RHEOTAXIS IN PLANARIA ALPINA

BY R. S. A. BEAUCHAMP.(Assistant Naturalist at the Laboratory of the Freshwater Biological

Association of the British Empire.)

(Rectivd 4th October, 193a.)

(With Seven Text-figures.)

INTRODUCTION.

THE rheotactic responses of fresh-water planarians have been studied more thanthose of any other invertebrate. Yet these records are for the most part incomplete,and at variance with each other. The only conclusions that can be drawn from themare that the stream-living forms are more inclined to give the positive rheotacticresponse than the lake-living forms. Also the stream-living forms are not alwayspositively rheotactic, at least under experimental conditions.

The earliest observation is by Johnson (182a), who records having seen anumber of " planarians " migrating upstream. It seems that Johnson's observationsrefer either to PUmaria alpma or PofyceHt contuta.

In 1900 Volz made a similar observation on PI. alpma in a spring near Aarberg.In 1903 Pearl worked in America with PI. dorotocephala, a species which is

found only in streams. He says: "The planarian is positively rheotactic to veryweak currents (as delivered by a fine capillary tube), the form of the reaction beingprecisely the same as that given to other weak stimuli." Working with the samespecies, Allen (1915) came to the conclusion that it was positively rheotactic only instrong currents.

Before considering the more recent experimental work, reference should bemade to the environmental factors which control the spread of these animal*. Thethree European species, PI. alpma, PoL contuta and PL gonocephala, are all limitedto the upper reaches of streams, since the summer temperature of the lower reachesis fatal to them.

Temperature is the chief controlling factor, but the rate of flow is also important(Beauchamp and Ullyott, 1932). Any fluctuation in the temperature of the streamshould shorten or lengthen the area over which these nnimaia occur, dependent onwhether the temperature of the stream is raised or lowered. The difference betweenthe summer and winter distribution shows this to be the case (Wilhelmi, 1904 andCarpenter, 1928). Voigt (1907) showed that PI. gonocephala migrated up tributarystreams, which had previously been colonised only by PI. alpma and Pol. contuta, onthe warming of the water, following the cutting down of woods.

The simple maintenance of position in a stream of water, quite apart from theseobserved migrations, demands some form of rheotactic response on the part of these

ii4 R. S. A. BEAUCHAMP

animals. This response was first demonstrated by Voigt (1904). He placed a numberof planarians (species not stated) in a large tube with two small tubes coming in atthe top; one brought pure water, the other water to which bait juices had beenadded. When the latter tube was turned on the animals were " alarmed " and crawledup the tube. He could obtain no response when the anifnuia were tested with purewater.

In 1913 Steinmann, using PL alpina, demonstrated positive rheotaxy in ordinarywater. The apparatus used was a sloping glass tube. Steinmann *s conclusions werethat 80 per cent of all the triclads tested were positively rheotactic.

Kafka (1914) and Olmstead (1917) have shown that PL gonocephala and PLmaculata are geotactic. Olmstead showed in PL metadata that the positivelygeotactic animal became negatively geotactic after feeding. The writer has con-firmed this observation for PL alpina. PI. alpina is strongly negatively geotactic for6 or 7 days after feeding. In the light of this knowledge all Steinmann's experimentsare open to the criticism that they measure geotaxis as well as rheotaxis, since heused a sloping tube.

Doflein (1925) made a careful study of the chemotaxis of PI. alpina, PI. gono-cephala, PL htgubris and Dendrocochtm lacteum. She found, that in still water, foodsubstances stimulated PL alpina to move about and to make "search movements."But, unlike the other three species, PL alpina was unable to orientate itself to thefood. Similarly in running water PL alpina could not appreciate the direction fromwhich the food stimulus was received.

Doflein's rheotactic experiments were done on a gently sloping glass plate, soof necessity there must be some confusion between geotaxis and rheotaxis. Shefound that 80 per cent of the PI. alpina were positively rheotactic, and consideredthat those which showed the negative response were either weakly or damaged.

Doflein states that the rheotactile organs are situated in the head. Using apipette, she found that the nnimaia only responded to a current of water when it wasdirected on to the head region. She further assumed that the receptors were situatedin the pair of head tentacles.

Koehler (1926) disproved this assumption by removing the head tentacles andfinding that the animals responded normally. His current consisted of a stream ofwater on an inclined plane and also of a jet of water from a fine pipette. In 1932he again concluded that PL alpina was strongly positively rheotactic and that therheoceptora were distributed over the whole body. In these later experiments heused only the pipette method for making his current.

