Behavior of the Lower Organisms

Is this what happens? Let us examine the facts as set forth in Romanes' own words: "Although in the experiment just described the manubrium is no longer able to localize the seat of stimulation in the bell, it nevertheless continues able to perceive, so to speak, that stimulation is being applied in the bell somewhere, for every time any portion of tissue below the cut a is irritated, the manubrium actively dodges about from one part of the bell to another, applying its extremity now to this place and now to that one, as if seeking in vain for the offending body. If the stimulation is persistent, the manubrium will every now and then pause for a few seconds, as if trying to decide from which direction the stimulus is proceeding, and will then suddenly move over and apply its extremity, perhaps to the point that is opposite the one which it is endeavoring to find. It will then suddenly leave this point and try another, and so on, as long as the stimulation is continued" (Romanes, 1885, p. 112-113).

From Romanes' description it is evident that the manubrium under these circumstances may not even move in the general direction of the point stimulated ; he says expressly that it may move toward the opposite point, or toward any other point. At times, he says, a manubrium moves from point to point, "without being able in the least degree to localize the seat of irritation." The considerations adduced by Loeb do not explain these facts; and his theory is quite inadequate to account for the behavior. Contraction occurs, not merely as a direct spreading from the point stimulated, but now in one place, now in another,

including even a region directly opposite that stimulated. The manubrium, having reacted once, does not cease, but in some way recognizes its failure and tries again. In other words, failure changes its physiological state, so that now it bends in a new direction. The whole account given by Romanes is as vivid a description of the method of reaction by the production of varied movements subjecting the organism successively to different conditions, as it would be possible to imagine under these circumstances.

It would be most interesting to determine whether the animal may thus by trial finally discover the irritated spot, and later through repetition come to bend toward it directly, as it did before the cut was made. In some sea anemones the presence of masses of waste matter on the disk leads to the performance of activities which result in the removal of the waste matter; this behavior we may call the rejecting reaction. Such behavior is well seen in the large sea anemone Stoichactis helianthus, found in the West Indies. This animal has a flat or concave disk 10 to 15 cm. in diameter, covered closely with tentacles about 8 mm. in length. If a quantity of dead plankton, or a mass of sand, or other waste matter, is placed on the disk, the animal sets in operation measures which remove it. Food placed on the disk of a specimen that is not hungry produces the same result. The behavior under such circumstances is complex, and the removal of the waste matter may be accomplished in more than one way.

The tentacles of that region of the disk bearing the waste body collapse, becoming thin and slender and lying flat against the disk. The disk surface in this region begins to stretch, separating the collapsed tentacles widely. As a result the waste mass is left on a smooth, exposed surface, the tentacles here having practically disappeared, while elsewhere they form a close investment. Thus the waste is left fully exposed to the action of the waves or currents, and the slightest disturbance in the water washes it off. Under natural conditions this must result in an immediate removal of the mass of debris. If this does not occur at once, often the region on which the debris is resting begins to swell, becoming a strongly convex, smooth elevation, thus rendering the washing away of the mass still easier.

But if the debris is not removed by the reaction just described, then new activities set in. If the waste body is near one edge of the disk, this edge usually begins to sink, while at the same time the tentacles between the edge and the waste mass collapse and practically efface themselves. Thus the mass slides downward off the disk. If this does not occur at once, after a time the region lying behind the mass begins to swell ; it often forms in this way a high, rounded elevation. The waste mass is now on a steep slope, and is bound soon to slide over the edge. Sometimes by a continuation of these processes the entire disk comes to take a strongly inclined position, with the side bearing the debris below. Often one portion of the edge after another is lowered successively till all of the waste matter is removed and the disk is thoroughly cleaned. The disk then resumes its horizontal position, with nearly flat or slightly concave surface.

Sometimes the edge bearing the debris cannot be lowered, owing to the fact that it is almost against an elevation in the irregular rock to which the anemone is attached. In this case (after perhaps an attempt to bend this edge downward) the part between this edge and the debris swells and rises, rolling the mass toward the centre, while at the same time the region beyond the debris sinks down. In this way the waste matter is rolled across the disk to the opposite side, and dropped over the edge. The process is slow, often requiring fifteen minutes to half an hour.

