Behavior of the Lower Organisms

In the extended animal the peristomal cilia are in continual motion. When finely ground India ink or carmine is added to the water, the currents caused by the cilia are seen to be as follows: The mouth of the animal forms the bottom of a vortex, toward which the water above the disk descends from all sides (Fig. 109). Only the particles near Fig. 109. — Stentor r&selii, showing the currents caused by the cilia of the peristome. the axis of the vortex really strike the disk; those a little to one side shoot by the edges without touching. Particles which reach the disk pass to the left, toward the buccal pouch, following thus a spiral course. Reaching the buccal pouch, they are whirled about within it a few times ; then they either pass into the mouth, at the bottom of the pouch, or they are whirled out over the edge of the pouch, at the mid-ventral notch. In thelatter case they usually pass backward along the midventral line of the body (Fig. 109, a), till they reach the edge of the tube. To this they may cling, thus aiding to build up the tube.

When stimulated, Stentor roeselii may contract into its tube, taking then a short oblong or conical form (Fig. no). Such contractions do not as a rule take place save in response to well-marked stimuli. When not disturbed in any way, the animal remains extended, with cilia in Let us try the effect of disturbing the animal very slightly. While the disk is widely spread and the cilia are actively at work, we cause a fine current of water to act upon the disk, in the following way. A long tube is drawn to a very fine capillary point and filled with water. The capillary tip is brought near the Stentor, while the long tube is held nearly perpendicular. The pressure causes a jet of water from the tip to strike the disk of the animal. Like a flash it contracts into its tube. In about half a minute it extends again, and the cilia

rceseia contracted into resume their activity. Now we cause the current to lts tubeact again upon the disk. This time the animal does not contract, but continues its normal activities without regard to the current of water. This experiment may be repeated on other individuals; invariably they react to the current the first time, then no longer react. The same results are obtained with other fixed infusoria: Epistylis and Carchesium. By using other very faint stimuli, such as that produced by touching the surface film of the water close to the organism, or by slightly jarring the object to which it is attached, the same results are obtained. To the first stimulus they respond sharply ; to the second and following ones they do not respond at all, even if long continued.

Thus the organism becomes changed in some way after its first reaction, for to the same stimulus, under the same external conditions, it no longer reacts. What is the nature of this internal change? The first suggestion that rises to the mind in explanation of such a cessation of reaction is that it may be due to fatigue. The distinction between fatigue and other changes of condition is an important one, for the following reason. Fatigue is due to what may be called a failure. It is an imperfection inherent perhaps in the nature of the material of which organisms are composed, preventing them from doing what might be to their advantage. Changes of reaction due to other causes might on the other hand be regulatory, tending to the advantage of the organism. Higher animals often react strongly by a "start," to the first incidence of sudden harmless stimuli, then no longer react, and this cessation is evidently a regulation of behavior that is to the interest of the organism. We must then determine whether the failure of the infusorian to react to the second stimulation is due to fatigue or to some other cause.

It seems improbable that the change of behavior is due to fatigue, since the change occurs after but a single stimulation and a single reaction. It could hardly be supposed that these would fatigue the animal to such an extent as to prevent further contractions. And if we use stronger stimuli, we find that the animal continues to contract successively every time the stimulus is applied, for an hour or more. It is evident that the failure to contract after the first stimulation cannot be due to fatigue of the contractile apparatus.

If we make the stimulation somewhat stronger than in our first experiments, as may be done by touching the animal lightly with a capillary glass rod, the behavior is a little different. The animal may react the first and second times, then cease to react, or it may react half a dozen times, or more, then cease. If we continue the stimuli, we find a change in the behavior. The animal instead of contracting bends into a new position, and it may do this repeatedly. This shows that the failure to contract is not due to a failure to perceive the stimulus, — in other words, to a fatigue of the perceptive power, — for the bending into a new position shows that the stimulus is perceived, though the reaction differs from the first one.

Our results thus far show that after responding once or a few times to very weak stimulation, the organism becomes changed, so that it no longer reacts as before, and that this change is not due to fatigue, either of the contractile apparatus or of the perceptive power. The behavior may then be of the same regulatory character as is the similar behavior in higher animals. Indeed, so far as the objective evidence goes, this behavior in Stentor precisely resembles that of higher animals, and is to the same degree in the interest of the organism.