Hubault (1927) experimented with PL alpina using a circular dish. He made hiscurrent by means of a jet at the side; the water was removed from the centre of thedish by a siphon. He stated that nearly 80 per cent were positively rheotactic. Thecurrent strengths used were very great judging from the fact that about 14 per centof the animals were washed away. None of his experiments lasted longer than 5 min.

Voute (1928) found that all the PL alpina collected from a stream and lakes nearOo8terbeck were decidedly negatively rheotactic. But putting food into the watermade them react positively. No description is given of the apparatus used.

Rheotaxis in Planaria alpina 115

Carpenter (1927) found that PI. alpina was always strongly negatively rheotactic.She also stated that if the animals were kept in still water for several days they nolonger responded to a current of water. It is possible that these observation* are theresult of unsatisfactory experimental conditions. It seems unlikely that any stream-living animal could be persistently negatively rheotactic, without sooner or laterdeserting its normal habitat.

Summarising we may say that the majority of workers have found that a largepercentage of the PI. alpina tested show a positive response to a current of water.The rate or extent of this reaction has in no case been determined. No work hasbeen done on the responses given by the same individual over a long period of time.Nor has any attempt been made to correlate the condition of individuals with theirbehaviour.

METHOD AND APPARATUS.

All the experiments described in this paper were done with PI. alpina.Since it is convenient to have a large number of animal* available in the labor-

atory a stock supply was kept Moreover, it is an advantage to have the animaiaunder known conditions previous to their being tested.

The water in which these stock animals were kept was well aerated or elsesupplied with a current of water from the tap. The temperature was kept belowio° C. At first some difficulty was experienced in keeping the animals in a healthycondition; later work showed that tap water was unsuitable owing to the presence ofdissolved iron.

In the end the most satisfactory conditions were found to be given by keepingnot more than fifty animals in a shallow open basin of spring water. Under theseconditions it was found unnecessary to aerate or circulate the water. A few stoneswere provided for the «nima1« to crawl under. The temperature was kept at a lowand constant value by leaving the basin in a cellar. Owing to the slope of theground the cellar received light from a window on one side, this ensured normaldiurnal light variations.

It was found that there was marked periodicity in the activities of the animals.During the day they remained under the stones, and in the evening they started tomove about But, under the quiet conditions in the basin, this periodicity was lostafter a few days although the daily changes of light were unaltered.

The rheotactic responses of these animnjn were tested in a long glass trough.This trough was 120 cm. long and 10 cm. wide. It was shaded from all sources ofdirect light and received only diffuse light from the interior of the room.

At first all the experiments were done with animals which were collected fromthe top of the streams. These animal* might well be expected to be positivelyrheotactic. Yet in almost every case the animals moved downstream; that is to saythey were negatively rheotactic. At the time it was felt that these results were theoutcome of unsatisfactory conditions.

The discovery that the tap water was injurious to these animals led to a radical

n6 R. S. A. BEAUCHAMP

change in the method employed. Clearly it was necessary to use only spring waterfrom where the animals were collected. This made it necessary to devise a circulatingapparatus which would give a fairly considerable and steady flow. It was essentialthat there should be no metal included in the apparatus which might contaminatethe water. This precluded the use of any sort of motor or even a mercury valve.

The general form of this apparatus is best understood by reference to Fig. i .From the vessel A the water flows through the trough used for the rheotactdcexperiments. From this trough the water pours into the periodic syphon B. Whenthe level of the water in B rises to the top of the siphon, the vessel automaticallyempties itself into the basin C.

prop

vrnhra

Fig. I. Diagrammatic view of the apparatus. The arrows indicate die directionin which the water Bow*.

From C the water is sucked into the chamber D from which the air is exhaustedby means of a filter pump. During this process the exit tube c is closed by the glassvalve.

When the amount of water delivered by the periodic siphon has been drawn upfrom C into the vessel D, the long sloping tube d fills with air. This releases thenegative pressure in D and the water escapes from this chamber by way of tube cthrough the glass valve into the vessel E. From E it siphons over into A.

The vessel X serves to minimise the changes of level in A. In addition thesiphon between E and A damps the rise in level following the emptying ofchamber D.