This whole reaction is characterized by great flexibility and variability. The debris sets in operation certain activities; if these do not put an end to the stimulation, other activities are induced, till one is successful. This is an excellent illustration of the general characteristics of behavior in the lower organisms. After contracting in response to stimulation, if the stimulus still continues, Hydra and the sea anemones usually set in operation other activities, having a more radical effect in separating the animal from the source of stimulation. We have examined certain cases of this character in the foregoing section on the rejecting reaction. We shall here consider such reactions as tend to remove the animal, or cause it to take a new position.

Hydra. — After contracting in response to stimulation, Hydra usually bends over into a new position and soon extends again in a new direction, just as happens in its spontaneous contractions (Fig. 114). This may be repeated many times, the animal occupying successively many different positions. In bending thus into a new position in response to a one-sided stimulus, does Hydra bend directly away from the source of stimulation? Wagner (1905) and Mast (1903) have answered this question experi-

mentally. Wagner tried stimulating one side of the body mechanically, while Mast raised or lowered the temperature of one side. Both authors agree as to the following results: The direction of extension after contraction bears no definite relation to the side from which the stimulus came; the animal is just as likely to extend toward the source of stimulation as in any other direction. In other words, when stimulated, Hydra merely changes its position, without special relation to the localization of the stimulating agent. The direction of bending and extension is determined by internal factors. If the stimulus is repeated, contraction occurs again, and the animal extends in still another direction. The analogy of these relations with those shown by the infusoria is evident; the latter when stimulated usually merely change the direction of movement, without regard to the direction from which the stimulus came. In the infusoria the internal factors (structural in character) which determine the direction have been determined; this has not yet been done for Hydra.

But repeated or strong continued contraction, with extension in a new direction, is not the final recourse of Hydra under strong stimulation. If the stimulation continues, the animal finally bends over, places its head against the surface to which it is attached, releases its foot, and moves away from the spot where it has been subjected to such objectionable experiences. The locomotion is usually of the sort illustrated in Fig. 117. This reaction has been observed by Wagner (1905) under mechanical stimulation, by Mast (1903) under stimulation by heat, and by the present author under stimulation by chemicals. In all cases it was found that the direction toward which the animal moves bears no definite relation to the direction from which the stimulus comes. Wagner stimulated one side repeatedly by striking it with a rod, and found that the animal was as likely to move toward that side as in any other direction. The experiments of Mast are particularly interesting in this connection. Mast placed a considerable number of Hydras in a fiatbottomed trough, and heated one end. At about 31 degrees C. the animals began to release their foothold and move about from place to place. But they were as likely to move toward the heated end as away from it. The results of a series of such experiments are shown in Fig. 127. In this figure are represented not only the movements of locomotion, but also the different directions in which the animal extended after contracting. The diagram shows clearly that both sets of movements are quite without definite relation to the direction from which the heat comes; their direction evidently depends on internal factors. When it experiences the high temperature, the animal merely changes its position, in a way determined by its structure or other internal fac-

tors. If the high temperature still continues, it changes position again, and thus continues till the high temperature ceases or the animal dies. The behavior resembles essentially that of infusoria under similar conditions. The reaction is very ineffective under the conditions shown in Fig. 127, owing to the slowness of the movements of Hydra. Most of the animals in the heated region finally die. But if the animals moved rapidly and far at each change of position, then those that moved away from the heated side would escape, and those that moved in the wrong direction the first time would, after one or two changes of direction, likewise get out of the heated region. The reaction would be of precisely the same character as that of the

infusoria. But the Fig. 127. — Diagram of the movements of a number of action SVStem of HvdrT Hydras when the trough containing them was heated at the end •* ^ to the left. Each of the small diagrams represents the moveis evidently adapted ments of a single Hydra. The figures i, 2, 3, etc., show the onlv for mpptinosuccessive different directions in which the Hydra extended * . & while remaining attached. The cross ( X ) between two numchanged Conditions Over bers indicates that here the animal released its foothold and

a VPrv limited arpa surh moved m the direction shown to a new point of attachment. slight, slow movement. When the changed conditions cover too large an area, the Hydra can only "try" its usual reaction; if this fails, it must die. A decrease of temperature does not cause Hydra to change position. As the temperature becomes lower, the animal merely becomes more sluggish, contracting more slowly and at longer intervals, till finally, near the freezing point, movement almost ceases (Mast).

