Behavior of the Lower Organisms

1 A simple apparatus of this sort is described and figured in Jennings, 1904. Mendelssohn found that the optimum temperature for Paramecium lies, under ordinary conditions, between 24 and 28 degrees C, and that when there is a difference of but 3 degrees C. between the two ends of a trough 10 centimeters in length, the Paramecia gather at the end of the trough nearest the optimum (see Fig. 54). If the end a has a temperature of 26 degrees, the end b 38 degrees, the Paramecia gather at the end a; if now the temperature of the two ends is interchanged, the Paramecia travel from a toward b, and collect there. The same results are produced if one end has a temperature of 10

degrees, the Other of Fig. 54. — Reactions of Paramecia to heat and cold, after 26 de°TeeS save that Mendelssohn (1902). At a the infusoria are placed in a trough, both ends of which have a temperature of 19 degrees. They are equally scattered. At b the temperature of one end is raised to 38 degrees while the other is only 26 degrees. The infusoria collect at the end having the lower temperature. At c one end has a temperature of 2 5 degrees, while the other is lowered to 1 o degrees. If Para- '^^le arumals now collect at the end having the higher temperature.

in this case the Paramecia gather at the end having the higher temperature, mecia are kept for some hours at a temperature of 36 to 38 degrees, the optimum becomes higher, — about 30 to 32 degrees; otherwise the phenomena remain the same. Observation of the movement of the individuals shows that the reactions in these experiments take place in the following manner. As one end of the trough is heated above the optimum, the Paramecia in that region are seen to become more active, darting about rapidly in all directions. Those that come against the sides or end of the vessel respond by the avoiding reaction ; they are thus directed elsewhere. Individuals that are swimming toward the hotter region likewise give the avoiding reaction, — at first in but a slightly marked form, stopping, swinging the anterior end about in a circle, as illustrated in Figs. 37-39, and "trying" forward movement in a number of different directions. This continues as long as they are moving toward the warmer region; but as soon as their direction of movement leads them toward the cooler region,

the avoiding reaction ceases, and they continue to swim in that direction. At that end of the trough which is cooled below the optimum, similar effects are produced, save that the reaction is less rapid, and the Paramecia therefore leave this region much more slowly than they do the heated end. Thus after a time the direction of movement of all the individuals in the hot or cold end of the trough has become changed, and all are moving, often in a well-defined group, toward the optimum region. Thus we may observe in these temperature reactions a well-defined common orientation of a large number of organisms ; all are headed toward the optimum. This orientation is brought about, as we have seen, by exclusion. That is, movement in any other direction is stopped, through the production of the avoiding reaction, so that all finally travel in this one direction. Or, to put it more accurately, the Paramecia try every possible direction, through the avoiding reaction (Figs. 37-39), till finally they all find the only one which does not cause stimulation; in this direction they continue to move. The method of reaction, by systematic trial of all directions, is such as to find any existing avenue of escape, no matter how narrow it may be.

To ordinary visible light Paramecium is not known to react in any way. If light is allowed to fall on the animals from one side only, or if one portion of the vessel containing them is strongly righted while the rest is shaded, this has no observable effect on their movements or distribution. But Hertel (1904) has recently shown that to powerful ultra violet light Paramecium does react. The ultra violet rays employed by Hertel came from a magnesium spectrum ; they were of a wave length of 280 fifx. When part of a drop of water containing Paramecia was subjected to this light, the animals in the lighted region at once began to move about rapidly. They therefore passed quickly into the region not lighted. Specimens moving about in this shaded region stopped at once on reaching the boundary of the lighted area, and turned away. It is evident that the reaction to light is by the usual avoiding reaction, though the details of the movement were not observed by Hertel.