Rheotaxis in Planaria alpina 117

The efficient working of this apparatus is dependent on a number of smalldetails. The design of the periodic siphon is important Its internal opening mustbe wide. This ensures that the column of water is broken at the end of the emptyingprocess and replaced by air. On the other hand the rest of the siphon must not betoo wide, otherwise the water will start to flow out as soon as it reaches the bottom ofthe curve at the top, that is to say at the level x. It will be seen that for greaterefficiency the bore of the siphon should be varied according to the current circulat-ing. The greater the current the greater should be the bore of the siphon tube.

In this particular case where the flow was usually about 1200 c,c. per min., thebore of the siphon tube was f in. The internal opening was 1 in. The vessel itself wasa Terry sweet-jar with the bottom removed. Removing the bottom is easily done bystanding the jar in } in. of cold water and then pouring a little boiling water into i tRubber bungs size 1 ^ in. fit the mouths of these jars.

It will be seen in Fig. 1 that the long sloping tube dis held at the bottom end by acord. It is of the greatest importance that a small length of elastic be included inthis attachment; without it the apparatus works very inefficiently1.

In the chamber D it was found necessary to bend the tube b slightly to one sideaway from the tube a, as water, falling off the roof of this chamber, was liable to besucked down a.

The glass valve was constructed by sealing a glass tube on to the side of a smallreagent bottle, of the type possessing a ground glass stopper combined with apipette. Both ends of this combined stopper and pipette were closed, but first thelower half of the pipette was filled with mercury to weigh it down. If this were notdone the stopper was apt to be blown right out of the bottle on the release of thewater from chamber D.

It sometimes happened that the stopper returned to its seating so abruptly thatthe weight of water in D was not sufficient to reopen it. This difficulty was overcomeby pulling out the stem of the pipette until it almost touched the bottom of thebottle and then fitting the end with a small rubber pad. Without the rubber padeither the pipette or the bottom of the bottle was liable to break.

Experience has shown that this apparatus will function satisfactorily for anindefinite length of time.

If the pressure of the tap water is low and if the apparatus is to be worked to itsfullest capacity it may be advisable to use two filter pumps in parallel. This apparatuscan circulate about 3 litres of water per minute.

Using this apparatus and the glass trough, better results were obtained and alarge percentage of the animpin tested showed the positive response. But thestraight sides of the trough made it possible for the animals to find shelter in theangle between the sides and the bottom. In addition, one was uncertain whether thebehaviour of an animal crawling on the vertical sides of the trough was comparablewith its behaviour on the bottom.

1 It ha* been found poaaible to dispense with the periodic siphon B, since a long elastic attach-ment causes excursions of d sufficient to maintain the periodicity necessary for the working ofthe apparatus.

u 8 R. S. A. BEAUCHAMP

Finally the glass trough was exchanged for a long wooden one. This trough was150 cm. long by 12 cm. wide, the depth was approximately 1 cm. The bottom wasflat and the sides were gradually curved up so that there were no corners or verticalsides. Fig. 2 shows one end of the trough, seen from above and in longitudinalsection. The water wells up from the bottom of the shallow pool at one end andoverflows into the main part of the trough by way of a gentle slope. The outflow isexactly similar to the inflow. This arrangement eliminated eddies, at least over therange of currents used in these experiments. The direction of current could bereversed by a system of glass Y-pieces and clamps.

The speed of the current had no appreciable effect on the behaviour of the; probably this is partly accounted for by the fact that changes in the current,

Fig. a. One end of the trough, teen from above and in longitudinal section.The two ends of the trough are exactly similar.

as measured by floating pieces of cork, were not followed to the same degree bychanges in the current immediately above the bottom of the trough.

As measured by floating pieces of cork, the current used in all experiments wasapproximately 2 J m. per minute.

The temperature of the water during all experiments was kept below io° C.This could only be brought about by cooling the whole laboratory. This was easilydone as the experiments were carried out during the winter months.

The initial experiments with this trough proved unsatisfactory. This was theresult of toxic substances dissolving out of the wood. The trough was made of teakand the difficulty was overcome by giving it a number of coats of cellulose paint.Black paint was used; this provided an additional safeguard against light errors.

The whole apparatus, consisting of the circulating apparatus and the paintedtrough, now proved entirely satisfactory.

Rheotaxis m Planaria alpina 119

Earlier workers studied the reactions of a large number of animals and expressedtheir results as percentage number of positively or negatively rheotactic forms. Itwas thought better to study the reactions of individual pninrmifr over a long period oftime and discover whether changes occurred in their behaviour. The actual move-ments of the animals were copied on long strips of paper, the same size as the trough.

