Rate of Movement and Rheotaxis in Planaria Alpina

In the present paper the influence of food and sexual development on the rate of movement is described and also the effect of a variety of stimuli on the rheotactic response. Together these results show how the rheotactic response is determined by the condition of the animal and the total amount of stimulation which the animal is receiving at any one time. It is shown how, under different circumstances, the animal may respond differently to the same stimulus. These results provide an explanation of the animal’s distribution and behaviour under natural conditions.

Fig. 1 (a) is a record of the rate of movement of an animal at 8·0° C. Each point on the graph represents the rate as calculated from the distance travelled in 1 min. The rate is maintained steadily at a value of approximately 4·9 cm. per min This animal was kept at 8·0° C. for a month, and on each occasion when readings were taken the rate recorded was approximately 5·0 cm. per min. (some of these readings are shown at the beginning of Fig. 2). For the rest of this experiment the temperature was kept at 8-o° C. On 2 May the animal was given a piece of Gammarus. After feeding on this the animal increased in size by about 20 per cent. This is a usual occurrence and is probably due to a considerable intake of water at the time of feeding. Two or three days after feeding animals have usually returned to their original size.

After feeding, this particular individual came to rest and remained at rest for 1 hour. It then started to move of its own accord. It moved about for 3 hours, during which time its average rate of movement was 5·4 cm. per min., which is slightly in excess of the previous speed. This slight increase in rate of movement immediately after feeding is usual and has been observed in many individuals.

After 2 days the animal was disturbed and readings during a period of 14 hours obtained. The initial rate was approximately 7·0 cm. per min. and this high rate of movement was maintained for 5 hours (see Fig. 1 (b)); after this time the rate gradually fell off during a further 7 hours to a value of 5·75 cm. per min. During this long period of activity the animal’s behaviour, apart from the greatly increased rate of movement, indicated that it was in a stimulated condition. The track made by the animal was more twisty than usual ; the animal frequently lifted its head, and on several occasions climbed from the bottom of the tray on to the surface film ; the variations in rate were also greater than normal.

Fig. 2 shows further rates recorded from this animal; the two points marked (a) and (b) were obtained from the records shown in Fig. 1,(a), (b). It will be seen that during a period of 10 days the rate of movement gradually fell from 7·0 cm. per min. to approximately the original rate. It should be noted that during all these periods of activity when the rate of movement was high the particular speed was maintained about a constant mean (as in Fig. 1 (b)).

Fig. 2 also shows the rate of movement of the animal when stimulated with Gammarus extract. The maximum rate (see Beauchamp, 1935) remains about the same throughout the period of the experiment. The response to food extract (Gammarus) 5 days after the animal had been fed, however, was not as great as it had been previously. This appears to be the result of some inhibitory process which affects the motor mechanism; application of Gammarus extract to animals 2 or 3 days after they have been fed has on several occasions caused them to come to rest.

From the above experiment it is clear that after a meal Pl. alpina behaves as though it were receiving some stimulation which causes it to move at a fast rate, to make a more winding track than usual, and to lift its head more frequently than usual. This effect of feeding is not observed immediately after feeding, but only after 1 or 2 days, consequently it seems that it is not caused by the ingestion of the food, but by the process of digestion and absorption from the gut. Further the degree to which the animal is stimulated appears to be proportional to the activity of the gut, at least this is suggested by the high rate of movement 2 days after feeding and the gradual decrease from day to day. It has been shown by Willier et al. (1925) that the process of digestion and absorption of food by Pl. maculata required about 6 days to be brought to completion, the temperature at which the experiments were conducted was not stated.

The effect of feeding on the responses of Pl. alpina to other stimuli will be discussed later when further data have been described.

For animals of a given size, sexually developed individuals appear to move faster than immature individuals, and during a single period of activity at any particular temperature the rates observed may vary considerably. This makes it impossible to determine accurately the effect of temperature or other factors on the rate of movement of these sexually developed animals, since for any given set of conditions the rates observed will vary from time to time.

Since these irregularities of speed were noticed in all the sexually mature individuals observed, it was decided to study the rate of movement of a single individual kept at a constant temperature while its development was controlled by the amount of food it was given. Pl. alpina under natural conditions is only found in a sexually mature condition at temperatures below 12° C. Consequently for these experiments the temperature was kept below 12° C. The temperature selected was 7° C. Since the experiment was bound to take a long time and entail a very large number of observations, it was considered best to study one individual very carefully rather than attempt to observe several individuals. The experiment to be described took 7 months to complete.