When the animals were unable to escape from the light, their movement became uncoordinated, and in ten to fifty seconds it ceased. The animals were dead. In the reactions which we have thus far considered, the infusoria do not become oriented in any precise way with relation to the direction of action of the stimulating agent. But to water currents, to gravity, and to centrifugal force the animals at times react in such a way as to bring about a definite orientation, with the body axis of all the reacting individuals in line with the external force. In a water current the anterior end is directed up stream ; under the influence of gravity the anterior end is directed upward, while when subjected to a centrifugal force the anterior end is directed against the action of the force.

How are these results produced, and why do the organisms take a definite axial orientation under the action of these stimuli, while they do not under most other stimuli ? In the reactions to water currents and to gravity, direct observation has shown that the orientation is produced through the movements which we have called the avoiding reaction. Under the action of a centrifugal force, observation of individuals is impossible, but beyond doubt the reaction is the same as that due to gravity.

The reactions to water currents can best be studied in a tube like that shown in Fig. 55. By covering the two open ends with rubber caps filled with air, and pressing on these, the water containing the animals in the tube Fig. 55. — Tube used in studying the reactions to water curthe narrow part of the tube with any desired velocity. With a certain velocity of current most of the individuals, both those that are free swimming and those that are resting against the glass, are seen to place themselves in line with the current, with anterior end up stream. Some of the individuals usually do not react. In those that do, the reaction is brought as follows : As soon as the current begins to act, producing a disturbance in the water, the animals give the avoiding reaction in a not very pronounced form. That is, a given individual swims more slowly or stops, and swerves more strongly toward the aboral side, thus swinging the anterior end about in a circle, as in Figs. 37 and 38, "trying" various directions. It then starts forward again in one of these directions. This reaction may be repeated several times, till the infusorian finally comes into a position with anterior

end directed up stream. The reaction then ceases, and the infusorian remains in this position, either swimming forward against the current, or at rest against the wall of the tube. Sometimes the reaction is a little more precise, the animal turning directly toward the aboral side till the anterior end is directed up stream. This is commonly the case with the individuals that are at rest against a solid. The reaction of the resting specimens is less easily observed, for the current easily carries them away from their attachment, when of course they behave like other free specimens.

What is the cause of the reaction to water currents ? Under natural conditions the cilia of Paramecium are beating backward, driving a current of water backward over the surface, especially in the oral groove. If an external current moves in the opposite direction, or in some oblique direction, it will of course act in opposition to the cilia on that part of the body which it strikes, tending to reverse or disarrange them, and to reverse or change the direction of the usual currents. It appears not surprising that such a disturbance acts as a stimulus, causing the usual avoiding reaction until the disturbance is corrected. The correction can occur only when the animal is headed up stream ; the current is then passing backward over the body in the usual direction. The reaction is essentially a response to a mechanical disturbance, comparable to that due to the touch of a solid body.

If this is the correct explanation, as seems probable, then there should be no reaction when the animal is completely immersed in a homogeneous current, — one moving at the same velocity in all parts. For as Lyon (1904) has pointed out, under these circumstances the animal is merely transferred bodily in a certain direction, along with the medium surrounding it, and at the same rate. Its relation to the enveloping fluid is the same as in quiet water; there is nothing to cause a disturbance. "Stimulation implies a change of relation between organism and environment. But if both in all their parts are moving at the same velocity, their relations do not change, and the conditions for stimulation are wanting" (Lyon, 1904, p. 150). ! The animal should then react only when either it is in contact on one side with a solid, or when the current is moving more rapidly on one side than on the other, producing a shearing effect, with the necessarily accompanying disturbing action. Whether this is true or not is very difficult to determine, but observation seems to indicate that it is.

1 This consideration, as well as the fact that individuals resting against a surface react to the current, shows the incorrectness of the theory put forward by the present author (1904 Ji), in which stimulation was supposed to be due to the variations in pressure produced through the varied movements of the animal in its spiral course. Certain authors (Dale, 1901, Statkewitsch, 1903 a) have reported that Paramecia sometimes swim with the current. But in these cases irregular currents have been used, such as are produced by stirring the water containing the animals. Using a tube, the present author has found the results to be practically uniform, the animals swimming up stream. If the reverse reaction actually occurs at times, it must be due to some change of internal condition, such as results in swimming backward under certain circumstances ; the direction of the current over the body would be the same in the two cases.

