the fatigue effects were more likely to appear during the second minute. The actual figures

The Reactions of Planarians to Light 61

for the above statement, based upon 240 two-minute trials under various intensities, are 0.645 mm. per sec., the average during the first minute, as against 0.713 mm. per sec., the average during the second minute.

Secondly, in these 240 trials, the percentage of turning under all intensities 1s greater during the first minute than during the sec- ond, being 87 per cent and 57 per cent, respectively. “This result may possibly be conceived to be due to a greater steadying influ- ence of the light during the second minute than during the first and to a consequent greater turning than during the first minute. But on the other hand a similar decrease of turning, although not so pronounced, took place during the second minute when the worms were in the dark. It must be admitted, therefore, that the fact of less turning during the second minute may have nothing to do with the interval of response.

Thirdly, on several occasions a notable piece of behavior was observed, which may have a bearing on the interval of response. The phenomenon in question AGars occurred in connection with a modification of the experimental held within the light machine to be more fully described later. Briefly this modification con- sisted in making a field of two distinct intensities of light, the latter being projected vertically from above in such a way that a sharp line of demarkation formed a boundary between the two areas. Ordinarily when the worms reached this boundary line as they glided from one intensity to another, they responded promptly to the stimulus caused by the change of intensity. Sev- eral times, however, they were observed to travel indifferently exactly along this dividing line for a distance of several centimeters with half the body in one intensity and half in the other. This curious fact lends itself to various interpretations, one of which is that the response to a new intensity may not be, in all cases, immediate.

Manner of Coming to Rest. During the experiments made in the non-directive light apparatus previously described, nor- mal worms could never be induced to come to rest in the light. If allowed to remain in the aquarium they would wander about until they reached the shadow under the diaphragm (Fig. 1, H),

62 Herbert Eugene Walter

where they finally stopped, usually in the angle formed by the parafhne wall and the bottom.

Loeb’s conclusion (’93b, p. 101) that planarians subjected to directive light come to rest in regions of least intensity, seems therefore to be equally true of planarians in non-directive light.

Summary. In non-directive light Planaria gonocephala moves faster, turns less and makes more “‘ definite’ but fewer ‘‘indefinite”’ changes than in the dark. Rate of locomotion; amount of turning; changes in the character of the course, as well as the amount of wandering, do not appear to be correlated with varying light inten- sities, unless in the following instance. Under the highest inten- sity employed, namely, 431 c.m., occurred the slowest rate; the least turning; the greatest number of “definite” and the fewest “indefinite”? responses, together with the straightest paths. The excess of clockwise over contra-clockwise turnings throughout the series of intensities is probably not attributable to light.

Continuous exposure to light results in a decreasing rate of loco- motion, although in the second minute of movement as compared with the first an increase in the rate of locomotion takes place, while fewer turnings occur.

Rate of locomotion is less influenced by differences in light inten- sity than by certain internal factors which go to make up what may be termed the individuality of different worms. Individ- ual worms may sometimes fail to respond for a considerable inter- val of time to light stimuli that ordinarily produce immediate effects.

Finally, planarians subjected to non-directive light come to rest in regions of lessened light intensity the same as they do in directive light.

“Gi Abrupt Changes in Intensity

Abrupt changes in intensity may be of two kinds: either with reference principally to time or to space. First, those changes are abrupt zm time in which light or dark is suddenly thrown upon the worm, and secondly, those changes are abrupt 1m space in which a moving worm passes immediately from an area of one intensity into a sharply defined area of a different intensity. ‘This topic

The Reactions of Planarians to Light 63

will be discussed here only in its relation to non-directive light, the effects of sudden changes in directive light coming more properly in a later section.

a Abrupt Changes of Light Intensity in Time

Whenever worms were left over night in the experimental aqua- rium completely shut off from light, a large proportion of them would be found at rest in the morning when the light in the hood was again turned on. By removing ie diaphragm (Fig. 1, H), under mie edge of which near the penaiine wall the worms were usually enllcceces it was possible without any mechanical disturb- ance to subject resting worms to sudden non-directive light after a prolonged period of complete darkness. This sudden stim- ulus rarely had an instantaneous effect. ‘The interval of response was often several minutes and frequently non-directive light alone proved insufficient to start the worms into activity.

No sudden increase of intensity ever proved powerful enough to throw a gliding worm into the more rapid method of crawling. Pearl (’03, p. 551) stated the same fact after subjecting planarians to much stronger intensities of light than were employed in the present experiments.

It was found that P. gonocephala showed a decided response— either some change in course or a wigwag motion of the anterior end—more frequently when suddenly subjected to dark than to light. By inserting a key into the electric circuit it was possible to control the light in the hood to a fraction of a second. Worms in complete darkness were by this means subjected to various intervals of sudden light and worms in light to intervals of sudden dark, the results being at once noted. While the worms were in the dark their behavior could not, of course, be directly observed, but by watching them closely just before the light was turned off and also the instant it was turned on again there was no great difficulty in determining whether a response had occurred during the interval. The results obtained from nearly a thousand trials are indicated in Table VIII.

It will be seen from this table that there are more responses than failures to respond and that the responses occur more fre-

64 Herbert Eugene Walter

quently when the worms are suddenly subjected to dark than to light.

It may be further noted that the excess of the responses in the dark over those in the light increases with the interval of exposure, indicating that the worm’s adjustment to a change in the light stimulus affecting it is not in all cases immediate.

The effect of previous exposure, whether to several hours of dark or light, is a factor in these results which will be considered more properly later on.

TABLE VIII

Percentage of the responses of P. gonocephala in various intervals of time when suddenly subjected to

dark and to light of 39 c.m.

Number of seconds exposed..............-- 5 | 10 15 20 25 30 | Average Percentage of responses in light............ 51 | 59 54 54 48 46 52 Percentage of responses in dark............. 63 66 73 75 71 71 7° Excess of responses in dark....... ROSA OS 12 7 19 21 23 25 18

It should be added that Bdelloura gives a remarkable response when enveloped in sudden darkness. It will frequently forsake its attachment under these circumstances and unattached in the water go through violent contortions. ‘This striking response can be called forth by an exceedingly brief interval of dark, namely, the shortest time required to turn the electric light off and on. Nagel (’94, p. 387) speaks of animals thus affected by sudden shadow as “skioptic.”’

The relation of Bdelloura to light falls into a somewhat different category, however, than that of the fresh-water planarians, since Bdelloura is positive to light, while fresh-water flat-worms are negative.

b Abrupt Changes of Light Intensity in Space

Several devices were employed to test the behavior of planarians passing abruptly from an area of one intensity of non-directive light into another. ‘The most successful device tried was that in which two lights of different intensities were mounted overhead

iy

The Reactions of Planarians to Light 65

in the hood of the apparatus already described in Fig. 1, the mingling of their rays being prevented by the insertion of a ver- tical diaphragm (Fig. 3, C), which extended from the region between the lights down to the surface of the aquarium. In order to place the diaphragm in position it was, of course, necessary to remove the heat screen (Fig. 1, C), the presence or absence of which, however, would not have affected the results sought since the water in the aquarium itself was nearly 2 cm. deep and thus

Fig. 3. A, stationary light; B, sheet iron walls of hood; C, vertical diaphragm separating the two lights; D, horizontal diaphragm; £, sheet iron roof of hood; F, plate glass aquarium floor; G, paraffine wall of aquarium; HH, diaphragm to shut off reflections from wall of aquarium; J, wall of reflector box; F, open side of box; K, mirror; L, black tunnel; M, black draperies cutting off side light; N, table sup- porting reflector box and end of tunnel; 0, movable light; P, track for movable light; 9, narrow, hori- zontal diaphragm attached at right angles tothe lower side of the diaphragm C, in order to prevent the light rays from the two sources of light, 4 and O, from overlapping.

constituted an efhcient heat screen. By keeping the hood sta- tionary and causing one of the lights (Fig. 3, O) to slide up and down at will, it was possible to bring about various contrasts of

66 Herbert Eugene Walter

intensity in the field below. The complete plan of the appa- ratus Is given in Fig. 3.

The principal variations in the behavior of Dendroccelum and Phagocata upon reaching the critical line separating the areas of two intensities are indicated diagrammatically in Fig. 4.

The dotted line represents the boundary separating two areas of different light intensities. The arrows represent the types of paths made by Dendroccelum and Phagocata. For the sake of simplicity the worms are represented as going in one direction; that is, into one of the two contrasting intensities, but the same types of paths resulted as well when the opposite direction was taken. The angles made in crossing the critical line were also more varied than those represented in the diagram.