Usually, animals were tested for several days in succession and then transferredto a labelled dish and later tested again. Occasionally an individual was kept as longas 6 weeks in the trough. During this time changes in its behaviour might occur, asexplained later, but on no occasion did changes occur owing to unsuitable experi-mental conditions. The experimental conditions were undoubtedly entirely satis-factory for the health of the animals; in no case did an animal become moribund orout of condition while in the trough.

EXPERIMENTAL DATA.

(a) THE SEXUAL CYCLE.

Once the above apparatus had been constructed satisfactory experimental datawere obtained. Errors due to possible changes in the chemical content of the waterwere eliminated and the animals were kept in water of optimum composition. Thetrough was evenly illuminated and kept in a low light intensity, although therheotactic response was always stronger than the phototactic. The temperature ofthe water was controlled. With these precautions, if the nnimpin re-orientated them-selves to the current when the direction of the flow was reversed, it was consideredthat the response could only be attributed to rheotaxis.

As a rule the animals were tested in the evening and sometimes late into thenight, that is to say during their normal period of activity. The individual which wasbeing tested was allowed during the day to rest under a stone in the trough. In theevening when it had come from under the stone of its own accord, the stone wasremoved and the current turned on. Before turning on the current the movementsof the animal in the still water were recorded.

Fig. 3 shows the actual path of an individual. The dotted line at A shows itsmovements before the current was turned on. These movements were quite at randomand demonstrated clearly that the animal was under no external influence which mighthave tended to direct its movements. The continuous line shows the path taken withthe current on. The animal now moved upstream in a straight line. At C and at D thedirection of the current was reversed and the animal responded by turning roundand moving upstream in the new direction. At E the current was shut off, with theresult that the animal again moved about in an entirely unonentated manner, as atthe beginning of the experiment The arrows indicate the direction of the animal'smovements and are marked in at minute intervals.

During the months of December and January it was found that almost all of theanimals collected from the top of the streams were positively rheotactic. An in-dividual was considered to be positively rheotactic only if it re-orientated itself each

120 R. S. A. B E A U C H A M P

time the current was changed and moved a minimum distance of three metresagainst the stream.

The rate of movement is dependent on the temperature. At o° C. the nnimaiamove at the rate of a cm. per minute. The rate of movement goes up as the tempera-ture is raised, at io° C. it is 8 cm. per minute.

One of the individuals collected in January from the top of the stream was testedto see how far it would move upstream before becoming fatigued. This test wascarried out at a temperature of 6° C , the rate of movement being 5 cm. per minute.Each time the animal reached the top of the trough the current was reversed, theanimal then re-orientated itself and proceeded in the opposite direction. Theexperiment was concluded after the animal had travelled 20 m. upstream in acontinuous effort lasting 7^ hours. The discrepancy between the rate of move-ment given, namely 5 cm. per minute, and the rate as calculated from the totaldistance gone, divided by the total time taken, is accounted for by the time taken re-orientating each time the current was reversed. Moreover, the 20 m. refers only to

A J

B

Fig. 3. The actual path of tn individual. The dotted line at A thowa it» movements in still water. Atof the current was reversed. At E the current was stopped. The arrows indicate the direction of theindicate the direction of the current.

the "progress" made in an upstream direction. Actually there were very fewdeviations, the animal moved up in almost a straight line, as in Fig. 3.

When individuals were tested for several evenings in succession, they were notremoved from the trough. The water was allowed to flow out from the pools at thetwo ends, so that the animal* were left isolated in the centre part of the trough.A stone was put into the trough, under which they were always to be found duringthe daytime. But in the evening they always came out from under the stone of theirown accord. That is to say the normal periodicity of the animals was continuedwhen they were stimulated to move about during the evenings. As already pointedout this periodicity is lost when the nnimnU are kept for a number of days in quietwater.

Animals tested on consecutive nights were quite consistent in their responses,that is to say those animals which were positively rheotactic one evening werepositively rheotactic the next The majority of those collected during December andJanuary were positively rheotactic but a few were found to be negatively rheotactic.These animals were, as a rule, just as consistent in their behaviour as were thepositively rheotactic individuals; that is to say they always went downstream.

Rheotaxis in Planaria alpina 121

As yet no one has studied the negative response; in some cases workers haveconsidered rheotaxis as synonymous with positive rheotaxis and have ignoredentirely the negative response, considering it to be the result of damage orweakness.

