Behavior of the Lower Organisms

The different physiological conditions are determined largely by the history of the individual worm, so that in this sense its behavior may be said to depend on its experience. The dependence of the reactions on the physiological state is in a given specimen very great, so that two individuals often react in opposite ways to the same stimulus. The same individual that reacts to a given stimulus positively may a little later react negatively, and vice versa. After long study of Planaria, Pearl concludes that "it is almost an absolute necessity that a person should become familiar, or perhaps better, intimate, with an organism, so that he knows it in something the same way that he knows a person, before he can hope to get even an approximation of the truth regarding its behavior." This remark might be extended to most lower animals.

As we have seen in a previous section (p. 236), the behavior of the flatworm shows certain well-defined reaction types, which might, taken separately, be called reflexes. But when we consider the various factors which determine the production and combination of these reaction types, we cannot consider the behavior of the flatworm as "purely reflex," if we mean by reflexes invariable reactions to the same external stimuli. On the contrary, the behavior is extremely variable in accordance with many conditions, internal as well as external.

A detailed analysis of the behavior of almost any of the lower invertebrates would show as many different physiological conditions on which behavior. depends as we find in the flatworm. In the earthworm, for example, the conditions are still more complicated than in the flatworm, so that the same external stimulus, acting with the same intensity, and applied to the same spot on the body, may produce any one of at least six different reactions. The variations of internal state as the animal moves about are what condition the "random movements" described by Holmes in the reactions to light, and by Smith in the reactions to other stimuli (see p. 247).

Of special interest are changes in state that lead to more or less permanent modifications in behavior. These are little known in the lower organisms. Most of the changes of physiological state described in the foregoing paragraphs are not known to last more than a short time. In Vorticella, Hodge and Aikins (1895) state that the modified behavior endured for five hours ; this perhaps needs confirmation. In the lowest organisms it is difficult to carry out experiments that shall determine how long modifications last. Perhaps the lowest animal in which an enduring modification of behavior has been demonstrated is the flatworm Convoluta roscoffensis. This is one of the lowest of the group, belonging to the division Accela, which includes the simple forms having no alimentary canal. The behavior of Convoluta, as described by Gamble and Keeble (1903), and by Bohn (1903 a), presents many features of the greatest interest ; into only a few of these can we enter. Convoluta is a small green worm that lives in immense numbers on the sand of the seacoast of Brittany, just above the water line. It forms thus large green patches. When the tide rises the water covers the region where

Convoluta is found, and the waves would wash the animals away, if their behavior did not prevent. As the water rises and the waves begin to beat on the sand near them, they go downward into the sand, where they are protected. As the water sinks, the animals creep upward and appear again at the surface. These upward and downward movements are reactions with reference to gravity, as is shown by placing the animals on smooth, inclined, or perpendicular surfaces. They go downward as the tide rises, upward as it falls. Bohn (1905) has shown that many littoral mollusks and annelids show similar movements with relation to the tides.

The peculiarly interesting fact concerning this behavior in Convoluta is the following: This periodical alternation of reactions, produced by an environmental factor, becomes so impressed on the organization of the animal that it occurs even when this factor is lacking. The alternation of movement has become habitual. If the worms are removed to an aquarium where the tide no longer acts upon them, they continue to go downward at the period of high tide, upward at the period of low tide. This continues for about two weeks, so that the worms may be carried far away from the shore, and may then be used for a time as tide indicators. But under such conditions the periodicity after a time disappears, showing that it was really due to the external factor, — the tides. This appears to be the lowest known case of what we call in higher animals a habit.

In some of the higher invertebrates, lasting modifications of behavior of a still more complex character may be induced experimentally. This has been accomplished in the Crustacea by Yerkes (1902), Yerkes and Huggins (1903), and Spaulding (1904). With the crayfish and crab, Yerkes and Huggins (1903) studied the modification of behavior in escaping from danger and in finding water. The crayfish was placed in one end of an inclined pen which opened at the other end into the water. The pen was partly divided by partitions in such a way as to leave two passages leading to the water (Fig. 139). Either of these passages could be closed at its end by a glass plate G. The animal was placed at T

region it might enter the blind pocket at G, thus not directly reaching the water, or it might go through the other passage straight to the water. After some preliminary experiments without closing either passage, showing that the animals were as likely to pass to the right as to the left, Fig. 139. — Pen used by Yerkes in experimenting with Crustacea. the partition was placed in the right passageway, as in Fig. 139. The crayfish which turned to the left on leaving T escaped at once to the water. But if it turned to the right it passed into the pocket G, and was compelled to explore the region, finally turning to the left and passing the partition P, before it could escape. Three individuals were given sixty trials each in the course of thirty days. In the first ten trials they went just as frequently into the blind passage as toward the water. In the second ten trials, the animals started in 60 per cent of the cases toward the open passage at the left. In the next ten trials this proportion had risen to 75.8 per cent; in the following ten, to 83.3 per cent. In the last ten trials of the sixty, very few mistakes were made. In 90 per cent of all cases they went straight for the open passage. In another series of experiments an individual, after four hundred trials, made only one mistake in fifty trials. Similar results were obtained by Yerkes (1902) in experimenting on the crab Carcinus granulatus.