At first the rate of movement of an immature individual 7 mm. long was determined. Fig. 3 (a) shows the rate of movement over a period of 3 hours ; each point on the graph represents the rate as calculated from the distance travelled in 1 min. It will be seen that the rate remains at about 4·5 cm. per min. Observations were continued for 7 days and the results (see first part of Fig. 4, curve A) show that the rate remained constant for that time. The animal was then given a small piece of Gammarus ; 5 days later another observation was taken. It will be seen that the rate (5·2 cm. per min.) is higher than it was previously. The effect of feeding on the rate of movement has already been described. The rate of movement 10 days after feeding was 4·7 cm. per min., and after 14 days was down to 4·5 cm. per min. and was maintained at that value during a period of 10 days.

One meal every 6 or 8 weeks is sufficient to keep an immature animal in good condition, but is not sufficient to allow it to develop sexually. But if the animal is given enough food and the temperature is below io° C. it will develop sexually. The individual at present being considered was given another meal 24 days after the first. Six days after this second meal readings were taken of the rate of movement. As is usual after feeding the rates recorded were high, but instead of falling after a few days to a lower value which is maintained steadily, it was found that the behaviour of this animal had become exactly similar to that observed in sexually developed animals. Fig. 3 (b) shows the rates recorded at the beginning of a period of activity which lasted 8 hours. It will be seen that the rate is high and irregular. This is typical of the behaviour of sexual individuals. Records obtained for the next 3 months were similar in form to Fig. 3 (b).

The determination of average speeds from these records would be of no comparative value, as the rate of movement is so irregular and does not vary about a constant mean value, but increases to a maximum towards the middle of a period of activity and then gradually decreases. In order, therefore, to compare the rate of movement of the animal during the course of the whole experiment, it was decided to plot the maximum speed attained during each period of activity. It will be seen that at the beginning of Fig. 4, curve A, the average speed of the animal is plotted. Curve B shows the maximum speeds attained. For the three months following the second feed of Gammarus only the points indicating the maximum speeds attained during periods of activity are recorded.

Other individuals whose behaviour was similar to the behaviour of this individual after it had been fed for the second time were found on sectioning to be sexually developed, therefore it was presumed that this individual was developed sexually. Further its general appearance suggested that of a sexually developed animal.

It is not always possible to determine from a superficial examination whether an individual is sexually developed or not, but a sexual animal is usually relatively broader than an immature individual ; also there is usually a slight indication of the presence of the genitalia just posterior to the pharynx.

After 3 months there was a change in the behaviour of this animal. The rates recorded became much lower than previously, but remained very irregular until towards the end of March (see Fig. 4 (b)). Following this period was a period during which the animal was extremely inactive. Even when disturbed in the usual way it was not possible to cause the animal to move.

When a mature individual of Pl. alpina is starved the gonads and genitalia are gradually absorbed and the animal reverts to an immature condition. At the time when this particular individual became inactive, it had been without food for 3 months. It is reasonable to suppose that this change in behaviour was associated with the process of tissue de-differentiation. At the end of this period of inactivity the animal became slightly more active, and it was possible to get occasional records of its rate of movement. It was now found that the rate was slower and that the irregularities of speed were much less. After another month the maximum rates recorded were in the neighbourhood of 4·25 cm. per min. and throughout the period of activity the rate remained fairly constant, so that it was again possible to estimate the animal’s average speed. This average speed was 3·8 cm. per min. The animal now behaved like a healthy immature individual. At the beginning of the experiment the average rate of movement was 4·5 cm. per min.

During the whole course of this experiment there was only a little change in the length of the animal. As a result of $512$ months without food, the length of the animal was reduced from 7 to 6·25 mm. Immature individuals of the same size after a similar period without food have decreased in length to about 5·5 mm. From this one must assume that the development of gonads and genitalia during a period when the animal has plenty of food provide a reserve, in addition to the normal ones, on which the animal can subsist for a considerable length of time.