Many individuals which were consistently negatively rheotactic were in nosense weakly. They were often bigger than those which showed the positive response;they were undamaged, while damaged animals were often positively rheotactic.Their rate of movement at particular temperatures was usually slightly higher thanthe rate of movement of positively rheotactic individuals at the same temperatures.

This negative response could not be explained on the grounds of unsuitablewater or other experimental conditions, since positively rheotactic individuals wereoften put into the trough at the same time as the negatively rheotactic ones. Theformer would migrate upstream while the latter would migrate, down. When thedirection of the current was reversed, both lots of animals would re-orientate them-selves and make their way respectively up and down the current

B the current m i turned on; the continuous line ihowa the path taken. At C and at D the directionanimal's movements and are marked in at minute intervals. The arrows immediately below the letters

Serial sections were cut of a number of these persistently negatively rheotacticindividuals, and in all cases they were found to be mature. The testes were large,both the testes and the vasa deferentia contained spermatozoa, but as a rule thetestes were not full and appeared to be past their period of maTimal production.The condition of the ovaries was always very much more characteristic. In a normalfully sexual individual each of the two spherical ovaries measure about 120/* indiameter; in all the negatively rheotactic individuals they were reduced in sizeto about 40^, and were clearly exhausted, showing that die animals had already laidtheir cocoons. Since all these animals had been collected from the very top of thestream it is evident that at one time they must have been positively rheotactic

Examination of the positively rheotactic individuals showed them to be eitherfully developed or else beginning to develop sexually. From this it seems clear thatanimal* which are developing sexually become positively rheotactic and remainpositively rheotactic until the sexual cycle is complete. When the cocoon is laid thebehaviour of the animal changes and it becomes negatively rheotactic. This changeover was witnessed in a particular case when the animal laid a cocoon while in theexperimental trough. Previous to laying the cocoon it had been positively rheotactic,after laying it became negatively rheotactic.

JM-Xii 9

122 R. S. A. BEAUCHAMP

(b) THE EFFECT OF STARVATION AND FEEDING.

It is well known that planariana can resist the effects of starvation for a very longtime. They absorb their gonads and become reduced in size.

A number of positively rheotactdc animals were kept for two months withoutfood. It was then noticed that they no longer gave their consistent positive re-sponses. At times they went upstream but rather more often they went down. Oftenwhen the current was reversed they did not re-orientate, but continued in thedirection they had previously been going, now of course the opposite direction withregard to the current

When travelling downstream these animals tended to keep a very much straightercourse than when going up. Often when going up they would zig-zag from side toside of the trough. This resulted in much greater progress being made in the down-stream direction than in the up.

This change in behaviour should be contrasted with that noticed on com-pletion of the full sexual cycle, when the animals become decidedly negativelyrheotactic.

Examination of the starved individuals showed that the gonads had beenabsorbed, but as yet the size of the am'maU was only slightly reduced. If theseanimals were fed with a piece of Gammons or Caddis larva, they were as a rule verydisinclined to move for 24 hours. But after that time they would react positively tothe current This reaction would last for about 6 days, and then they would occa-sionally go downstream again, reverting to their previous undecided behaviour.

After a second feed the animal continued positively rheotactic for a very con-siderable time, that is to say well over a month, after which time the effects ofstarvation would be repeated.

The first reaction after feeding, namely, the positive rheotaxy lasting 6 or 7 daysmay be called " temporary " positive rheotaxy. The reaction after a second feed maybe called "permanent" positive rheotaxy.

Often a single feed would be sufficient to render the animal "permanently"positively rheotactic.

Feeding an individual, which had laid a cocoon and become persistentlynegatively rheotactic, resulted in it too becoming "temporarily" positively rheo-tactic. This reaction lasted 6 or 7 days, as in the case of starved individuals; or,sometimes it lasted for rather a shorter time, the animal then reverting to its previousbehaviour.

It was possible to produce "permanent" positive rheotaxy in these individuals.But it took longer than in individuals that had been starved and had never completedtheir sexual cycle.

Clearly there are two effects of feeding negatively rheotactic animals. The firstis almost immediate and results in the ^nimg1« becoming positively rheotactic forabout 6 d^ys. The second is the development of "permanent" positive rheotaxy,brought about, presumably, by the re-development of the sexual charactersresponsible for positive rheotaxy.