Type 4 represents a passage without re- sponse; B,an angular change of course made at the critical line; C and F, aloop-like return effected after a short excursion into the new intensity, and G,a sharp turning aside, while A indicates a halt at the critical line, as if a barrier had been encountered. Finally D and E represent a temporary pause on the part of the worm accompanied by wigwag move- ments of the anterior end of the body. In the case of D the wigwagging is immediate, but E typifies a case when there occurred in the response an interval of such a nature that the

' significant movements were not made until Fig. 4 the worm had advanced at least its own length into the new area.

Of these types all, with the exception of 4, are to be regarded as reactions to differences in intensity encountered. The most questionable are the infrequent types C and F, which may be otherwise explained as arcs in a curving course which might have occurred in a field of uniform intensity. By far the commonest type was D, plainly the least doubtful of the series.

The Reactions of Planarians to Light 67

As a result of over 3000 observations on the manner in which the critical line separating the two intensities was passed, three facts become evident. First, responses were considerably more

TABLE IX

Kind and percentage of responses of Dendrocalum and Phagocata in passing from one intensity of non-

directive light to another

Turn-backs| No Wigwags and full Loops Angular Total Character of course responses) (Types | stops | (Types courses reece (Type 4) Dand E) | (Types | C and F)| (Type B) G and H) Going into greater intensity, PETCEN Greys eteressfulshessreleat ests 79 II 6 2 2 2I Going into lesser intensity, PEM CODG gararaintatelstetar lererstaroks 5° 36 5 8 I 50 Average responses, per cent.| 64.5 tals 5-5 | 5.0 Aly 35°5

frequent when the worms were passing into the lesser intensity than they were when entering the greater intensity. Secondly, lack of response is more frequent than a visible response of any kind since 64.5 per cent of the crossings made over the critical line were of the type 4. Thirdly, the responses at the critical line were more frequent when the worm was upside down, 1. e.. moving on the surface film, than when it was on the floor of the aquarium. This latter point was illustrated most fully by Phagocata, which, being an active worm, takes quite readily to the surface film, so that it was possible with this species to get a series of observations in which the behavior when crossing the critical line on the bottom of the aquarium could be compared with that when the same line was encountered at the surface film. Table X contains the results of these observations.

The doubling of responses when the worm is on the surface film is probably not due to an unequal receptivity of light stimulus by the dorsal and ventral surfaces of the planarian as might at first thought seem possible. As will be shown further on, the worm’s rate of locomotion on the bottom of the aquarium is nearly the same whether the light comes from below or from above, pro-

68 Herbert Eugene Walter

vided the amount of light in both cases is equal. Planarians, as Pearl has emphasized, are strongly thigmotactic. Naturally, then, their response to contact is much greater when they are on the glass bottom of the aquarium than when they are suspended on the less resistant surface film. In other words, the less the worm is influenced by the stimulus of contact the freer it is to respond to the stimulus of light.

TABLE X

Percentage of the responses made by Phagocata at the critical line separating two intensities of non-direc-

tive light either on the bottom of the aquarium or on the surface film

| Number of No response Response | observations per cent per cent Ounithe surfacedilia!acones chs apeanastosuiss cess 740 45% 544 On: thetbottome. oancrsosen ee eee aneiacg cee ai 1664 76 24 MMotalisiepsannc te oxelateernvctsy se corers/o\sralsinyo: Se co-fiassiecto 2404 60} 39%

Finally, a series of experiments was tried in which the contrast between two intensities was varied by raising or lowering one of the lights in the hood. It was found that the responses made by Phagocata under these circumstances increased with the increase in contrast between the two intensities as shown on the bottom line of Table XI, where these contrasting intensities are expressed in a ratio between the constant light taken as unity and the movy- able light.

The fact that responses by no means invariably occur when bright light and complete darkness are suddenly substituted for each other (see Table VIII) rendered a further extension of this series unnecessary. The contrasts here used form probably a much greater range of intensity contrasts than the worms ever encounter in nature.

Attention to the details presented in Table XI brings to light the fact that, although the number of responses is correlated in a general way with an increase in the contrast between the two illuminated areas, as shown in the bottom line of the table, yet the percentage of the responses is further influenced by the actual degree of the intensities employed. For example, when the two

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IX ATAVL

70 Herbert Eugene Walter

areas of light were respectively 33.16 and 68.18 c.m. the ratio was practically the same as when the two intensities were 16.3 and 33-16 c.m., yet the percentage of responses in the two cases is decidedly different, being 10.5 per cent in the former, and 56 per cent in the latter case W hen ve lesser of the two lights was 33.16 c.m. there were invariably fewer responses than aa the lesser light was 16.3 c.m. The latter intensity is undoubtedly nearer the planarian’s optimum intensity, and the apparently inhibitive action of the higher intensities agrees perfectly with certain facts already detailed, as, for instance, that the activities of Planaria gonocephala were less pronounced at 431 c.m. than in lesser inten- sities; and, again, that all planarians show more responses on going into a lesser than when going into a greater intensity.

Attempts were made in some other ways to subject planarians to areas of contrasted intensities and, although the results were less satisfactory in general than those obtained by the method of using two buerhead lights of different intensities just described, yet certain facts were ibeatie out that may be worth recording.

In the first of these attempts two concentric rings of white paper, each about two centimeters wide and having between them a space of a couple of centimeters, were fastened to the under sur- face of the aquarium floor. The white paper thus arranged reflected the light upward and made areas of gradually increased intensity as compared with the remainder of the aquarium floor through which the light passed without reflection. Worms placed in the center of these circles would consequently be obliged to pass from one intensity of light directly to another, ee er the direction of the radius they might be taking. When worms were actually tested, it was found that they exhibited considerable modification in their movements, particularly when approaching the edge of the paper backgrounds.

Owing to the considerable thickness of the plate-glass floor of the aquarium as well as to the fact that white paper is a surface which scatters the light falling upon it, it was found that there was formed, not a sharp line of demarkation between two intensities, but rather a penumbra-like margin of intermediate light. This apparatus was therefore abandoned as unsatisfactory.

The Reactions of Planarians to Light 71

The difficulties presented by paper as a reflecting surface largely disappeared by the substitution of a plain mirror in its place, since the surface of a mirror is such that all the light striking it at right angles is reflected at right angles. When, therefore, an unmounted mirror was brought into contact with half of the under surface of the aquarium floor the whole field was thereby divided into two regions sharply separated from each other. Of these one was supplied with light from above only, while the other received the same light plus Aeanl: an equal amount reflected from the mirror below. With the aid of this device an increase of II per cent was gained over the responses obtained when white paper instead of a mirror was used as a reflector. Both Phagocata and Dendro- ccelum were tried by this method. In 76 per cent of the trials made, 7. ¢., in 125 cases out of 165, the worms showed a visible modification in their behavior on reaching the boundary of the two areas of light. It was nevertheless decided that this method was an uncertain test of behavior, since the body of the worm, although fairly translucent, would by no means allow all the light that fell upon it to pass through and be reflected, and consequently the difference of the two intensities to which it was being sub- jected could not be easily estimated.

Summary. When sudden light or dark envelops planarians (Dendroccelum, Phagocata and Planaria) the response, if any occurs, is often not immediate.

No one of the intensities of light which were employed in these experiments when introduced suddenly was sufficient to make the worms forsake gliding for crawling.

Sudden dark calls out more responses than sudden light, while the number of responses increases with an increasing interval of exposure to the stimulus. Bdelloura is decidedly “‘skioptic.”’

Worms encountering the edge of a reflecting area which increases the intensity of the light without introducing any other barrier, show a marked degree of response. ‘The percentage of response is considerably larger when a mirror instead of white paper is used to produce the reflecting surface. If worms are allowed to pass from one intensity to another sharply separated from it, their responses are more frequent upon passing into the lesser intensity

72 Herbeit Eugene Walter

than when going into the greater. The average number of fail- ures to respond to these contrasts of intensity reaches about two out of three.

Phagocata, at the critical line separating two contrasting inten- sities, responds oftener when on the surface film than when glid- ing over the bottom of the aquarium.

The number of responses increases with the increase in the con- trast between the two intensities employ ed, but the percentage of response is greater, regardless of ratio, when one of the lights is of low intensity (13.6 c.m.) than when both are of higher intensity

(33 + c.m.) 2 PHOTOTAXIS

The term “phototaxis” was introduced by Strasburger (’78) in a study of certain swarm-spores, to indicate movements which were parallel with incident light rays. The term has since been extended by several authors to include similar movements on the part of animals. Any organism is said to be positively phototac- tic when it moves toward the source of light in the direction of the rays and negatively phototactic when it goes in the opposite direc- tion.