If the explanation of the reaction to water currents above given is correct, this reaction is clearly analogous to the compensatory movements of higher animals, as Lyon (1904) has brought out for other organisms. It is a response to unusual relations with the environment, and tends to restore the usual relations. In the reaction to gravity the animals place themselves with anterior end directed upward, and as a result swim to the top of the vessel containing them, forming a collection there (Fig. 56). If the tube is inverted after the collection is formed, so that the infusoria are now at the bottom, they again direct the anterior end upward, and swim to the top. These results follow in the same way whether the upper end of the tube is open or closed, and they take place equally well when the

To determine the way in which the reaction occurs, it is necessary to direct the lenses of a microscope of long focus upon a region where the animals are taking up the position with long axis in the direction of gravity. Fig. 57. — Tube used in observing the way in which Paramecium reacts to gravity. This may best be lected at the top accomplished by placing the ° animals in a U-shaped tube, sen (1893). at first with the free ends upward. After the animals have become grouped at the two free ends, the tube is inverted (Fig. 57). The Paramecia now move upward, reach the cross-piece of the U, and

move across it to the opposite side. Reaching this, they at first continue the course by swimming obliquely downward, to the point x. Here the reaction occurs; the animals turn around and swim upward again. Studying the movements of the Paramecia at this point, one observes that the forward motion becomes slower, while the spiral course becomes wider. The animals swerve more strongly than usual toward the aboral side, so that the anterior end swings about in a circle, as in Figs. 37 and 38. Thus the animals are giving the avoiding reaction, "trying" successively many different positions. This is continued or repeated till after a time they come into a position with anterior end upward. The strong swerving then ceases; the animals swim upward in the usual spiral course.

The position of individuals at rest against a solid is usually quite independent of gravity. The body axis may be placed at any angle with the pull of gravity, with either end higher. The contact reaction interferes with the reaction to gravity, preventing it almost completely. Yet there is a tendency, even when in contact with a solid, to take a position with anterior end above. If Paramecia are placed in clean water in a clean, upright glass tube, in the course of time many individuals come to rest against the perpendicular walls. It will now be found, in some cases, that a considerable portion of the animals, though by no means all, are resting with the body axis nearly in line with gravity and with anterior end upward. When a swimming individual places itself in contact with the wall, it is often seen to make a sudden turn toward the aboral side, just as it comes to rest, till the anterior end is upward ; then it remains in that position. The proportion thus oriented with reference to gravity is in some cultures sufficiently great, amounting perhaps to half the individuals, to show that the position is not accidental. In other cultures there may be almost no indication of any influence of gravity on the position of the attached specimens.

The precise nature of the determining factor in the reaction to gravity is very obscure. Jensen (1893) held that the reaction is due to the difference in pressure between the upper and lower portions of the organism. The cilia on the side where the pressure was greatest (the lower side) were supposed to beat more rapidly, thus turning the animal directly upward. But, as we have seen above, exact observation of the movements of the individuals shows that the reaction does not take place in this way. Moreover, the difference in pressure between the two sides of the organism is in certain reacting infusoria only one millionth of the total pressure, and this difference seems beyond question too slight to act as an effective stimulus.

Davenport (1897, p. 122) held that the reaction to gravity is due to the fact that the resistance in moving upward is greater than the resistance in moving downward, owing to the fact that the animal is heavier than water. To the changes in resistance as it swims up or down, the animal reacts. This view was accepted and elaborated by the author of the present work (Jennings, 1904 h). But to this can be made an objection analogous to that which is fatal to the corresponding view for the reaction to water currents. Under the uniform action of gravity, as Radl (1903, p. 139) has pointed out, it is not apparent how any such difference of resistance could be perceived by the organism. The animal would, with the same action of the cilia, and overcoming the same resistance, move somewhat more rapidly downward than upward. But it is very questionable if this slight comparative difference in rate could be perceived by the organism, — though this is of course not impossible. In any case, the fact that resting individuals may react to gravity appears fatal to the view at present under consideration.