Rheotaxis in Planaria alpina 123

The first, namely "temporary" rheotaxy, may be compared with the negativegeotaxy induced by feeding. This latter reaction was observed by Olmstead (1917),working with PL maculata. Olmstead noted that the duration of this response wasabout 5 days. The writer has observed an exactly similar reaction in PL alpina; inthis case the reaction usually lasted 6 or 7 days. We may suppose that the absorptionof the digested food acts as a general stimulus; L. Hyman (1919) has shown that therate of metabolism of the whole animal goes up after feeding. This general stimula-tion appears, in some way, to affect the geotactic and rheotactic responses.

The "permanent" positive rheotaxy induced by feeding starved animals is lessdifficult to understand. Starvation caused these animals to de-differentiate and losetheir positive rheotaxy. Feeding enabled them to re-develop those characters whichwere responsible for their original positive rheotaxy.

This positive response is closely associated with sexual development and thenegative response following the completion of the sexual cycle is very striking, yet itwas found that feeding induced the "permanent" positive response before theanimals had re-developed their gonads. It seems, therefore, that the factor re-sponsible for the positive response develops before the gonads are differentiated.

It was noted above that it took longer to induce "permanent" positive rheotaxyin individuals which had completed their sexual cycle and become negativelyrheotactic than in individuals which had been starved. It is evident from this thatafter laying a cocoon some factor is developed which inhibits the positive responseand causes the persistent negative response to reappear once the immediate andtemporary effect of feeding has worn off.

It is equally clear that this inhibiting factor is gradually lost, since the "per-manent" positive response can be induced after a certain length of time.

The negative response following the laying of a cocoon, though definitely ob-served in a large number of cases is not absolutely invariable. One individual whichwas well fed previous to laying did not become negatively rheotactic after laying itscocoon. This animal was seen copulating on February 6th; on February 29th it laida cocoon. All this time the animal had been positively rheotactic. It had been fed atintervals of about 7 days. This amount of food is greatly in excess of what theanimal« are likely to get in their normal habitat, and may explain why the usualchange to negative rheotaxy did not occur.

OBSERVATIONS AND EXPERIMENTS IN THE FIELD.

During the autumn, winter and spring three streams in the immediate neigh-bourhood of the laboratory were kept under observation. They were typicalmountain streams. Two of them flowed into Windermere and the other into BlelhamTarn.

These three streams which contained PL alpina appeared to have a more constantwater supply than other similar streams which did not contain these animals.

It was found impossible to visit all three streams at frequent intervals. It wasdecided, therefore, to visit one of the streams more frequently than the other two.The more occasional visits (every 3 or 4 weeks) to the other two sufficed to show that

I 2 4 R. S. A. BEAUCHAMP

exactly similar migrations occurred in them as occurred in the stream which wasmore closely observed. The latter was visited about every 10 days. It was thoughtthat more frequent visits would disturb the stream too much.

Since there are no permeable rocks in which a supply of water can be stored, thesource is not very constant, either with regard to the quantity of water or to tem-perature. But the water-holding capacity of the soil is sufficient to insure that thestream does not dry up in dry weather.

Fig. 4 shows the variations in temperature. The curve with the points marked bytriangles shows the temperature of the water at the source. The curve with thepoints marked by squares shows the temperature of the water 300 m. from the

14-

8 -

8EPT. OCT. NOV. DEC. JAN. FEB. MAR. APR. MAY JUNE JULY1931 1932

Fig. 4. Temperature records. The point* marked by triangle* are temperature* recordedat the source. The square* are temperature* recorded 300 m. from the source.

source. The temperature variations at the source, though they would be consideredUrge for a true spring, are considerably less than those at 300 m.

The number of PL aJpma found at particular stations down the stream wererecorded and especial care was taken to determine the lowest limit at which thisspecies was found. During October and November a few individuals could alwaysbe found about 400 m. from the source. But by December none could be found sofar down the stream, and by the end of January the lowest limit was about 225 m.from the source. That is to say, the lowest point at which this species could be foundwas by the end of January about 175 m. higher up the stream than it was duringOctober. There are two possible explanations to this; either that the individualsbelow 225 m. from the source had been killed or else they had migrated up-stream. The huge increase in the numbers found at the source show the latter

Rheotaxis in Planaria alpina 125

suggestion to be correct Fig. 5 is a graph showing both the lowest limh of P/.a^>oiaand alto the number of animals found on a particular area (roughly \ sq. m.) at thetop of the stream. These two graphs alone make it quite clear that during thewinter months there was a general migration upstream.