The purpose of this section is to consider the phototactic move- ments of planarians, as distinct from their photokinetic behavior, (A) when the light remains constant, (B) when the light is changed either (a) in intensity or (b) in direction, and (C) when phototaxis is combined with responses of a different kind.

A In Constant Directive Light

Orientation. With the exception of Bdelloura all the planarians studied are, under normal conditions, negatively phototactic so far as their first movements in directive light are concerned. To obtain quantitative data for this statement it was necessary to construct an apparatus in which the worms to be tested could be placed quickly and with as little mechanical disturbance as possible in the center of a unit circle with the long axis at right angles to the direction of incident light. The circle was marked off into degrees so that by noting the place at which a worm made its exit a quan-

The Reactions of Planarians to Light 73

titative measure of the amount of turning toward or away from the source of the light under the given conditions was obtained.

The apparatus finally utilized for this experiment was based upon a device employed by Parker and Burnett (’oo) in testing the relative behavior of normal and eyeless planarians when sub- jected to directive light. Its arrangement is shown in Fig. 5.

On the top of a table (4) in the dark room was placed a rectan- gular aquarium (BCDE), the ends of which (BE and CD) were

Fig. 5 A, Top of table; BCDE, rectangular aquarium; BE, glass end; FG, round swinging aquarium; H, copper wire attached to ceiling and supporting the swinging aquarium FG; J, movable light; 7, diaphragm; K, surface of water in outer aquarium; L, surface of water in inner aquarium; M, lens.

made of glass while the floor and sides were of wood painted with camera-black. Within this aquarium a second cylindrical one (FG), made entirely of thin glass and measuring 20 cm. in diam- eter, was suspended from the ceiling by means of a fine wire (/7) attached to a swivel to allow turning. On the floor of the outer aquarium and directly beneath the inner one was drawn a circle

74 Herbert Eugene Walter

10 cm. in diameter and marked off plainly into arcs measuring 5 degrees each. An incandescent lamp (/), placed on the table at approximately the height of the inner aquarium floor, could be manipulated at any desired distance, while a diaphragm (Ff) prevented much of the light from reaching either the upper surface of the water contained in the two vessels or the floor of the outer aquarium whence it would be reflected. A biconvex lens was then so interposed as to make the light rays practically parallel upon theiremergence fromit. ‘Their course through the inner aqua- rium was kept parallel by means of the medium of water on both its inner and outer sides. A nearly uniform intensity over the entire floor of the swinging aquarium was thus obtained and the objection arising when the inner aquarium is used in air, viz: that it acts as a converging lens, was obviated. Side reflections were eliminated by enclosing the light (/), together with the interven- ing space between it and the diaphragm, with black screens.

When a worm introduced into the inner aquarium began to glide, it could with slight mechanical disturbance be quickly cored) by means of moving cen aquarium, into any desired posi- tion with reference to the light, and then swung so as to bring its posterior end exactly over the center of the stationary circle below.

Various species of planarians were started in this manner at right angles to the light. Out of 386 cases, 371, or 96 per cent, emerged from the 10 cm. circle at a point farther away from the light than that toward which they were originally directed. ‘This is taken to mean that 96 times out of a hundred the worms were negatively phototactic. If, however, the method of reckoning negativeness employed by Parker and Arkin (’o1) on the earth- worm is used, the foregoing per cent would be somewhat less. These authors assume (’o1, p. 28) that the apparently positive responses of a normally negative animal, such as the earthworm, may be due to causes other than light, in which case an equal number of responses of like nature might be expected to occur on the negative side as well as on the positive. A number equal to the sum of these apparently positive responses should therefore be subtracted from the total of the apparently negative responses

The Reactions of Planarians to Light 75 in order to obtain approximately the amount of unquestionable negativeness. By following this method in the case just given, the per cent of negativeness would be g2 instead of 96, but since this method assumes that normally negative worms are never posi- tive, which is contrary to the evidence to be given later, the most accurate estimate of negativeness would probably fall somewhere between these two percentages.

Bdelloura, on the other hand, behaves in the same way only three times out of ten, therefore showing itself to be positively phototactic.

This difference in orientation becomes more marked if the total number of degrees, that is, the amount of positiveness and nega- tiveness of emergence from the circle is used as the basis of reckon- ing, instead of only the number of times of emergence. Such a quantitative computation is shown in Table XII.

TABLE XII

Amount and kind of orientation to directive light exhibited by various species of planarians in 396 trials

| Number of Total de- | Total de- |Percentage of Percentage of

trials grees positive |grees negative) degrees neg. | degrees pos. Negative worms (Dendroce- lum, Planaria, Phagocata). 386 | 566 10157 94.7 ces Positive worms (Bdelloura) .. .| 10 397 50 11.2 88.8

Although the actual number of trials for Bdelloura in this table is small, they are characteristic of what was observed in a large number of unrecorded instances.

The amount a planarian may deviate from the direction in which it is pointed, depends upon the direction of the light imping- ing upon it. A negative species deviates from a straight course least when headed away from the source of the light and most when headed toward it, while an intermediate degree of deviation occurs when the direction of the light is at right angles to the long axis of the worm. In the case of Bdelloura the converse is true, as shown in Table XIII.

Rate of Locomotion. In obtaining the rate of locomotion of worms subjected to directive light, the double aquarium apparatus

76 Herbert Eugene Walter

just described was used. After the worm to be tested had been placed in the inner aquarium and had begun gliding, it was so oriented that the tip of its posterior end came precisely over the center of the subjacent circle 10 cm. in diameter. The exact time of its departure from the center of the circle was then noted and the instant thereafter that the tip of the posterior end passed over the circumference of the circle was again taken and the worm’s course plotted at once on a duplicate circle sheet. Each worm was given four trials in this manner, being started in four different directions, toward the light, away from the light, and with the long axis of the body at right angles to the light, first with one side to the light and then with the other.

TABLE XIII Amount of average deviation tn 2400 trials expressed in degrees of a circle, exhibited by negative planarians, (Dendrocelum, Planaria and Phagocata), and a positive one (Bdelloura) when pointed toward,

away from, and at right angles to the source of light

Direction in which the worm was pointed with regard)

tOsthesl ser hiti tess /alejararacstcvatevaveracchey shia. ’eps/s a wierd ontynusith one At right angles Toward Away from Negative planarians, degrees.........-.-+++++++s0-- 48.1 128.7 2759 Positive planarians, degrees..... : eveuarsvele Waxege 49. 39-3 132.1

The time of the worm’s emergence from the circle was not taken with a stop-watch because the observer’s hands were otherwise occupied. Instead a small clock, ticking half-seconds, was placed conveniently near. By counting the number of ticks during the interval of the worm’s transit from the center to the circumference of the circle the time consumed could be determined within less than a half-second. After tracing the worm’s course on a dupli- cate circle sheet and measuring the same by means of a map measurer, a unit of distance was obtained, which together with the known unit of time consumed in covering this distance, fur- nished all the data necessary for computing the rate of locomotion.

Ten representatives of Dendroccelum lacteum, Planaria macu- lata, Phagocata gracilis and Planaria gonocephala respectively were given four trials apiece by the method just explained. ‘The results are presented in Table XIV. From the 160 records thus obtained it becomes evident that the average rate of locomotion

The Reactions of Planarians to Light fh

is greatest when the worms are pointed toward the light, and least when they are pointed in the opposite direction, while an inter- mediate rate occurs when they are started at right angles to the light.

This result is at variance with the findings of Parker and Bur- nett (’00, p. 381), who incidentally reported that Planaria gono- cephala when started away from the light traveled faster than when started toward the light.

TABLE XIV

Average rate of locomotion, expressed in mm. per sec., of various species of planarians when started torward, away from, and at right angles to the source of directive light of 27 c.m. intensity.

: Dendroceelum | Planaria |Phagocata Planaria Total Species |

lacteum | maculata | gracilis | gonocephala| average

Direction in which the worm was

pointed with reference to the light

At right angles'.....5....2.% 0.855 1.475 1.445 0.980 1.19 LGR Eid logoseoopagnaouneder 0.g10 1.505 1.430 1.205 1.26 wma ye ir OM cra )axerereesey~layate/2)<yei2 0.795 | 1.440 1.310 1.090 1.16

It was further found that, regardless of the direction in which the worms were started, there was a gradual decrease of the rate dur- ing the four successive trials. “The order in which different worms were oriented during the four trials was arranged so as to neutral- ize the possible effect of the sequence in the direction started. In Table XV the data for 200 trials are arranged to express this slowing down of the rate.

TABLE XV

Average decrease in rate of locomotion for 50 planartans during four successive trials while subjected to

directive light of 27 c.m.