With still stronger stimulation, produced by touching the animal with the capillary glass rod, another curious phenomenon often shows itself. The animal may react to each of the first half dozen strokes, then cease to react; then after a few more strokes react again, then cease to react till a large number have been given, and so continue. A typical series, giving the number of strokes before contraction is produced, is the following, obtained from experiments with an individ-^ ual of Epistylis: —

During such experiments the organism, when it does not contract, continually changes its position, as if trying to escape the blows. The reason for the contraction at irregular intervals which become longer as the experiment continues, is not clear. Possibly fatigue may have something to do with this matter. The stimuli with which we have thus far dealt are not directly injurious, and do not interfere in the long run with the normal functions of the organism, so that the power of becoming accustomed to them and ceasing to react is useful. Let us now examine the behavior under conditions which are harmless when acting for a short time, but which, when continued, do interfere with the normal functions. Such conditions rriay be produced by bringing a large quantity of fine particles, such as India ink or carmine, by means of a capillary pipette, into the water currents which are carried to the disk of Stentor (Fig. in).

Under these circumstances the normal movements are at first not changed. The particles of carmine are taken into the pouch and into the mouth, whence they pass into the internal protoplasm. If the cloud of particles is very dense, or if it is accompanied by a slight chemical stimulus, as is usually the case with the carmine grains, this behavior lasts but a short time; then a definite reaction supervenes. The animal bends to one side — always, in the case of Stentor, toward the aboral side. It thus as a rule avoids the cloud of particles, unless the latter is very large. This simple method of reaction turns out to be more effective in getting rid of stimuli of all sorts than might be expected. If the first reaction is not successful, it is usually repeated one or more times. This reaction corresponds closely with the "avoiding reaction" of free-swimming infusoria, and like the latter, is usually accompanied by revolution on the long axis, — the animal twisting on its stalk two or three times as it bends toward the aboral side. Fig. in.— A cloud of car- If the repeated turning toward one side

currents passing to the mouth does not relieve the animal, so that the parti - of stentor. c[es 0f carmine continue to come in a dense cloud, another reaction is tried. The ciliary movement is , suddenly reversed in direction, so that the particles against the disk and in the pouch are thrown off. The water current is driven away from the disk instead of toward it. This lasts but an instant, then the current is continued in the usual way. If the particles continue to come, the reversal is repeated two or three times in rapid succession. If this fails to relieve the organism, the next reaction — contraction — usually supervenes.

Sometimes the reversal of the current takes place before the turning away described first ; it may then be followed by the turning away. But usually the two reactions are tried in the order we have given. If the Stentor does not get rid of the stimulation in either of the ways just described, it contracts into its tube. In this way it of course escapes the stimulation completely, but at the expense of suspending its activity and losing all opportunity to obtain food. The animal usually remains in the tube about half a minute, then extends. When its body has reached about two-thirds its original length, the ciliary disk begins to unfold and the cilia to act, causing currents of water to reach the disk, as before.

We have now reached a specially interesting point in the experiment. Suppose that the water currents again bring the carmine grains. The stimulus and all the external conditions are the same as they were at the beginning? Will the Stentor behave as it did at the beginning? Will it at first not react, then bend to one side, then reverse the current, then contract, passing anew through the whole series of reactions? Or shall we find that it has become changed by the experiences it has passed through, so that it will now contract again into its tube as soon as stimulated?

We find the latter to be the case. As soon as the carmine again reaches its disk, it at once contracts again. This may be repeated many times, as often as the particles come to the disk, for ten or fifteen minutes. Now the animal after each contraction stays a little longer in the tube than it did at first. Finallv it ceases to extend, but contracts repeatedly and violently while still enclosed in its tube. In this way the attachment of its foot to the object on which it is situated is broken, and the animal is free. Now it leaves its tube and swims away. In leaving the tube it may swim forward out of the anterior end of the tube ; but if this brings it into the region of the cloud of carmine, it often forces its way backward through the substance of the tube, and thus gains the outside. Here it swims away, to form a new tube elsewhere.

While swimming freely after leaving its tube, Stentor shows the characteristic behavior of the free-swimming infusoria, such as Paramecium. Upon this, therefore, we need not dwell, passing at once to the behavior in becoming reattached and forming a new tube. On coming to the surface film of the water, or the surface of solid objects, the free-swimming Stentor behaves in a peculiar way. It applies its partially unfolded disk to the surface and creeps rapidly over it, the ventral side of the body being bent over close to the surface. It may thus creep over a heap of debris, following all the irregularities of the surface rapidly and neatly, seeming to explore it

thoroughly. This may last for some time, then the animal may leave the debris and swim about again. Other heaps of debris or the surfaces of solids are explored in the same way. Finally, after ten or twenty minutes or more, one of these is selected for the formation of a new tube. It may be seen that as the Stentor moves about a viscid mucus is secreted over the surface of the body. To this mucus particles of debris stick and are trailed behind the swimming animal. In a certain region, perhaps between two masses of debris, the animal stops and begins to move backward and forward with an oscillatory motion, through a distance about two-thirds its contracted length. This movement, in precisely the same place, is kept up for about two minutes, while the mucus