The average size of the animals at the top of the stream during December andJanuary was larger than the average size of the animals 200 m. from the top. That isto say, the larger animals were the ones which had migrated upstream. By the end ofJanuary a few newly hatched individuals were to be found; these were all in theupper part of the stream.

400-1

800-

800- r-aoo

-400 .

OOT. DEO. JAN. FEB. MAR. APfl.NOV.1031 1032

Fig. 5. The top graph show* the lowest point in the stream where Pi. atpata was found at differenttime* of the year; the bottom graph show* the number of anirniU found on a particular area at thetop of the stream.

The great number of animal* at the top of the stream during January andFebruary was very remarkable. On one occasion as many as a6o were found on theunder surface of a single stone; this surface measured about 250 sq. cm.

As already stated experiments showed that almost, all of the anirnaia collectedfrom the top of the stream during January were positively rheotactic. But by themiddle of February only about 50 per cent of them were positively rheotactic, andby the first week in March only 5 per cent were positively rheotactic. The greatmajority of the remainder proved to be individuals which had been starved and hadde-differentiated and had then lost their positive rheotaxy, as explained in thepreceding section. The rest were individuals which had completed their sexualcycle and showed the more definite negative response.

126 R. S. A. BBAUCHAMP

One would naturally suppose that as soon as these animals became negativelyrheotactic they would start to migrate downstream. They were prevented fromdoing so in the early part of this year by the shortage of water in the stream. Fromthe last week in January till the first week in March there was scarcely any rain andin consequence by March the streams were very dry. But on March 5th and 6ththere was heavy rain, and immediately afterwards it was found ttujt almost all theplanarians had disappeared from the top of the stream. This is shown at the pointX on the bottom graph, Fig. 5. Most of these animals migrate downstream of theirown accord. But some get washed away and carried down to where the stream runsmore quietly or to where it flows into the lake. Here they are deposited. An accountis given by the writer (1932) of the conditions under which these individuals mayestablish themselves on die lake shore.

In short, during December and January there was a general migration upstreamon the part of the PL alpma population. During February and the beginning of

300-

200-

OOT NOV. DEO. JAN. FEB. APR.

Fig. 6. The length of stream occupied by PL alpata. The width of the black area* indicate* thedensity of the population. By April the population was smaller, since some individuals were washeddownstream.

March these animals either completed their sexual cycle or else were starved, withthe result that they became negatively rheotactic. But by this time the streams wereso dry that they were unable to start migrating downstream until the rain came onMarch 5th and 6th.

Fig. 6 shows the length of stream occupied by PI. alpma between October andApril. The width of the black areas indicates the density of the population.

Laboratory experiments (see above) showed that if individuals, which had beenstarved and which had de-differentiated and lost their positive rheotaxy, were fedthey regained their positive rheotaxy. This return to positive rheotaxy was sodecided that an experiment was attempted to show the effect of feeding in the field.The result was very striking.

The experiment was done on the stream which had previously been the mostclosely observed. Feeding was commenced almost immediately after the suddenmigration downstream on March 5th and 6th. Every 2 or 3 days for a fortnightabout 150 insect larvae and Gammons were collected from other streams. Thesewere killed and put into the stream. The first lot of food was put in at the lowest

Rheotaxis in Planaria alpina 127

point where PL alpina m i to be found, at that time about 300 m. from die source.Successive lot* of food were put in, higher and higher upstream, with the idea that,in this way, the greatest use would be made of the food. The counts of the number ofanimals found on the particular area at the top of the stream were continued (seeFig. 7, a, the continuous line, with the points marked by triangles). At the start of theexperiment this area and 5 m. of the stream below it were cleared of all PL alpina.This was done so as to eliminate errors due to the wandering of individuals in theimmediate neighbourhood. On each occasion when the counts were made, all theanimals found were either taken back to the laboratory for examination or else werekilled. In this way it was hoped to obtain a really accurate idea of the numbersmigrating upstream.

800-

700-

400-

300-

2 0°"100-

FEB. MAR. APR.

Fig. 7. Number of Pi. alpina recorded from particular area* at the top of two stream*.a m fed during the period of time marked by a thick black line.

The first sign of a migration upstream was evident towards the end of a fort-night. But, as shown by the graph (Fig. 7, a), the full effect was not shown till later.

The dotted curve, b, shows numbers of PL alpina counted at the top of one of theother streams which was used as a control. A comparison of the two curves a and bshows the effect of feeding. The thick black line indicates the time when stream awas supplied with food.