IN rina exo fi tari all sa sjoyefays)-\sh-00 2 abate ace nayoxste = lava'e) yafelen sie First | Second Third Fourth i} |

Averapexatenmimm.) per SeGer cieefeleiers ware laleistaiets 1.140 1.130 1.075 1.070

Various factors influencing the rate of locomotion, such as the intensity of light, the size and species of the worm, the amount of pigment present in the body and the general physiological state of the animal under experimentation, will be more suitably dis- cussed in other connections.

78 Herbert Eugene Walter

Change in Character of Course. When several specimens of Phagocata were placed in a square aquarium which received light solely from one side, their first movements were plainly negative, that is, away from the light. After a brief interval, however, it was seen that apparently as many worms were going toward the light as in the opposite direction. In fact an actual count showed that in a certain interval of time 43 worms passed a central point going toward the light while 44 passed the same point in the oppo- site direction. This apparent change in the character of the course was probably due, not to any change in the degree of nega- tiveness of the animal, but rather to the fact that the impulse to keep moving in some direction is stronger than the impulse to neg- ative phototaxis. Consequently when the limit of the aquarium in a negative direction is reached a worm, since it normally travels in straight lines or sweeping curves and does not turn around and around in one spot, continues its locomotion in the direction of least resistance, namely, back toward the light. It will be remembered that Loeb (’93b) has called attention to this fact by saying that planarians are not negatively “heliotropic”’ in a strict sense because they do not remain as far away from the source of light as they can get.

Among various observations made with other ends in view, there were numerous incidental cases of a normally negative worm making an unexpected positive response even from the first moment of being subjected to the light stimulus. ‘This occasion] positiveness is clearly apparent from the general fact already noted that four times out of a hundred the average negative pla- narian turns toward the light.

Two definite instances of a reversal in the character of response may be cited.

The first was the case of a Phagocata in the double aquarium, which became increasingly positive through twelve successive trials. Its average emergence from the circle for the first four trials was 45°, which is a normal negative result, since go° represents com- plete indifference. In the next four trials, however, the average was 100°, that is, slightly positive, and in the last four, 124°, which is decidedly positive, as shown diagrammatically in Fig. 6.

The Reactions of Planarians to Light 79

In the other instance an individual worm, Planaria gonocephala, made the erratic average emergence from a circle of 145°, just 35° short of absolute positiveness. ‘This worm was carefully isolated and tested again four days later under identical external conditions when it was found to have returned to a normal nega- tive condition by showing an average record of 56°.

Accuracy of Orientation. It was found to be frequently the case that when negative worms were subjected to directive light their first movement instead of being directly away from the source of light formed a path in a diagonal direction. “This tendency to

rege GO

90° AS

Light 180° O°

90° Fig. 6 The arrow at the left represents the constant direction of the light. In eachof the three sets of trials each worm was headed successively toward 0°, the upper (in the diagram) 90°, 180°, and the lower go°. The point of average emergence for the first set of trials—supposing the records of the lower semicircle to have been transferred to the upper semicircle—was at 45°, of the second set, at 100°, and of the third set, at 124°.

travel diagonally away from the light has also been noted in the case of the earthworm by Smith (’02, p. 469).

If the negative phototaxis of planarians is to be explained on the theory of tropisms, and if, moreover, the eyes, as Hesse (97) maintains, are the principal organs which, when unequally illumi- nated, cause the directive response, it may be shown that possi- bly the arrangement of the crescentic pigment shields around the sensory cells of the eyes is such that equal stimulation of both eyes is just as certainly received by the worm when it is in a position diagonal to the light as when it is pointed directly away from the

light.

80 Herbert Eugene Walter

By reference to Fig. 7, in which the relative size of the eyes 1s somewhat exaggerated and made diagrammatic for sake of clear- ness, it will be seen that no more light reaches the sensory cells of either eye from position 4, the diagonal position, than from posi- tion B, and that it is only when the light comes from some source more lateral than 4 that the left eye receives more illumination than the right.

This view may furnish a possible explanation of the diagonal paths representing imperfect orientation among planarians, but it can in nowise apply to the case of earthworms since in them direc- tion eyes are absent.

Fig. 7 A, diagonal direction of light; B, posterior direction of light; C, location of sensory cells;

D, pigment shield.

Degree of Wandering. ‘The degree of wandering decreases with an increase of intensity. It may be found approximately through the degree of error in orientation in a unit space under different intensities of light, for perfect orientation signifies the minimum of random wandering and, conversely, the greater the error of orientation the greater the probable wandering.

The error of orientation expressed in percentages was computed as follows. With a negative worm emergence from the circle at

The Reactions of Planarians to Light SI

a point directly opposite the light was reckoned as 0 per cent of error, whereas emergence at a point directly toward the light was reckoned as 100 per cent, or a maximum of error in orientation. The orientation value at these two extremes having been estab- lished, the percentage of error which occurs when the worm emerges at any intermediate position on the circumference of the circle may be easily determined.

TABLE XVI

Average degree of error in orientation made by various species of planarians during 300 trials in directive light of different intensities

| Percentage of error in orientation

iene 4 When started

Mea etarte! = away from the Average ward the light light 3-3 candle meters......- poopdso ss OnoeandeDane 34 11.5 22.7 FLOIGALI GAG TTNCLENS = ans Vaperalsy/atece(ye/<tatsisraial svete ave4e <ns/e 32 | 12 22.0 BAL Cand lewn etersemrstar sate rsiateisieie/s aleleteteiateiefeinve -| 31 | 10 20.5 | ; MAVCLA RE otatateletaieveyeteteCayetecerctetenesctcus crs| sobs she, ze1 32 II

From this table it appears that there is three times as much wandering, or error of orientation, by worms headed toward the light, as by those headed away from it. This doubtless indicates that orientation is a more complicated process in the former case than in the latter.

Duration of Activity. Superficial observation is sufficient to establish the fact that different species of planarians when set into activity in directive light show decided differences with regard to the length of time they normally continue in motion before com- ing to rest. Among the forms experimented upon, Bdelloura

came to a stand-still in light soonest and Phagocata latest.! Fatigue in itself is by no means the inevitable result of continued activity on the part of anorganism. For instance, Hodge and Aikens (95) observed a Vorticella continuously for 36 hours, during which time its regular ciliary and contractile movements continued unin- terruptedly, while Radl (’or) found that the eye of Daphnia when

82 Herhert Eugene Walter

light was flashed upon it vibrated as vigorously after the experiment had been repeated 410 times in close succession as it did at first. An attempt was made with Planaria maculata to see how long activity would continue in a succession of trials in directive light. The worm was started on the middle of an aquarium floor and allowed to glide in any direction. As soon as it stopped and assumed the relaxed contour of the resting worm, the time required for the journey being noted, it was immediately returned to the starting point. Subjected to this treatment, the worm made 39 trips, which in general occupied an ever decreasing length of time, ranging from 18 minutes to 1? minutes, or an average of 5 minutes and 53 seconds each. When returned to the starting point the fortieth time the worm refused to start. Although in this experi- ment, which lasted 43 hours, the worm became gradually less responsive to the mechanical stimulus of the brush by means of which it was transferred to the starting point, its fatigue did not materially affect the negative character of its response to light. Time Required to Leave a Unit Circle. In obtaining the data on this point, the apparatus and method already described (p. 73) were employed. It was found that when worms of different species were subjected to three different intensities in immediate succession the degree of intensity did not prove to be as important a factor as fatigue in determining the average number of seconds necessary for the worm’s exit from a circle 10 cm. in diameter. During the series of experiments upon this point care was exer- cised so to vary the succession of intensities that the effect obtained could not be attributed to any cumulative increase or decrease of intensity. “Thus, on one day the order of intensities was I, 2, 3, on the next 2, 3, 1, and on the third, 3, 1,2. In Table XVII the data obtained are arranged on the left with reference to the actual intensities employed and on the right with reference to the suc- cession of trials made upon the various species which are desig- nated in the middle column. The averages in the table are each made up of four records. It will be noted that Phagocata gracilis and Planaria gono- cephala are, according to these figures, less subject to fatigue than Dendroccelum lacteum or Planaria maculata.

The Reactions of Planarians to Light 83

Manner of Coming to Rest. Loeb (’93b) and others have shown that planarians under the influence of directive light generally come to rest in regions of lessened intensity. A few experiments were made bearing on this point. By means of screens and back- grounds, both black and white, a rectangular glass aquarium was arranged so that the area of least intensity was plainly localized and could be varied in different ways. In Fig. 8 are shown (1) the places where worms (P. gonocephala) which had been started together in the middle of the dish finally came to rest; (2) the num- ber of worms in each locality; and (3) the different combinations of backgrounds and screens used in each of the experiments.