The view having the greatest probability is perhaps that suggested by Lyon (1905). The animal contains substances of differing specific gravity; this Lyon has demonstrated. The distribution of these substances must change with the various positions taken by the animal. When the anterior end is directed downward the redistribution of internal substances thus induced acts as a stimulus, causing the usual reaction. The animal "tries" new positions till it reaches one with anterior end upward; then the reaction ceases and the animal remains in the position so reached.

Whatever the cause for the reaction to gravity, the stimulation it induces is evidently very slight, and its effect is easily annulled by the action of other agents. As we have seen, the contact reaction usually prevents the reaction to gravity. The same is true of most other stimulating agents. Almost any other stimulus that may be present produces its usual effect without interference from gravity, so that the reaction to gravity is seen clearly only in the absence of most other stimuli. Thus, if the walls of the vessel containing the animals are not clean, or if the water contains many solid particles in suspension, often no reaction to gravity can be observed.

Furthermore, the reaction to gravity becomes reversed under certain conditions. Sometimes nearly all the individuals in a given culture swim downward instead of upward. This result may be produced in cultures having originally the more usual upward tendency, in a number of different ways (Sosnowski, 1899 ; Moore, 1903). These will be mentioned in our section on reactions to two or more stimuli. Conditions similar to those due to gravity may be produced by a centrifugal force, and Paramecia then react, as might be expected, in the same way as to gravity. Jensen (1893) shows that if a tube containing Paramecia is placed in a horizontal position on a centrifuge and whirled at a certain rate, the infusoria tend to swim toward that end of the tube next to the centre. In a tube 12 cm. long, with the inner end 2 cm. from the centre, the phenomena were well shown when the tube was whirled at the rate of four turns per second, for ten or fifteen minutes. In such a tube the Paramecia at the outer end, where the movement is fastest, are carried by the centrifugal force, against their active efforts, to the outer end of the tube ; this is of course a purely passive phenomenon. The remainder of the Paramecia swim toward the end of the tube next the centre and collect there ; this is the active part of the reaction.

This movement toward the inner end of the tube is doubtless due to the same causes, whatever they may be, that produce the upward movement in the reaction to gravity. Lyon (1905) has shown that the body contains substances of varying specific gravity, some of which collect, under strong centrifugation, at that end of the animal which is at the outer end of the tube. This redistribution is probably the cause of the reaction to centrifugal force. If the passage of such substances into the anterior end should act as a stimulus to the usual reaction, this would produce the results actually observed.

We are now in a position to define the difference between these orientation reactions and the others that we have described, and to see why the result of the avoiding reaction is to produce a certain position of the body axis in one set of cases, while it does not in the others. In the reactions to mechanical stimuli, chemicals, osmotic pressure, heat and cold, and powerful light, the avoiding reaction is caused by the transition from one external condition to another; by a change in the intensity of action of some agent, — the change being of such a character as to lead away from the optimum. As a result, the organism tries repeated different directions of movement (in the avoiding reaction) till it hits upon one in which the transition is toward the optimum instead of away from it ; in this direction it continues. This does not require the body axis to take any definite orientation, since as a rule there are various directions in which the animal can move and be on the whole approaching the optimum. Furthermore, the body axis might be in any position, provided the movement were on the whole toward the optimum.