As we have seen on page 194, an internal condition — hunger — may induce the same locomotor reactions as are produced by continued external stimulation. This is a matter which we shall take up again in the account of food reactions. Sea Anemones. — In some sea anemones, as in Hydra, repeated strong stimulation causes the animal first to contract, then to bend into new positions, and finally to move away. Each of these reactions may be repeated several times before the succeeding one occurs. There are certain features of this behavior that are of much interest, since they lead to results analogous to habit formation in higher animals. The facts have been most carefully studied in Aiptasia annulate.

Aiptasia is a rather slender, somewhat elongated actinian living in crevices beneath and between stones. If stimulated by touching the disk or tentacles with a rod, it contracts strongly. It then extends in the same direction as before. When it is fully extended we repeat the stimulus. The animal responds in the same way as at first. This continues usually for about ten or fifteen stimulations, the animal extending each time in the same direction as at first. But at length, when stimulated anew, the polyp contracts, bends over to one side, and extends in a new direction. As the stimuli are continued, the animal repeats for a number of times the contraction and extension in the new direction, then finally turns and tries a still different position.

This change of position may be repeated many times. But in the course of time the reaction becomes changed in a still different manner. The anemone releases its foothold and moves to a new region. This same reaction is produced in Cerianthus, as we have seen, by hunger. Aiptasia frequently extends in most awkward turns, the body taking and retaining an irregular and even crooked form. This is evidently due to its life in irregular crevices and crannies. In order that its disk may protrude into the open water, it is compelled to extend in the irregular ways mentioned, and to retain the crooked shapes thus produced. "When removed from its natural habitat, it still retains these irregularities of form and action, so that a collection of Aiptasias shows all sorts of right-angled and zigzag shapes. It would appear that these irregularities must have arisen as a result of the way in which the animal extends in its natural surroundings. From this it would appear that a method of extension frequently repeated must in the course of time become stereotyped, forming what we are accustomed to call in higher animals a habit.

If this is the case, then it should be possible to produce new stereotyped reaction forms, by so arranging the conditions that the animal shall be compelled to extend always in a certain way (differing from its former way), and to retain the form thus induced. In some specimens this result is obtained with the greatest ease, and in a very simple manner. Thus, in a certain case, an individual attached to a plane horizontal glass surface was bent in extension far over to the left. Stimu-

lating it repeatedly, it contracted at each stimulation, then bent, in extending, again to the left. But after some fifteen stimulations it turned away, and bent over to the right. Now when stimulated it contracted as before, then bent regularly, in extending, over to the right. It seemed to have acquired a new method of behaving, bending to the right instead of to the left. Close examination showed that the cause of this phenomenon is as follows: When it contracts in response to stimulation, it does not regain a completely symmetrical structure, but remains a little more contracted on the side that is concave in extension. In extending anew, this side still remains a little more contracted than the opposite one, so the animal takes a curved form, concave toward the same side as in its previous extension. In other words, the structure conditioning the curved form is not completely lost even when the animal contracts, and it becomes evident again on a new extension.

Thus in Aiptasia the formation of a stereotyped method of action depends upon very simple conditions. Yet there can hardly be a doubt that the permanent individual peculiarities of form and action found under natural conditions, as mentioned above, have risen in exactly this way. It thus plays the part taken by what is called habit formation in higher animals. The facts set forth in the present section show clearly that the ccelenterates do not always react in the same way to the same external stimulus. Internal conditions of the organism, as determined by past stimuli received, past reactions given, and various other factors, are of equal importance with external conditions in determining behavior. We shall see many further illustrations of this fact in the reactions toward food.

Besides the changes in behavior under constant stimuli that we have described in the last section, there are certain others which may perhaps be classed as acclimatization to stimulation. In sea anemones a light stimulus that is not injurious may cause at first a marked reaction, then on repetition produce no reaction at all, or a very slight one. Thus, a drop of water is allowed to fall from a height of 30 cm. on the surface of the water just above the outspread disk of Aiptasia annulata. The animal at once contracts completely. After the animal has expanded, another drop is allowed to fall in the same way. As a rule, there is no response to this or to succeeding drops. Sometimes there is a reaction to the first two or even three drops, but usually reaction ceases after the first one.

