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 than those 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 them are that the stream-living forms are more inclined to give the positive rheotactic response than the lake-living forms. Also the stream-living forms are not always positively rheotactic, at least under experimental conditions. The earliest observation is by Johnson (182a), who records having seen a number of " planarians " migrating upstream. It seems that Johnson's observations refer 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 very weak currents (as delivered by a fine capillary tube), the form of the reaction being precisely the same as that given to other weak stimuli." Working with the same species, Allen (1915) came to the conclusion that it was positively rheotactic only in strong currents. Before considering the more recent experimental work, reference should be made to the environmental factors which control the spread of these animal*. The three European species, PI. alpma, PoL contuta and PL gonocephala, are all limited to the upper reaches of streams, since the summer temperature of the lower reaches is 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 stream should shorten or lengthen the area over which these nnimaia occur, dependent on whether the temperature of the stream is raised or lowered. The difference between the summer and winter distribution shows this to be the case (Wilhelmi, 1904 and Carpenter, 1928). Voigt (1907) showed that PI. gonocephala migrated up tributary streams, which had previously been colonised only by PI. alpma and Pol. contuta, on the warming of the water, following the cutting down of woods. The simple maintenance of position in a stream of water, quite apart from these observed 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 number of planarians (species not stated) in a large tube with two small tubes coming in at the top; one brought pure water, the other water to which bait juices had been added. When the latter tube was turned on the animals were " alarmed " and crawled up the tube. He could obtain no response when the anifnuia were tested with pure water. In 1913 Steinmann, using PL alpina, demonstrated positive rheotaxy in ordinary water. The apparatus used was a sloping glass tube. Steinmann *s conclusions were that 80 per cent of all the triclads tested were positively rheotactic. Kafka (1914) and Olmstead (1917) have shown that PL gonocephala and PL maculata are geotactic. Olmstead showed in PL metadata that the positively geotactic animal became negatively geotactic after feeding. The writer has con- firmed this observation for PL alpina. PI. alpina is strongly negatively geotactic for 6 or 7 days after feeding. In the light of this knowledge all Steinmann's experiments are open to the criticism that they measure geotaxis as well as rheotaxis, since he used 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, food substances 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 the food. Similarly in running water PL alpina could not appreciate the direction from which the food stimulus was received. Doflein's rheotactic experiments were done on a gently sloping glass plate, so of necessity there must be some confusion between geotaxis and rheotaxis. She found that 80 per cent of the PI. alpina were positively rheotactic, and considered that those which showed the negative response were either weakly or damaged. Doflein states that the rheotactile organs are situated in the head. Using a pipette, she found that the nnimaia only responded to a current of water when it was directed on to the head region. She further assumed that the receptors were situated in the pair of head tentacles. Koehler (1926) disproved this assumption by removing the head tentacles and finding that the animals responded normally. His current consisted of a stream of water on an inclined plane and also of a jet of water from a fine pipette. In 1932 he again concluded that PL alpina was strongly positively rheotactic and that the rheoceptora were distributed over the whole body. In these later experiments he used only the pipette method for making his current. Hubault (1927) experimented with PL alpina using a circular dish. He made his current by means of a jet at the side; the water was removed from the centre of the dish by a siphon. He stated that nearly 80 per cent were positively rheotactic. The current strengths used were very great judging from the fact that about 14 per cent of 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 near Oo8terbeck were decidedly negatively rheotactic. But putting food into the water made 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 no longer responded to a current of water. It is possible that these observation* are the result of unsatisfactory experimental conditions. It seems unlikely that any stream- living animal could be persistently negatively rheotactic, without sooner or later deserting its normal habitat. Summarising we may say that the majority of workers have found that a large percentage 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 has been 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 their behaviour. 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 animaia under known conditions previous to their being tested. The water in which these stock animals were kept was well aerated or else supplied with a current of water from the tap. The temperature was kept below io° C. At first some difficulty was experienced in keeping the animals in a healthy condition; later work showed that tap water was unsuitable owing to the presence of dissolved iron. In the end the most satisfactory conditions were found to be given by keeping not more than fifty animals in a shallow open basin of spring water. Under these conditions it was found unnecessary to aerate or circulate the water. A few stones were provided for the «nima1« to crawl under. The temperature was kept at a low and constant value by leaving the basin in a cellar. Owing to the slope of the ground the cellar received light from a window on one side, this ensured normal diurnal 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 to move about But, under the quiet conditions in the basin, this periodicity was lost after 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 of direct light and received only diffuse light from the interior of the room. At first all the experiments were done with animals which were collected from the top of the streams. These animal* might well be expected to be positively rheotactic. Yet in almost every case the animals moved downstream; that is to say they were negatively rheotactic. At the time it was felt that these results were the outcome 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 water from where the animals were collected. This made it necessary to devise a circulating apparatus which would give a fairly considerable and steady flow. It was essential that there should be no metal included in the apparatus which might contaminate the 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 rheotactdc experiments. From this trough the water pours into the periodic syphon B.