But in the reactions to water currents, gravity and centrifugal force, it is a certain position of the body that results in stimulation ; displacement of the cilia, or of certain internal constituents, occur in certain positions of the body, causing disturbances to which the animal reacts, as usual, by the avoiding reaction. This reaction consists in successively "trying," not only different directions of locomotion, but also different positions of the body axis, as a glance at Figs. 37-39 will show. As soon therefore as a position is reached in which the disturbance causing the reaction no longer exists, the reaction of course stops ; the animal therefore retains this axial position.1

A comparison of the reactions to these two sets of agents brings out strongly the general adaptiveness and effectiveness of the reaction method of the infusorian. The avoiding reaction is of such a character as to bring about in a systematic way (1) different directions of movement; (2) different axial positions; (3) different environmental conditions (of temperature, chemicals, etc.). If any one of these puts an end to the disturbance which caused stimulation, the reaction of course stops at that point, and the animal retains the direction of movement, axial orientation, or environmental condition thus reached. If a certain axial orientation must be reached before the stimulating disturbance ceases, then the result of the reaction will be to produce this orientation. If the disturbance ceases before a common orientation of all the individuals is reached, then no common orientation will occur. In other words, the method of reaction is such as to bring about any condition whatsoever that is required in order to put an end to stimulation, — provided of course that this condition is attainable. It will therefore produce in some cases a certain direction of movement, in other cases a certain axial orientation, in other cases the retention of a certain environmental condition, just as circumstances may require.

1 It is worthy of note that the position of orientation is not one in which a median plane of symmetry takes up a definite position with reference to the external agent, as is sometimes set forth. The infusorian when oriented continues to revolve on its long axis, so that no more can be maintained than that the longitudinal axis (in reality the axis of the spiral path) is in line with the orienting force. BEHAVIOR OF PARAMECIUM {Continued) Reactions to Electricity and Special Reactions

The reactions of Paramecia to electricity are more complex than those to other stimuli. This is owing to certain factors peculiar to the action of the electric current, which interfere with the usual reaction method. The gross features in the behavior under the action of electricity may be seen as follows. The Paramecia are placed in a watch-glass or other small vessel, and through the water containing them an electric current is passed (Fig. 58, A). Unpolarizable electrodes should be used, though the gross features in the reaction may be observed with platinum electrodes. A current such as is produced by six or eight chromic acid cells is needed. As soon as the current begins to pass, all the Paramecia swims toward the cathode or

Fig. 58. — A, General appearance of Paramecia reacting to the electric current. After Verworn (1899). The current is passed by means of unpolarizable brush electrodes through a cell with porous walls. The infusoria have gathered at the cathodic side. B, Magnified view of a portion of the swarm as it moves toward the cathode. After Verworn. negative electrode. The swarm of infusoria all moving in the same direction present a most striking appearance (Fig. 58, B). If while all are swimming toward the cathode the direction of the current is reversed, the Paramecia at once turn around and swim toward the

new cathode. If the electrodes are small points, the Paramecia swim in curves, such as are known to be formed by the current (Fig. 59). If while all are moving toward the cathode the current is interrupted, the group breaks up and the Paramecia scatter in all directions. not react snarply, only a ICW pointed electrodes are used. B, Collection of Paraof them swimmin0" toward mecia behind the cathode, when the electrodes are the cathode. When the P^ed close together. After Verworn (,899).

strength of the current is increased, more of the animals react and the movement is more rapid, till at a certain strength of current practically all are swimming rapidly to the cathode. With a further increase in the current, the rate of progression toward the cathode becomes slower. As the increase continues, the rate of swimming decreases till progress nearly or quite ceases. The animals now remain in position, with anterior ends directed toward the cathode, but not moving in either direction. Increasing the current still farther, the animals begin to swim backward toward the anode. At this time each Paramecium is seen to have become deformed, being short and thick. If the current is farther increased, the animals burst at one end and go to pieces. These remarkable phenomena were first observed by Verworn (1889 a). How is this striking behavior brought about? Why do the Paramecia first all go to the cathode, then in a stronger current stop, then swim backward to the anode ?