During the whole of this experiment no food was put into the top 50 m. of thestream. This invalidates any suggestion that food substances in solution " attracted "the animals to the top of the stream.

With the exception of a few very recently hatched individuals, all the planariansfound at the top were well fed. One can only suppose that they had taken the foodlower down the stream and had redeveloped and, becoming positively rheotactic,were forced to migrate upstream.

128 R. S. A. BEAUCHAMP

CONCLUSIONS.

Sexual development in PL alpine is associated with low temperatures, seeSteinmann (1913) and Carpenter (1928). It is certain that sexual individuals arerarely found at temperatures above 10° C.

In cold springs where the temperature is less than io° C. sexual individuals canusually be found at any time of the year. That is to say, there is no fundamentalseasonal rhythm in the reproduction of this species. But in the winter, when thetemperature of the whole stream is lowered, sexual development is made possiblefor the whole population.

It has been shown that positive rheotaxy accompanies sexual developmentConsequently, if the planarian population of a stream develops sexually, a generalmigration upstream is bound to follow.

This migration leads to dense overcrowding at the top of the stream, with theresult that there is a shortage of food. Consequently, most of the animals whichreach the top of the stream are eventually starved. A few are able to complete theirsexual cycle and these lay cocoons and then become negatively rheotactic.

Both those that have been starved and those that have completed their sexualcycle migrate downstream.

The starved animals if they find food, soon re-develop and become positivelyrheotactic again; they then migrate upstream again and may lay cocoons or theymay again be starved.

Temperature and the amount of available food are clearly the two most im-portant factors which control the sexual development of PL alpina and its migrationsup and down the stream.

SUMMARY.

1. PI. alpina is normally active in the evening and quiescent during the day.2. PI. alpina is shown to become positively rheotactic when developing sexually,

but becomes negatively rheotactic on completing the sexual cycle.3. Starvation leads to de-differentiation and the loss of positive rheotary.4. Feeding produces a temporary positive rheotaxy and a temporary negative

geotaxy.5. Continued feeding leads, if temperature conditions are suitable, to the re-

development of those characters which produce positive rheotaxy in sexual in-dividuals.

6. The development and consequently the behaviour of PL alpina is controlledby temperature and the food supply.

7. Considerable migrations up and down the stream are brought about bychanges in the animal's behaviour.

Rheotaxis in Planaria alpina 129

REFERENCES.

AIXIN, G. D. (1915). Bid. Bull. Mar. Bid. Lab. Woods HoU, 29, m .BBAUCHAMP, R. S. A. (193a). Journ. Animal Ecology, 1, 175.BEAUCHAMP, R. S. A. and ULLTOTT, P. (193a). Journ. Ecology, 20, aoo.CAsraNTO, K. (1937). Brit. Journ. Esp. Bioi, 5, 197.

(1908). Journ. Ecology, 16, 105.DOFLMN, I. (1935). Z. wist. Bid. Abt. C, 3, 60.HUBAULT, ET. (1937). Bull. Bid. Suppl. q, 354.HTMAN, L. H. (1919). Amtr. Journ. Phyt. 49, 377.JOHNSON, J. R. (1833). Phil. Tram. Roy. Soc. 112, 437.KAFKA, G. (1914). Emfakrung in du Turptychologu auf ExptrvmtnU&Ur und Ettmologischtr Gnmd-

lag*. Leipsig, p. 151.KOBHLIR, O. (1936). Vtrhl. d. d. Zod. Gtt. *u Kid. 31, 183.

(1933). Zmt.fOr vtrgl. Phyl. 18, 605.OLMSTOAD, J. M. D. (1917). Journ. Animal Bt/taviour, 7,81.PEARL, R. (1903). Quart. Journ. Microtcop. Sci. 40, 509.STSINMANN, P. (1913). Vtr. Nat. On. Batd, 24, 136.STUNMANN, P. and BHMLAU, E. (1913). Momagrapkun mnhmmuchtr Titr; 5. Leipzig.VOIOT, W. (1904). Vtrh. not. Vtr. pram. RJmnl. Wtttf. 81, 103.

(1907). BtrkhU Vmamml. Bot. Zod. Vtr. RhtM. Wtttf. 64,93.VOLZ, W. (1900). Miami, dtr Naif. Gm. Btm, 74.Vofrra, A. D. (1938). Tijdtehr. ntdtrl. dUrk. Vtr. 1, 69.WILHKLMI, J. (1904)- Zod. Ant. 27, 355.