Sometimes a slight reaction of a different character supervenes after the stimulus has been repeated many times. The animal begins to shrink slowly away from the region where the drops are falling, so that in the course of time the disk has been withdrawn much farther below the surface, though no decided reaction has occurred to any one stimulus. In the foregoing sections we have taken up reactions to mechanical stimuli, heat and cold, and chemicals; we shall have occasion to consider some of these further in the account of food reactions. There are certain other classes of external stimuli which may play a part in determining behavior in these animals ; these we will take up separately.

Induction shocks have been much employed in experimental work on contraction in ccelenterates. The results of such stimulation do not Fig. 128. — Reaction of an attached Hydra to a constant electric current of moderate intensity. 1-5, successive stages in the reaction. After Pearl (1901). differ greatly from those produced by other forms of stimulation (mechanical, etc.), local or general contractions occurring in dependence on the strength of the current. These may be followed by locomotor movements.

The effects of the constant electric current are more peculiar and of greater interest. They have been studied in Hydra by Pearl (1901); in the medusa Polyorchis penicillata by Bancroft (1904). Hydra. — In Hydra the constant current causes local bendings of the body similar to those produced by sharply localized chemical and thermal stimuli. If a weak current is passed through the water transversely to the Hydra, the animal contracts on the anode side, at a point a little above the foot, thus bending the body (Fig. 128). At the same time or a little before, the tentacles which were in line with the current contract (Fig. 128, a). Sometimes, further, there is a contraction on

the anode side just below the base of the tentacles. As a result of the contraction on the anode side, the Hydra bends toward the anode. As soon as it comes into a position wi^h the anterior end directed toward the anode, the entire body contracts, since a Hydra in this position is stimulated more than in any other (Fig. 128, 5). In a stronger current the complete contraction takes place first, then the animal slowly bends over toward the anode. If, as sometimes happens, the foot is free while the head is attached, the bending takes place as usual on the anode side.

Fig. 129. — Successive stages in the reaction of a Hydra to the electric current when the foot is unattached. The foot becomes directed toward the anode. After Pearl (1901). The result is necessarily that the foot becomes directed toward the anode, so that in this case the orientation of the animal is the reverse of that found in the specimens attached by the foot (Fig. 129). This result shows clearly that the orientation to the electric current is due to the direct local contractions caused by the current on the anode side, and is not due to an attempt on the part of the animal by anything like a process of trial to come into a certain definite position.

In a Hydra placed transversely to the current, the tentacles contract in a peculiar way. A weak current causes only the tentacles which are in line with the current to contract, and of these, that extending toward the cathode contracts more quickly and more completely than Fig. 130. — Fffects of the constant electric current on pieces of Polyorchis. After Bancroft (1Q04). A, meridional strip passing through the manubrium. B, similar strip stretched out in line with the current. C, isolated tentacles.

that directed toward the anode (Fig. 128, a). If the Hydra is lying parallel with the current, the body contracts much more readily when the anterior end is directed toward the anode than when it is directed toward the cathode. In either of these positions the tentacles usually remain extended, and somewhat inclined toward the cathode (Figs. 128 and 129). But if a very strong current is used, both body and tentacles contract strongly. Pieces of the animal react in essentially the same way as the entire organism, and young buds (with tentacles) react in the same way as adults, but are more sensitive to the current (Pearl, 190 1).

Medusa. — If strips of various shapes are cut from the medusa Polyorchis, and subjected to the action of the constant current, the tentacles and manubrium bend toward the cathode (Fig. 130, a, b). This takes place even with isolated tentacles (Fig. 130, c). If the current is long continued, such isolated tentacles partially relax, then contract again. This is repeated, so that an irregular rhythmic contraction is produced by the constant current.

The position of the body and the direction of locomotion are partly determined in some of the Ccelenterata by gravity. There is great diversity among different members of the group in this respect. In some, gravity is an almost constant determining factor in the behavior. In others it plays only an incidental part, affecting the behavior under certain circumstances, while in still other cases it seems to have no effect on the movements whatsoever.