TABLE XVII

Relative effect of fatigue (at right of table) and change in intensity of light (at left of table) as shown b i g g §' y &£ y the average number of seconds required for individuals of various species of planarians to leave a circle Io cm. in diameter

Wherever shaded borders are indicated the aquarium was surrounded on five sides by black screens and likewise on the sixth side except for a narrow space admitting the light, the direction of which is indicated by arrows; in a similar fashion, where unshaded borders appear, light-reflecting screens enclosed five sides.

It will be seen at a glance that the great majority of the worms placed in directive light come to rest as far from the light as pos- sible. That this is due to the directive power of light is at once apparent by comparing 4, B and C with D, where the light was non-directive. The darkened area was selected whenever the directive force of the light did not prevent, as in 4, C and D,

84 Herbert Eugene Walter

The five worms coming to rest on the lighter side of D were carefully examined and found to be mutilated or fragmented individuals, while the same was not true of the others.

The reason why the worms in B failed to arrive in the darkened area is probably that, being started near the middle boundary line, their first movements were normal, 7. ¢., away from the light, and carried them into the area of greatest intensity, whence they were unable to escape. In this case the effect of the directive light seems to have more than counterbalanced the locomotive

| | Light vertical

A B C D

Fig. 8 Planaria gonocephala. The arrows represent the direction of the light. The dotted areas were surrounded by black backgrounds, except for a space on the side toward the light, and the clear areas similarly by white backgrounds. The figures represent the number of planarians that came to rest in any particular locality.

energy exerted by the worms. Had the species experimented upon been Phagocata gracilis, instead of Planaria gonocephala, the result might have been different, for in the former species, as already shown (p. 78), the phototactic response is secondary to the tendency to a general wandering.

It was frequently observed that worms when fatigued after a period of activity apparently lost their phototaxis, with the result that the final movements of a tired worm would sometimes be made toward the light. Such behavior is probably not to be con- sidered as a reversal of phototaxis, but rather as indifference to

The Reactions of Planarians to Light 85

photic stimuli, due to the worm’s lowered physiological state and a chance turn toward the light. In fact the final position taken by 49 fatigued worms with reference to the source of light, showed that only five of them, or 10+ per cent, pointed away from the light while 15 (30+ per cent) were headed toward the light and 29 (59+ per cent) stopped indifferently at right angles to tae tars quite probable that among the external fictors that influence a worm to come to a halt, light plays an exceedingly insignificant role, as compared with the stimulus of contact or some stimulus, prob- ably chemical, given out by other worms in close proximity.

One curious instance was observed, however, in which light was apparently of more importance than contact or other stimuli in determining the place of coming to rest. A large crystallizing dish half full of water was left over night with a few planarians in

Fig. 9

it. Floating on the surface of the water in this dish was a small Petri dish, in which a few more planarians were isolated. In the morning the worms in both vessels were found grouped at the same region on the outside and inside of the smaller dish, as shown in Fig. 9.

This curious distribution on both surfaces of the Petri dish could not be due to chemical stimulus exerted by one group of worms on the other, and there seems to be no particular reason why a thigmotactic reaction should have caused them to assemble in such a way. The locality chanced to be one, however, where the intensity of the light was considerably reduced; this seems to offer a reasonable explanation of the observation.

Bdelloura in coming to rest shows an entirely different behavior. When left over night free to wander in an aquarium half of which

86 Herbert Eugene Walter

had been previously covered with black cloth to exclude most of the light, this species was found in the morning in the light area, a behavior exactly the reverse of that shown by fresh-water pla- narians. Another peculiarity of this species is that individuals in coming to rest arrange themselves in compact rosettes with the anterior end of the body pointed toward the circumference of the rosette, while the sucker-like posterior end remains attached near the center of the group. ‘They are so delicately responsive to me- chanical stimuli that any slight disturbance of one member of such a rosette is sufficient to throw the whole group into activity. The advantage to the individual worm of such a habit of arrangement in coming to rest, is evident.

Finally, Bdelloura was repeatedly seen on taking the resting position to point directly toward the light with the anterior end a the body raised and the posterior end Hatencd out into a sucker- like expansion.

Summary. Fresh-water planarians (Dendroccelum, Planaria and Phagocata) are negatively phototactic while Bdelloura is posi- tively phototactic.

Negative planarians deviate most from the direction in which they are started if pointed toward the light and least if pointed away from the light, an intermediate deviation occurring when they are pointed at right angles to the light.

The rate of locomotion is greater when worms are headed toward the light than when they are headed away from it.

During successive trials the rate of locomotion decreases.

Negative planarians frequently take an apparently positive course because the impulse to move in any direction is greater than the phototactic impulse.

The normal negative phototaxis of a worm may change tem- porarily to positive by reason of some physiological state ania is not obviously referable to external stimuli.

The greater the intensity of the light the less worms wander in their course. When they are Peaded away from the source of light, there is less error in the precision of their orientation than when they are started toward it.

Planarians frequently travel away from the source of light diagon-

ally instead of directly.

The Reactions of Planarians to Light 87

Bdelloura continues activity in the light for a much shorter time than Phagocata.

When subjected to successive trials the period of a planarian’s activity decreases.

Change in the intensity of light is less important than the effects of fatigue in determining the time required for a worm to leave a unit circle. When fatigued, worms often become indifferent to light, coming to rest less frequently in an oriented position with reference to the light than in an unoriented one.

Fresh-water planarians come to rest as far away from the source of light as possible and, if the directive stimulus does not prevent, in the region of least illumination.

Bdelloura candida, on the contrary, comes to rest in regions of greater rather than of less illumination; usually worms of this species arrange themselves in compact rosettes with the anterior ends pointed outward.

B In Changing Directive Light

The light acting upon planarians in their natural habitat must necessarily be a variable factor of great complexity, since its inten- sity changes constantly throughout the day, while the position of the sun relative to various surfaces which reflect light is also continually shifting.

The fact that planarians, to a great extent, keep out of the light, does not diminish the force of this statement, for whatever the part played by light in their behavior, it must always be an exceed- ingly varied and complex one.

Changes in the Intensity. When a worm is gliding away from a source of light it shows a more marked response to change of ' intensity when the change is made suddenly than when it is made gradually. In fact, it is possible by exercising patience and care to change the intensity of directive light to a considerable degree so gradually as to produce no corresponding response on the part of the worm, whereas a comparatively slight change, if abruptly effected, immediately results in the animal’s performing some one or more of the acts in its repertory of behavior, such as halting, wigwageging, etc.

88 Herbert Eugene Walter

In all the experiments made upon the effects of change of inten- sity in directive light, more responses were found to occur when the intensity was decreased than when it was increased. This is in agreement with the experiments already described relating to the critical region between two intensities, in which it was found that worms show a greater number of responses when going from a higher into a lower intensity than vice versa.

Bdelloura is particularly sensitive to changes in intensity. It is necessary to throw a shadow on a moving worm only momen- tarily tg cause it to perform vigorous wigwag movements or to change the direction of its course.

Whitman (99), writing of Clepsine, suggests that the extreme agitation of this animal when a shadow is thrown upon it may be the result of natural selection, since any sudden shadow cast upon it in its natural environment may be caused by a turtle swimming overhead, to which the leech, if it is quick enough, may become attached. It may be that Bdelloura, which is also an ecto-parasite, has developed this extreme responsiveness to sudden decrease of intensity in a similar way.

Changes i in Direction. The precision with which all the pla- narians in a dish may be made to pass back and forth by shifting a directive light from one side to another is astriking g phenomenon, which is sure to impress anyone who sees it. By careful manip- ulation of the light, it is possible even to make an individual planarian follow a predetermined path in the most undeviating manner. For example, when two lights, placed near the ends of an aquarium, are alternately turned on and off, the worm will zigzag across the field, at right angles to the direction of the lights, while under a moving light it may be made to turn around and around, almost as if its posterior end were a pivot, to trace figure 8’s and curves of various patterns, or to turn abruptly at right angles an imaginary corner.

Unlike the changes in intensity previously described the degree of abruptness in any change in the direction of the light made no apparent difference in the quale, of the reaction, since any change in direction, however gradual,-met with an immediate response on the part of the worm. Indeed it was necessary to abandon an

The Reactions of Planarians to Light 89

attempt to illuminate one side of the worm alone because the ani- mal invariably turned faster than it was possible to regulate the light.