from the surface is rapidly secreted. The movement compacts this mucus into a short tube or sheath, — the tube in which the Stentor is to live. The process is represented in Fig. 112. Next the tip of the foot is pressed against the debris at the bottom of the tube. There it adheres by means of Fig. 112. — Oscillating movement of nne pseudopodia sent out from the in- i-2, alternating positions, o, the secreted ternal protoplasm. Now the Stentor mucus; b, masses of debris. extends to full length, and we find it

in the usual attached condition, with the lower half of the body surrounded by a transparent tube of mucus. The Stentor has thus moved away from the place where it was subjected to the mass of carmine particles, and has established itself in another situation. The behavior just described shows clearly that the same individual does not react always in the same way to the same stimulus. The stimulus and the other external conditions remaining the same, the organism responds by a series of reactions becoming of more and more pronounced character, until by one of them it rids itself of the stimulation. Under the conditions described — when a dense cloud of carmine is added to the water — the changes in the behavior may be summed up as follows : —

(1) No reaction at first: the organism continues its normal activities for a short time. (2) Then a slight reaction by turning into a new position, — a seeming attempt to keep up the normal activities and yet get rid of the stimulation. (3) If this is unsuccessful, we have next a slight interruption of the normal activities, in a momentary reversal of the ciliary current, tending to get rid of the source of stimulation. (4) If the stimulus still persists, the animal breaks off its normal activity completely by contracting strongly — devoting itself entirely, as it were, to getting rid of the stimulation, though retaining the possibility of resuming its normal activity in the same place at any moment.

(5) Finally, if all these reactions remain ineffective, the animal not only gives up completely its usual activities, but puts in operation another set, having a much more radical effect in separating the animal from the stimulating agent. It abandons its tube, swims away, and forms another one in a situation where the stimulus does not act upon it. The behavior of Stentor under the conditions given is evidently a special form of the method of the selection of certain conditions through varied activities, — a form which we have not met before. The organism "tries" one method of action; if this fails, it tries another, till one succeeds. Like other behavior based on this method, it is not a specific reaction to any one stimulus, but is seen whenever analogous conditions are produced in any way. Thus we may use in place of carmine other substances. Chemicals of different kinds produce a similar series of reactions. A decided change in osmotic pressure has a somewhat similar effect. There are variations in the details of the reaction series under different conditions. Sometimes one step or another is omitted, or the order of the different steps is varied. But it remains true that under conditions which gradually interfere with the normal activities of the organism, the behavior consists in "trying" successively different reactions, till one is found that affords relief. The production of any given step in the behavior cannot be explained as a necessary consequence of the preceding step. On the contrary, the bringing into operation of any given step depends upon the ineffectiveness of the preceding ones in getting rid of the stimulating condition. The series may cease at any point, as soon as the stimulus disappears. Moreover, it is evident that the succeeding steps are not mere accentuations of the preceding ones, but differ completely in character from them, being based upon different methods of getting rid of the stimulation.

All our results on Stentor then show clearly that the same organism may react to the same stimulus in various different ways. It may react at first, then cease to react if the stimulus does not interfere with its normal activities; it may react at first by a very pronounced reaction (contraction), then later by a very slight reaction (bending over to one side) ; or it may respond, if the stimulus does interfere with its normal functions, by a whole series of different reactions, becoming of a more and more pronounced character. Since in each of these cases the external conditions remain throughout the same, the change in reaction must be due to a change in the organism. The organism which reacts

to the carmine grains by contracting or by leaving its tube must be different in some way from the organism which reacted to the same stimulus by bending to one side. No structural change is evident, so that all we can say is that the physiological state 0} the organism lias changed. The same organism in different physiological states reacts differently to the same stimuli. It is evident that the anatomical structure of the organism and the different physical or chemical action of the stimulating agents are not sufficient to account for the reactions. The varying physiological states of the animal are equally important factors. In Stentor we are compelled to assume at least five different physiological states to account for the five different reactions given under the same conditions. We shall later find much occasion to realize the importance of physiological states in determining behavior.

These relations may be stated from another point of view, which leads to interesting questions. The present physiological state of an organism depends upon its past history, so that we can say directly that the behavior of such an organism as Stentor under given conditions depends on its past history. This statement we know is markedly true for higher organisms. What a higher animal does under certain conditions depends upon its experience : — that is, upon its past history. In the typical and most interesting case we say that the behavior of the higher organism depends upon what it has learned by experience. Is the change in the behavior of Stentor in accordance with its past history a phenomenon in any wise similar in character to the learning of a higher organism? In judging of this question we must rely, of course, entirely upon objective evidence ; — upon what can be actually observed. When this is done, it is hard to discover any ground for making a distinction in principle between the two cases. The essential point seems to be that after experience the organism reacts in a more effective way than before. The change in reaction is regulatory, not merely haphazard. And this is as clearly the case in Stentor as in the higher organism. It is true that, so far as we can see, the behavior of Stentor shows in only a rudimentary way phenomena that become exceedingly striking and complex in higher organisms. Stentor seems to vary its behavior only in accordance with the experience that either (1) the stimulus to which a strong reaction is at first given, does not really interfere with its activities, so that reaction ceases; or (2) that the reaction already given is ineffective, since the interference with its activities continues, so that another reaction is introduced.1 If the changes in