During this investigation the rate of movement of the animal when fully stimulated was also observed (see Fig. 4 (c)). In an earlier paper (Beauchamp, 1935) it was shown that at any given temperature the rate of movement of an individual could be increased to a certain maximal value. It was found that the most effective stimulus in causing this maximal response was a weak extract of Gammarus. The same stimulus was used in the experiments at present being described. Before the first meal of Gammarus the maximum speed of the individual was 7·5 cm. per min., and after the second meal the maximum speed was almost the same as previously (see Fig. 4, curve C). In an earlier paper (Beauchamp, 1935) it was shown that the maximum speed bore a definite relationship to the normal speed. This relationship varies somewhat with temperature. Here it will be seen that the normal speed is approximately 60 per cent of the maximum speed. In the earlier experiments the same relationship was found between the maximum and normal speed at a temperature of 7° C. It will be seen that during the period of 3 months when the animal was developed sexually the maximum speed was about 8 cm. per min. This is only a slight increase in speed over that of 7·5 cm. per min. as recorded for the stimulated animal before feeding. From this it is clear that though the behaviour of the animal when unstimulated altered considerably, the change in behaviour when fully stimulated altered only slightly. This slight increase in the speed of the animal when fully stimulated may be the result of a slight increase in size.

Since it has been found that the maximum rate of movement bears a definite relationship to the normal rate of movement, it is possible to calculate from the maximum speed (8 cm. per min.) what would be the normal speed for this animal if it had not developed sexually. This estimated value is 4·8 cm. per min., and is only slightly in excess of the animal’s previous normal speed when immature. This estimated value is indicated in Fig. 4. From this it will be seen that during the sexual period the animal moved at a speed considerably in excess of the estimated normal speed of an animal which is not sexually developed, and at a speed approaching that of a fully stimulated animal. In short the animal behaved as though it were receiving some form of continuous stimulation. Further evidence for this has been obtained from a study of the animal’s behaviour in a current of water.

It was noticed that the response by mature individuals following stimulation with Gammarus extract was somewhat different in form to that shown by immature individuals. The stimulus used was in all cases the same, namely, 0·3 c.c. of an extract made from one Gammarus and diluted to 100 c.c. In an immature animal this stimulus causes the animal to show an increase in rate of change of direction and an increase in rate of movement. This increased rate of movement reaches a maximal value almost at once and is gradually reduced during a period of from 20 to 40 min. to the normal value. In the case of the sexually developed animal, however, it took from 2 to 3 hours before the rate of movement fell to the rate at which the animal had previously been moving. From this it appears that the general sensitivity of the sexual animal is higher than that of the immature individual, and that conditions of central excitation are maintained longer in the sexually developed individuals than in the immature individuals. This agrees with the earlier observations that the mature individual behaves as though it were receiving some form of continuous stimulation. Coincident with sexual maturity there appears to arise a condition of central facilitation as indicated by the increased rate of movement and by the prolonged response to Gammarus extract. If this condition exists, an increased responsiveness to other forms of external stimulation should also be observed. This has been shown to be so with regard to the rheotactic responses of the animal.

In an earlier paper (Beauchamp, 1933) it was shown how, under natural conditions, Pl. alpina migrates towards the head of the streams in the winter. This movement was shown to be associated with sexual development. The effect of feeding on this migration was also indicated.

The present paper describes the effect of various stimuli on the rheotactic responses, and outlines the mechanism which causes both positive and negative rheotaxy. (The apparatus used for the study of rheotactic responses was described in Beauchamp, 1933.)

It has been found that, in a weak current of water (speed less than 3 metres per min.), immature individuals are usually indifferent to the current, sometimes going upstream, sometimes down or across. If the direction of the current is reversed no corresponding change in the orientation of the animal may occur. If, however, the strength of the current is increased, such immature individuals nearly always react positively, and move up the current. This indicates that immature individuals are able to orientate themselves to the current. They are however very unresponsive in comparison with sexually developed animals. A sexually developed animal will react to extremely weak currents, whereas immature individuals react only to currents flowing at a speed of about 4 metres a minute or over.