The quickness with which this delicate response to any change in the direction of the light occurred was found to increase upon suc- cessive trials. A square aquarium was arranged so that it could be illuminated instantly at either end, in a room otherwise dark. With one light on, a planarian was allowed to move until it had assumed the characteristic negative direction, whereupon the source of illumination was instantly changed 180° by turning this light off and the one at the other end of the aquarium on. ‘The time required for the worm to become headed about was noted and then a reversal of lights repeated and the interval necessary for re- adjustment again recorded. Ina typical experiment of this kind the number of seconds required by the worm, Planaria maculata, to accomplish re-orientation were for 16 successive orientations as follows: 260, 70, 100, 60, 65, 110, 60, 85, 70, 105, 80, 60, 50, 40, 45, 35. The sum of the first eight is 810 sec., that of the last eight, only 485 sec.

Summary. Planarians show a greater response to sudden change of intensity than to gradual change. ‘This response 1s more pronounced when the intensity is lowered than when it is rgised.

Bdelloura is particularly affected by sudden changes of in- tensity.

Planarians respond with great precision to changes in the direc- tion of the light, and as promptly when the change is gradual as when it is abrupt.

The period required for re-orientation to changes in the direc- tion of light, diminishes upon repetition.

C [In Combination with Other Responses

It is impossible to subject planarians to the influence of light alone. ‘The best that can be done is to render extraneous factors as uniform as possible. For example, so long as a moving worm is kept upon a horizontal surface there can be no directive geotactic stimulation, because the worm is moving in a plane at right angles

go Herbert Eugene Walter

to the force of gravity. The moment the worm begins to glide up the sides of an aquarium, however, the relation of the axes of its body to the center of the earth changes and directive geotaxis results.

No systematic attempt was made to analyze compound stimuli, for such a study would overstep the boundaries set for the present inquiry. Nevertheless certain facts bearing on this point were incidentally noted and these may properly be detailed here.

Geotaxis. Ina majority of cases, Planaria gonocephala seems, after several hours of exposure to the dark, to be positively geo- tactic, and after several hours of exposure to light, negatively geo- tactic, as shown in the following series of observations.

A cylindrical aquarium jar 20 cm. in diameter and 40 cm. high was placed before a moderately lighted window and stocked with a freshly obtained supply of about 300 worms. No stones, sand, or water-weeds, which would afford places of concealment, were introduced. At intervals during the next 10 days the distribution of the worms was recorded and these records are brought together in Table XVIII.

TABLE XVIII

The distribution of about 300 planarians (Planaria gonocephala) in an aquarium, as observed forenoons and afternoons during 10 days. The figures express percentages

PLACE IN THE AQUARIUM | Top SIDEs | Bottom Time of day a.m. | p-m. a.m. p-m. | a.m. p.m. PSRs aan en ae oncmanenoeoeede arin 5I i | | 38 PA pri o.7erotasecshsfalatesasabelel Seriorarcielee sete aeet 61 13 26 ANpral 28/25. he, grace enatanacet ere a re4ay suesetonssaveostanataiera 74 3 II Is 15 82 IMayitiese nts 72 29 6 7 22 64 IW IEW oa sanntis HPtIg OD RAB OS Criehao OO GOde. oe 63 39 12 | 20 25 41 IMA ypegictstere cia any aitianaxagass ts ahsuive sigiersiaseia\elaters 67 43 16 | 41 May 4.. 50 16 | 34 DMA yi Sara stoseyeteve/=tsferere' foie st eeratsista etotosehs 6 eles e 31 13 56 ——— | ISVET APES yaye ye Soe seiko michele Sie si siete 60.5 30.6 12-3). |j| UI¥-20 || e27 ee 56.2

‘The forenoon census was taken about 8 o’clock, when the worms were re-arranging themselves after the darknessof the night, while the afternoon records were made about 4 o'clock, when the worms

The Reactions of Planarians to Light gi

had been all day in the light. The average at the bottom of the table indicates, first, that an approximately equal percentage of worms was found on the sides of the aquarium at both times of day, which may therefore be left out of the reckoning, and, secondly, the occurrence of a significant migration during the interval be- tween 8 a. m. and 4 p. m., demonstrated bythe distribution of the worms at the top and bottom of the jar respectively. Accord- ing to the data obtained, at least 30 per cent of the worms in the top group must have become positively geotactic and gone to the bottom during the day.

A later set of experiments in which an aquarium was kept swathed in black cloth during the day showed less migration. The conclusion naturally follows that geotaxis is more likely to occur in the presence of light than in its absence. Whether there is a regular diurnal vertical migration among planarians in nature, as Birge (’97) and Schouteden (’02) found to be true for fresh- water entomostraca, and various authors‘ for different forms in marine plankton, remains unknown. It is probable, however, that planarians ordinarily remain quiescent on the under sides of stones or in other shaded places for considerable intervals of time, coming under the influence of light only when started into activity through some other stimulus.

A worm placed in an aquarium with square sides and left free to travel undisturbed on the bottom or the sides occupies the sides more frequently than the bottom.

In a trial to test this point, an aquarium was used, the bottom area of which measured approximately five times that of the sides. The course pursued in this aquarium by one worm (P. gonoceph- ala) in directive light and covering 1340 cm., was plotted and the percentage of distance traveled on the sides was found to be prac- tically equal to that traveled on the bottom, notwithstanding the fact that the animal was started in the middle of the bottom, where it had five times as much available territory to travel over as on the sides. Other things being equal, therefore, this worm showed itself five times as ready to travel on the sides of the aquarium as on the bottom.

*Groom and Loeb (’g0), Loeb (’93a), and Parker (‘o2).

g2 Herbert Eugene Walter

The existence of such a decided geotactic tendency should not be forgotten when trying to determine the part light plays in planarian behavior.

Again, it was found that there was less accuracy of orientation to directive light while the planarians were on the sides of the aquarium in a position parallel to the light rays than while they were on the bottom.

Their behavior in the former case was the resultant of at least two known stimuli, gravity and light, whereas gravity was prac- tically eliminated when they elided on the floor of the aquarium. In the experiment cited under the preceding paragraph 92 per cent of the distance traversed by the worm on the bottom of the aquarium was in a direction in general away from the light, as contrasted with only 79 per cent when it was traveling on the sides of the aquarium. This difference of 13 per cent may represent roughly the necessary correction for geotaxis, in order to ascertain the influence of light alone.

Thigmotaxis. Contact with the substratum is an almost con- stant condition of planarian activity. Occasionally worms may be seen dangling free at the end of a mucus-thread, as commonly occurs among many fresh-water snails; sometimes they may fall helplessly from the surface-film to the bottom, but definite con- tact with something firm is the rule during their ordinary loco- motion.

A change in the degree of this contact, and consequently a pro- duction of thigmotactic stimulation, may come about in two ways the surface on which the animal glides may present irregularities, such as increased roughness or a different degree of solidity, or the worm itself may vary in the extent of Cae -surface which it brings into contact with the substratum. ‘This latter method of causing thigmotactic stimulation applies especially to Bdelloura, which has the habit of frequently alternating a leech-like looping movement with ordinary gliding, thus ae its contact rela- tions and probably producing a Antec ppling. in conse- quence.

As already mentioned, Bdelloura, when subjected to sudden dark, usually detaches itself from its support and wriggles vio-

The Reactions of Planarians to Light 93

lently in the water. It is uncertain how far this behavior is attrib- utable to light alone or to some combination of light and thigmo- taxis.

This phenomenon of compound stimulation occurs in a less pro- nounced way whenever a change of light intensity results in the “wigwagging” response common to planarians. ‘The same uncer- tainty prevails as to how far the subsequent behavior of the worm may be due to the direct stimulation of light and how far to thig- motactic stimulation primarily and to hal stimulation second- arily. It is evident, then, that under any circumstances there is such a close interrelation of stimuli that an accurate analysis of the consequent behavior 1s difficult.

Further evidence of the close relation between different kinds of stimuli is afforded by the fact that planarians are more respon- sive to the mechanical stimulus of a slight jar when the entire ven- tral surface of the body is in contact with the substratum than when the anterior end is lifted up and waving about. Apparently the greater the degree of contact the greater is the effect of a jar- ring mechanical stimulus.

This point was demonstrated by means of a small aquarium mounted on a turntable, such as is used in “ringing”? micro- scopic slides, in such a way that it could be rotated with great ease and delicacy. A light from one direction only was projected upon the single pianeian placed in the aquarium. Any attempt to change the. angle of light by rotating the aquarium ever so slightly pesnltecl instantaneously i in a momentary halt on the part of the worm, provided it happened to be gliding with its ventral surface entirely in contact with the floor of the dish. If, however, the rotation was made when the anterior end of the worm was lifted, the halt did not so readily occur. This response was of such deli- cacy that with a little practice it was possible to halt the anterior end of a worm without disturbing the continuous progress of the posterior end! That this halting was due to thigmotaxis rather than to any rheotaxis induced by the movement of the animal against the relatively stationary water particles, is shown by the fact that the reaction was more pronounced when the anterior end of the body was held flat than when it was raised and so brought more under the possible influence of a water current.