1 It is to be noted that nothing is said in this statement as to the Stentor's perceiving these relations. The statement attempts merely a formulation of the observed facts in such a way as to bring out their relation to what we observe in higher organisms. the behavior of Stentor were not regulatory, becoming more fitted to the existing conditions, a comparison with the behavior of higher animals in learning would be out of place. But since the changes clearly are regulatory, in the one case as in the other, it would be equally out of place to deny their similarity, in this respect at least.

In another important feature the behavior of Stentor falls, so far as our present evidence goes, far below the level of that found in the learning of higher animals. The modification in the behavior induced by experience seems to last but a very short time. Immediately after reacting in one way, which proves ineffective, it reacts in another. But a short time after it apparently reacts in the same way as at first.1 As a rule, it is evidently to the interest of an organism living under such simple conditions as Stentor to return to the first method of reaction when again stimulated after a period of quiet, for as a rule this first method is effective, and it would be most unfortunate for the Stentor to proceed to the extremity of abandoning its tube without a trial of simpler reactions. But the difference between behavior which is modified only for a few moments after an experience, and that which is permanently modified, is undoubtedly important. The latter would nevertheless be developed from the former by a mere quantitative change, so that the variation in duration does not constitute a difference in essential nature.

We may sum up the results of the present section as follows : The same individual does not always behave in the same way under the same external conditions, but the behavior depends upon the physiological condition of the animal. The reaction to any given stimulus is modified by the past experience of the animal, and the modifications are regulatory, not haphazard, in character. The phenomena are thus similar to those shown in the " learning " of higher organisms, save that the modifications depend upon less complex relations and last a shorter time.

We have thus far dealt chiefly with the behavior of infusoria under experimental conditions. In experiments the conditions are usually 1 This matter cannot be considered definitely settled. It is exceedingly difficult in practice to devise and carry out experiments which shall actually determine the length of time that the modified behavior lasts. A thorough, definitely planned investigation should be directed precisely upon this point. Hodge and Aikins (1895) report that Vorticella, which at first took yeast as food, later rejected the yeast, and that for "several hours" it refused to take the yeast again. But unfortunately no further details are given. We do not know whether the Vorticella was injured and took no food at all, or what other conditions were present, so that we can build little upon this observation.

made as simple as possible. All sources of stimulation save one are excluded, in order that we may discover the precise effects of that one. In our account of Paramecium we have seen that when more than one source of stimulation is present, the behavior is determined by all the existing conditions, so that often the behavior cannot be characterized as a precise reaction to a definite stimulus. That this is true also for other infusoria we have seen in a number of instances, particularly in our account of the contact reaction. It would be possible to add many other examples to these, making a special chapter on "Reactions to Two or More Stimuli," but this would add no new principle to what we have already brought out. The general statement may be made, that to account for the way an infusorian behaves at a given time, it is as a rule not sufficient to take into account a single source of stimulation, but all the conditions must be considered.

We shall now look at certain features of the behavior of infusoria under the conditions that are supplied by the environment, in all their variety and complexity. We wish to see how the natural "wild" organism behaves. Our account cannot be exhaustive, for the natural history of the thousands of species of infusoria remains largely to be worked out. We shall merely examine certain typical features of the behavior, devoting especial attention to the food reactions.

In our chapter on the "Action System" we have seen some of the chief variations in the natural behavior of infusoria. We have there seen that the infusoria can be divided, according to their methods of life, into three main groups : those that are attached, those that creep over surfaces, and those that swim freely. The behavior in these different groups necessarily differs much. Yet, as we have seen, every possible gradation exists from one group to another, and even the same individual may at different periods represent each different group. The behavior is simplest and least varied in the free-swimming organisms ; more varied in those which habitually creep along a surface; most complex in those which live attached. The reason for this seems to be as follows : In the open water the conditions are exceedingly simple. The free-swimming organism may escape an injurious stimulus simply by swimming away. In the fixed organism, on the other hand, the conditions are more complex. At any moment both the solid and the free fluid are acting on the organism. For a fixed animal to obtain food and escape injurious conditions, varied devices are necessary. It cannot at once solve any difficulty by departing, as the free organism can. We find, then, that such fixed organisms have developed varied reaction methods (see the preceding chapter).