If, however, an immature individual in a weak current is momentarily stimulated by a dilute solution of food extract (Gammarus) it responds by becoming positively rheotactic and for 40–60 min. will continue to respond by moving upstream. The food extract is applied in the neighbourhood of the animal’s head, and consists of 0·3 c.c. of a solution made from one Gammarus diluted to 100 c.c. The stimulus from the food extract can only last for a few seconds, but the animal is instantly aroused and appears excited and reacts positively to the current. The rate of movement is increased which agrees with the experiments done on the effect of stimulation by food extract on the rate of movement of comparable animals in still water (Beauchamp, 1935). In still water it was found that the rate of movement fell to the original value after an hour; for a similar period stimulated animals react positively to the current. The production of rheotactic orientation by the application of a non-orientating stimulus suggested the idea that any stimulus by exciting the central nervous system might facilitate centrally the paths concerned with the translation of sensory stimulation by a current of water into the rheotactic response. Experiments showed that the application of a large number of weak chemical solutions (1/100 per cent hydrochloric acid, 1/5 per cent citric acid, 1/10 per cent sodium bicarbonate, 1/10,000 per cent ammonium hydroxide, 1/200 per cent carbonic acid) caused immature individuals to respond positively to a weak current. Water at a temperature five degrees higher than the water in which the animal was and a weak intermittent light have the same effect.

A sexually developed individual normally reacts positively to a current, and, as has been shown above, the animal’s rate of movement is faster and more irregular than that of an immature individual, and its general behaviour is similar to that of a slightly stimulated animal. If we assume that the developed sexual apparatus produces an effect, comparable with that of external stimulation, on the central nervous system, we have an explanation of the positive rheotaxy of sexual individuals, and can bring the behaviour of these individuals into line with that of immature individuals which are receiving some form of external non-orientating stimulus.

Immature animals which have been fed are positively rheotactic for about 10 days. Ten days is the period during which the rate of movement is above the normal, which supports the view that this positive response is brought about in a way similar to that in sexual individuals; namely, by facilitation through the central nervous system.

In an earlier paper (Beauchamp, 1935) it was shown that, at temperatures above 12⁰ C., some inhibitory mechanism was damaged, with the result that the animal’s rate of movement increased. The longer the animal was kept at the particular temperature, the greater the rate of movement became until it had reached the fully stimulated rate. After that the rate of movement decreased and the animal might eventually die. Above 18° C. the initial rapid rate of movement was followed by a decline in speed, a decrease in the length of the period of activity, and ultimately by the death of the animal.

Experiments showed that after a period at temperatures above 12° C. (the higher the temperature the shorter the period) all individuals become positively rheotactic. This fact provides the explanation for the distribution of this animal under natural conditions. Pl. alpina lives in cold spring-fed streams and only extends to a point where the summer temperature of the water is 15° C. It will be seen that animals which wander downstream and enter the water at a temperature above 12° C. will soon become positively rheotactic and start migrating upstream.

It was observed (Beauchamp, 1933) that following strong mechanical stimulation (e.g. pipetting) animals were negatively rheotactic, that is to say, they went downstream. This response was quite as definite as the positive response, for each time the current was reversed the animals re-orientated themselves.

In these earlier experiments it was also noticed that in the original troughs which had not been suitably painted and in glass troughs when tap water was used the animals were negatively rheotactic. That is to say, certain chemical substances in solution changed the behaviour of these animals and caused consistent negative rheotaxy.

It has already been shown above that weak acids and alkalies induce positive rheotaxy; all the previously mentioned chemicals at slightly higher concentrations (1/10 per cent hydrochloric acid, 1–2 per cent citric acid, $12$ per cent sodium bicarbonate, 1/2000 per cent ammonium hydroxide, 1/50 per cent carbonic acid) cause negative rheotaxy.

These experiments with stronger chemical solutions suggest that it is merely the degree of excitation produced which causes the negative response and not the kind of stimulation.

One form of stimulation, however, was found not to produce negative rheotaxy. Stimulation with water at a temperature 5° C. above that at which the animal was living was shown to cause positive rheotaxy; it was expected that stimulation by water with a higher temperature would cause negative rheotaxy. This however did not occur; water at 15 and even 20 degrees higher did not cause negative rheotaxy, and after a slight indefinite reaction the animals continued to move upstream. It would appear from this that Pl. alpina is very insensitive to temperature changes. This statement does not of course apply to the effect of temperature on the general metabolism of the animal, but merely refers to its sensory appreciation.