94 Herbert Eugene Walter

Finally, it may be recalled that in a preceding section data were given (Table X, p. 68) to show that there is more response to light while worms are upside down on the surface-film than when they are in contact with the bottom of the aquarium, a difference probably referable in large measure to the different thigmotactic relations in the two cases.

Gomiotaxis. Goniotaxis is a term introduced by Pearl (’03, p. 561) to define a particular kind of thigmotactic response in which the “different parts of the body are brought into such positions that they form unusual angles with each other,” as when a plana- rian occupies the angle formed between a side and the bottom of an aquarium.

There is no doubt that the peculiar movements resulting from the goniotactic stimulus directly modify the phototaxis of the worm. Once in the angle of an aquarium a planarian becomes increasingly indifferent to light. In one series of records, show- ing how a considerable number of planarians came to rest, it was found that the majority came to rest in an “‘angle” and that out of this number 78 per cent failed to orient to the light. The stim- ulus of the “angle” was greater apparently than the stimulus of light.

Furthermore, it is to be noticed that goniotaxis is always more effective if the worm is in a lowered rather than ina heightened physiological state, for whenever a planarian is freshly intro- duced into an aquarium and is in an aroused condition on account of the mechanical stimulation necessarily given it in transference, it will pass over angles and crevices with total indifference, all the while responding plainly to light. As soon as it has become fatigued, however, if its path chances to cross an angle or crevice it exhibits goniotaxis at once by slowing down and remaining in the new situation, as if caught in a trap, with complete disregard of the continued action of directive light.

Chemotaxis. Pearl has made an extensive study of this phase of planarian behavior and suggests that the well-known planarian habit of collecting in groups may be explained on the supposition that a resting planarian is surrounded by a halo of chemical ema-

The Reactions of Planartans to Light

\O al

nations which serve as a direct stimulus to other planarians, attract- ing them and causing them to come to rest in groups.

In this connection it is worth mentioning that several times when Dendroccelum lacteum was put im an aquarium with other species of planarians, the individuals of this species would later be found gathered into aseparate group bythemselves. “This manner of isolation was also repeatedly noticed in examining on the under sides of stones taken from the pond at Falmouth, Mass. A similar segregation of species in the case of P. alpina and P. gonocephala, was noted by Collin (’91). He says (’g1, p. 180) “‘liyjima fand diese beiden Arten zusammenlebend, wahrend sie im Harz stets getrennt vorkamen; auch in der Gefangenschaft schien die P. alpina die grossere P. gonocephala in demselben Behalter zu meiden und thr angstlich auszuweichen.”” It would be difficult to explain how these planarians avoid each other so as to fraternize in this fashion, except on the basis of some delicate chemotactic response which caused them to halt when they entered the chem- ical halo of their own kind, but not to do so in the different chem- ical halos of other species. As in the case of goniotaxis, the mani- festations of phototaxis may be entirely superseded by the effect of feeding (Chemotaxis). When once a hungry planarian, driven by directive light into the neighborhood of a crushed snail, becomes subjected to the chemical stimulus arising from the fluids of that object as they are disseminated through the water, it seems to become suddenly indifferent to the light, owing to the greater influence of the chemical stimuli.

The same inhibition of the influence of light by a chemotropic response to food has been observed by Parker ('03) on the mourn- ing-cloak butterfly, Vanessa antiopa L. He says (’03, p. 457)

“when a butterfly alights on a bough, it orients in the sunlight with the usual precision. Should the sap be running from a near stem, the insect is very soon attracted to the spot, begins feeding, and moves about from that time on with no reference to the direc- tion of the sun’s rays. “Thus, when feeding or near food the but- terflies do not respond phototropically.”’ Furthermore Darwin (81, p. 23) observed that earthworms are less disturbed by light while feeding or during copulation than at other times.

go Herbert Eugene Walter

The foregoing examples illustrate only a few of the many modi- fications of light responses due to the interference of some other stimulus.

Summary. In judging the effect of any stimulus upon an ani- mal it is necessary to have constantly in mind the accelerating or inhibiting effects of other stimuli which may be influencing the organism at the same time. In the case of planarians some ai the responses known to be intimately connected with phototaxis are geotaxis, thigmotaxis, goniotaxis and chemotaxis.

Planaria gonocephala shows itself to a certain extent negatively geotactic after several hours of dark and positively geotactic after a similar interval of light.

When given horizontal and vertical surfaces of equal extent, worms travel more on the vertical surfaces.

Their accuracy in orienting themselves to light while subjected to geotactic stimulus on a vertical surface is less than when they are traveling on a horizontal surface, where the directive geotactic stimulus is eliminated.

Thigmotactic stimulus may result either from an environmental change in the substratum, or a change in contact caused by the worm itself whereby its relation with the substratum is varied.

There is a close interdependence of the various stimuli which may be acting on an animal at the same time.

Behavior may be the direct consequence of light or the indirect result of light combined with the direct effect of a thigmotactic stimulus indirectly brought about by some change in the intensity of the light.

The greater the degree of contact with the substratum the more responsive a planarian becomes to the mechanical stimulus of jar- ring, but the less to the stimulus of light, as shown by comparing the behavior of worms on the surface film with their behavior on the aquarium floor.

Goniotaxis has an inhibitive effect on phototaxis; this effect becomes more apparent as the worm reaches a condition of fatigue, phototaxis meanwhile becoming less apparent.

Dendroccelum lacteum exhibits a remarkably delicate response (Chemotaxis ?) in frequently coming to rest in the neighborhood of its own kind.

The Reactions of Planarians to Light 97

Hungry planarians in the presence of food have their photo- taxis entirely obscured.

3 KINDS OF BEHAVIOR

In the two preceding sections, treating of Photokinesis and Pho- totaxis, respectively, animal behavior, as illustrated by the effect of light upon planarians, has been taken up from the point of view of the stimulus. In the two following sections, on the other hand, the reactions of planarians will be dealt with from the stand- point of the animal rather than from that of the stimulus.

To this end a classification of the behavior of planarians in light is here presented based upon (A) generic and specific differences, and (B) individual differences.

That there are morphological differences which fall naturally within the lines of this classification has long been recognized, indeed, the criteria used in classification by systematists are based almost exclusively upon such differences, while relatively little importance has been attached to differences in the behavior of animals.

As already mentioned in the historical review, Loeb (94), in dealing with the differences of behavior which characterize the two genera, Planaria and Thysanozoon, pointed out that decided physiological variation may appear in forms closely related mor- phologically. The same fact had been previously emphasized for the case of the pulmonates by Willem (’g1). Obviously such physiological variations do not furnish reliable criteria for the systematist, since they are so largely dependent upon environ- mental causes, and furthermore the work of the systematist is usually done upon dead animals. Nevertheless some interesting relations between behavior and systematic position await the stu- dent who approaches the study of animal behavior from this direc- tion.

Strictly speaking, all behavior is individual behavior. In this sense it is manifestly incorrect to speak of the behavior of a genus or of a species per se.

The behavior of individuals may, nevertheless, be classified into responses which are characteristic of all the members of a genus,

98 Herbert Eugene Walter

or again into responses which are characteristic of only one species of a genus and not necessarily of other species of the same genus, and, finally, into those peculiar to the individual as such, which may not in all particulars be shared by other representatives of the species to which the individual in question belongs. It is in this sense of the terms generic, specific and individual, that behav- ior will be taken up in the present section.

A Generic and Specific Behavior

In the present inquiry a basis for generic comparisons is afforded by a study of the behavior of edad: of four different genera, namely, Planaria, Dendroccelum, Phagocata and Bdelloura, while some idea of specific differences is ee possible by compar- ing the behavior of individuals of the two species Planaria macu- Tet and Planaria gonocephala. In the cases of Dendroccelum, Phagocata and Bdelloura it is obvious that the conclusions drawn are based in each instance upon the behavior of representatives of a single species under each genus. ‘The question may be properly raised as to how far such conclusions indicate generic behavior and how far specific behavior. Conceding that from the data obtained exact deductions may not be drawn, the fact still remains that the three species, Dendroccelum lacteum, Phagocata gracilis, and Bdelloura candida, are separated from each other by generic gaps, such that the differences exhibited by these species may be regarded as generic in degree. The point unestablished, then, is whether other species of the genera in question if examined might not show that the behavior, which in these single representative species seems generic in nature, is not characteristic of other species of the same genus as well.