In order to find out whether the facilitation of the positive response or the production of the negative was determined solely by the degree of central excitation, experiments were made, combining various stimuli, which alone induced positive rheotaxy, to see whether, together, they produced the negative response.

Individuals were stimulated with an intermittent light strong enough to induce positive rheotaxy, but not strong enough to induce negative rheotaxy. Weak chemical solutions, as above, were then added; alone these solutions would have facilitated the positive response, but when applied together with the flickering light they caused negative rheotaxy. It had been felt that possibly stimulation by food extract would be a special case, and that the response to it might be so specific as not to fall in line with other forms of stimulation ; but it was found that application of Gammarus extract at the same time as the animal was being stimulated by a flickering light also produced negative rheotaxy.

Once negative rheotaxy has been induced nothing will call forth the positive response. Only after the animal has remained undisturbed for several hours or been at rest for a period will it again respond positively.

An attempt has also been made to locate the position occupied by the chemoreceptors. From a histological examination it was seen that a very large number of sensory cells were situated along the anterior border of the head. It was suspected that the cells were chemo-receptive. The anterior border was removed from a number of individuals; in all cases the animals could react normally to a current but were indifferent or extremely irresponsive to chemical stimulation. In one experiment fourteen animals were in the trough, seven of which had the anterior border removed. When all fourteen were moving upstream sufficient ammonia was added to the water to bring about the usual negatively rheotactic response. This response was only given by the seven undamaged individuals, the other seven continued upstream and only after several minutes did they show any reaction to the ammonia.

From the above data one may conclude that all individuals of Pl. alpina are capable of orientating themselves to a current of water, or in other words they possess a sensory mechanism which when suitably stimulated will bring about orientation. The behaviour of immature individuals in a weak current shows that this mechanism is not always effectively stimulated. But if the current strength is increased they orientate themselves in such a way as to move upstream, that is to say, the first response once the mechanism is stimulated above its threshold value is positive.

In immature individuals positive rheotaxy to a weak current follows stimulation by dilute solutions of chemical substances, light of low intensity or a momentary temperature change, and continues for a time similar in length to the time during which in still water the animal would have shown an increased rate of movement. In order to produce a change in the rate of movement these weak stimuli must have affected the condition of the central nervous system and produced a state of greater excitation. This central excitatory state, one must presume, facilitates the passage of stimuli received from the rheo-receptors, and enables individuals to respond to a weak current to which previously they were unable to respond.

In a similar way it appears that the processes of digestion and assimilation and the sexual condition act as a mild stimulus facilitating rheotactic responses.

At temperatures above 12° C. the inhibitory mechanism controlling the rate of movement breaks down, and presumably makes easier the passage of nervous impulses with the result that all individuals become positively rheotactic.

Negative rheotaxy appears as the result of excessive stimulation, and is induced by the addition of strong stimuli or the combined effect of stimuli which alone would have facilitated the positive response. This negative response can be explained if one imagines that when the animal is orientated so as to move downstream, the rheo-receptors are less stimulated than when the animal is moving upstream. Thus an individual which has been over-stimulated avoids those positions which lead to further stimulation and becomes orientated so as to move downstream.

At temperatures above 12° C. it is more difficult to induce negative rheotaxy than at lower temperatures. There appears to be no obvious explanation for this fact.

1. The rate of movement in Pl. alpina is increased during a period of about io days after feeding.

2. Sexual individuals move at a faster and more irregular rate than do immature individuals.

3. Immature individuals do not respond to a weak current but are positively rheotactic to strong currents. Various weak non-orientating stimuli facilitate the positively rheotactic response of these animals.

4. Sensory appreciation of temperature differences is slight.

5. After feeding, immature individuals respond positively to a weak current.

6. Sexual individuals respond positively to a weak current.

7. Above 12° C. Pl. alpina responds positively to weak currents, this fact explains their distribution under natural conditions.

8. Strong stimuli or combined weak stimuli cause negative rheotaxy.

9. After negative rheotaxy has been induced the positive response only occurs after a period of rest or after a period of time long enough for the excitatory state of the central nervous system to have become reduced.

10. The nature of the rheotactic response depends on the excitatory state of the central nervous system, and under different circumstances the same individuals will respond differently to the same stimulus.