It will be convenient to present the data of both generic and specific behavior at the same time.

Percentage of Negativeness. The manner of obtaining this _ criterion of behavior has been explained in the section on Phototaxis (p. 72). It will be remembered, too, that in Table XII a comparison was made between positive and negative worms, showing the degree of their orientation to directive light. The

The Reactions of Planarians to Light 99

data there used are rearranged for the present purpose in Table

XIX.

TABLE XIX

Percentage of generic and specific negativeness in worms started at right angles to incident light, as deter- mined at the circumference of a circle 10 cm. in diam. by the average amount of their deviation from

the directions in which they were started

SPECIFIC

GENERIC DIFFERENCES DIFFERENCES

F Planaria Mende eee Boe hpi aried |iEdellowealy seca gono- coelum cata maculata | cephala Number of observations........... 78 80 158 10 78 | 80 Total number of degrees positive... 155 238 165 397 5 | 160 Total number of degrees negative...) 2112 1964 4070 50 | 2102 | 1968 Percentage of negativeness........ 93-1 89.6 96.1 II 99-9 84.6

Comparing the figures given in this table, a greater range of difference is seen to obtain between the two species of Planaria (P. maculata and P. gonocephala) than between the genus Planaria and either of the other negative genera, namely, Dendroccelum and Phagocata. Although not indicated in this table, similar results appear when the number of times the worms went in a negative direction is used as a basis of comparison, instead of the total num- ber of degrees of negative deviation.

Character of the Course in Directive Light. When worms were placed on the middle of a rectangular aquarium floor and sub- jected to a directive light their movements showed both generic and specific differences. By experimenting with one worm at a time it was possible to plot on a sheet of paper with sufficient accuracy for general comparison the entire course of the worm during a considerable period. This was done many times and typical records of such observations are given in Figs. 10-14. — In such instances the worm was exposed to a light of approximately 147 c.m., placed so as to correspond to the right side of the figures. The central rectangular area bounded by the broken lines indicates the limits of the floor of the aquarium, while the smaller exterior adjacent areas represent its vertical sides so rotated as to

100 Herbert Eugene Walter

bring them into the plane of the floor. The course taken by a planarian is indicated by the tortuous line. The full line shows the course taken on the solid surface of the aquarium; the dot-

Fig. 10 Dendroccelum lacteum. The shortness of the path shows comparatively little persistence

in locomotion, and the direction, considerable indifference to the source of light.

and-dash lines, the course of the worm on the surface film. The dotted line indicates a hiatus in the path, made necessary by the attempt to represent on a flat surface a continuous line which tra- verses vertical as well as horizontal surfaces. A succession of

The Reactions of Planarians to Light IOI

abrupt kinks in the line signifies that at that point the worm exe- cuted decided wigwag movements with its anterior end. The figures are reduced in size from the original records.

In Fig. 10 is given a specimen record of a Dendroccelum’ which

4

Fig. 11 Phagocata gracilis. This path shows great activity on the part of the worm, and, although it is mostly laid down away from the source of the light, it shows that the worm experienced no great difficulty in moving toward the light.

came to a standstill after 18 minutes of locomotion. ‘The first movement of this worm was diagonally away from the light, but It soon came back toward the light traversing almost the entire

102 Herbert Eugene Walter

width of the aquarium and in doing so showed considerable indif- ference to the directive influence of the light. Its susceptibility to goniotactic stimulus is plainly shown by its behavior upon reach-

Fig. 12 Bdelloura candida. This path was traversed with much ‘‘wigwagging;” there was indif- ference to the source of light and locomotion was not of long duration.

ing the angle formed at the junction of the sides and floor of the aquarium, as well as by its manner of finally coming to rest. A typical Phagocata (Fig. 11), on the other hand, exhibited

The Reactions of Planartans to Light . 103

almost no goniotaxis, although the worm repeatedly crossed the line of the angle. The response to the directive influence of the light, too, was in this case even less than that of the

Fig. 13 Planaria maculata. Considerable activity was shownover this course and a decided

inability to approach the source of the light beyond about the middle of the aquarium.

Dendroccelum just described, as is evident from the general wan- dering character of the course. Although the Phagocata in ques- tion frequented both sides of the aquarium—that which was toward

104 Herbert Eugene Walter

the light, as well as the opposite side—its wanderings were in the main on the side away from the light. An hour’s activity is chron- icled in the record, at the close of which the worm was apparently as energetic as ever.

Fig. 14 Planaria gonocephala. The path is of the same generic type as with Planaria maculata (Fig. 13), and is easily distinguishable from those of Dendrocelum, Phagocata and Bdelloura.

Fig. 12 gives a characteristic record of the way in which Bdel- loura behaves. The first movement of this specimen was more

The Reactions of Planarians to Light 105

toward than away from the source of the light, but very soon wig- wagging motions set in, and after every exercise of these move- ments, which were apparently in the nature of explorations, a change in the direction of the course was effected. As might be expected, such abrupt changes in direction were more difficult of execution when the worm was on the surface-film.

Characteristic movements of individuals of the genus Planaria are shown in Figs. 13 and 14. From these two typical records it would be difficult to select any diagnostic points which would dis- tinguish the behavior of P. gonocephala from that of P. maculata. There is no doubt, however, that taken together the behavior of representatives of these two species presents a distinct (generic) dif- ference from that of the representatives of the other genera studied. The most striking feature of the Planaria records ( (Figs. 13 and 14) is the high degree of response exhibited by members of this genus to the decane action of light. Although many attempts were made by the individual worms to penetrate the half of the aqua- rium nearer the light, yet they seemed as unable to keep to that direction as they would have been had a solid barrier been inter- posed between them and the light. This characteristic respon- siveness to directive light helps to explain why (as shown in Fig. 8, B, p. 84). P. gonocephala was unable to come! to rest In the area of lessened illumination as it would naturally have been expected to do.

From the cases cited in this section, at least, it may be afirmed that the generic differences are so pronounced that one could take a miscellaneous, unidentified assortment of such records and correctly assign the great majority of them to the proper genera.

Duration of Movement. When worms of different genera are subjected to the same light intensity there is considerable variation in the time required to bring them to a standstill. Bdelloura is usually the first to stop, followed in order by Dendroccelum, Pla- naria and Phagocata. Of the two species of Planaria, P. gono- cephala, although averaging somewhat smaller in size, usually keeps in motion for a longer time than P. maculata. The individ- ual records of the duration of movement given in Figs. 10-14 may be taken as a typical set of records. “They were as follows:

106 Herbert Eugene Walter

Fig. 10. Dendroccelum, 18 min.

Fig. 11. Phagocata, 60 min. (still moving).

Fig. 12. Bdellourz, 15 min.

Fig. 13. Planaria maculata, 47 min.

Fig. 14. Planaria gonocephala, 60 min. (still moving).

Woodworth (’97) in contrasting the activity of Planaria macu- lata, P. gonocephala and P. dorotocephala states that individuals of the latter species remain in motion longer than individuals of the other two—an observation confirmed by Pearl (’03).

Degree of Wandering. fa worms started at the center of a circle parallel to the direction of the light and pointing away from its source, then the more devious its course the more it may be said to wander. Both generic and specific differences were obtained bear- ing upon this phase of behavior. Selected instances of such dif- ferences are given in Table XX, expressed in average degrees of deviation upon emergence from a circle 10 cm. in diameter.

TABLE XxX

The average generic and specific differences between individuals of four genera and two species of Janarians expressed in degrees of deviation upon leaving a circle 10 cm. in diam. In ever y

instance the worm was started away from the source of the light

SPECIFIC GENERIC DIFFERENCES DIFFERENCES Ded Pl PI _ | Planaria os 248°" | Planaria Bdelloura ae gono- celum cata maculata cephala Average degree of deviation....... 9-4 30.0 24.6 132.0 21.6 27-7 Number of observations........... 56 46 gz 10 46 46

The remarkably large deviation shown by Bdelloura is due to the fact that it is a positive worm. When pointed toward the light its deviation was only 39.3°, a number which would perhaps be more justly comparable with the other records in this table. But even so, it will be seen that Bdelloura, of all the forms observed, is the least oriented by directive light. Specific differences in the degree of wandering are in general less marked than the generic differences, according to the records in Vable XX.

The Reactions of Planarians to Light 107

Rate of Locomotion. As regards rate of locomotion the records of specific differences exhibit a wide range, although not as great as that of the generic differences existing between Dendroccelum and Phagocata.

TABLE XXI