Thus at the beginning of the experiment the animals were as likely to go to the right as to the left, while at the end they went almost invariably to the left. Since the external conditions had not changed, the animals themselves must have changed. Their internal condition now differed in some way from the original condition. Yerkes and Huggins (1903) endeavored to determine how easily this acquired condition could be modified or destroyed. After the crayfish had learned to go through the open passageway so as to make a mistake in only one case in ten, the experiments were discontinued for two weeks. On the fourteenth day the animals were still inclined to go straight to the open passage, though the habit had become dulled, and they now made mistakes in about three cases out of ten.

In other experiments, after the animals had acquired the habit of escaping through the right passage, the partition G was changed, so as to block up this passage, but leave the left one open. At the next trial the animal made a long-continued attempt to escape by the right-hand passageway, following the path shown in Fig. 140. — Path followed by a crayfish which has Yw IJ.O It Wandered hand passageway, when this passage is closed and the about tor utteeil minutes

left one opened. After Yerkes and Huggins. before disCOVCrino" the Open way. But in the next trial it turned to the left, and thereafter it turned almost as regularly to the left as it had before turned to the right. This habit formation took place in the same manner when the floor of the pen was carefully washed out after each trial, showing that the animals were not merely following a path marked by an odor from the previous passage along it. It was evident that the customary direction of turning played a large part in the behavior. When the left passage was closed, the crayfish that had erred into this passage escaped by turning to the right, as indicated by its path in Fig. 141. When after the establishment of this habit, the right passage was closed (Fig. 140), the animal tried persistently to escape from this passage by turning to the right, as it had previously done.

Spaulding (1904) studied the modifiability of behavior in the food reactions of the hermit crab. These animals tend to remain in the lighted parts of the aquarium. They were fed by placing a small, dark screen with a fish beneath it in a certain part of the aquarium. The diffusion of juices from the fish set the crabs to moving about actively, and in the course of time some passed beneath the screen. Here the food was found. At first it took the crabs a long time to find it under these conditions. On the first day only three out of thirty succeeded in fifteen minutes. But by the

Fig. 141. — Path followed by crayfish while being trained to avoid the left passage. On erring into this passage, it escapes by passing to the right, thus forming the habit of turning to the right. After Yerkes and Huggins. third day, twenty of the thirty had passed beneath the screen fifteen minutes after it was introduced. At the end of the eighth day, twentyeight out of the twenty-nine present had passed beneath the screen inside of five minutes. The crabs had become so modified that they went quickly beneath the screen as soon as it was introduced.

Now the experiments were varied by placing in the aquarium the screen alone, without the food. Most of the animals passed beneath it as before. Thus, on the thirteenth day of the experiments, twentyfive specimens out of twenty-seven present had passed under the screen within five minutes. After they had entered they were fed, in order that the association between the screen and food might not be destroyed. Phenomena of this character are usually spoken of as learning, or as the formation of habits or associations. The facts may be expressed in a purely objective way as follows: When subjected to the stimulus of the screen and the food, the animals reacted to the food by gathering about it — incidentally of course gathering under the screen. After many repetitions of such stimulation, the animals had become changed

so that they responded to the dark screen alone by the reaction proper to food. We shall analyze these phenomena more fully in our general discussion of behavior (Chapter XVI). These processes, by which behavior becomes more or less enduringly modified, are known to play a large part in the behavior of higher invertebrates, such as ants and bees, and in the vertebrates. As investigation progresses, we find analogous processes lower and lower in the animal scale. It was only eight years ago that Bethe (1898) could deny their occurrence even in ants and bees ; now they have been fully demonstrated in these and much lower animals. The study of these matters has hardly begun, and it is not too much to say that no experiments have been carried through on the lowest invertebrates that would show this lasting modifiability, even if it exists. We are therefore still in the dark as to how far downward such modifiability extends; time may show it to be a universal property of living things.

The importance of this modifiability for the understanding of behavior is obviously great. Where such modifiability exists, the definite " reflex" is not to be considered a permanent, final element of behavior. On the contrary, it is something developed, and it must differ in individuals with different histories. Two specimens of Convoluta side by side might show at the same moment, one "positive geotropism," the other "negative geotropism," depending on their past history. Whether a hermit crab will pass beneath a dark screen, or will avoid it, is not determined by the permanent properties of its colloidal substance; this can be predicted only by knowing the history of the individual.

The process by which an organism acquires a definite reaction which it before had not is, of course, nothing mystical, but an actual physiological one, whose progress is open to investigation as is that of any other. It needs to be studied and analyzed in the same objective way as the circulation of the blood. The power of changing when acted upon by outer agents, in such a way as to react differently thereafter, is one of the most important properties of living matter, and it is misleading to ignore this property and deal with animals as if their reactions were invariable. How the modifications occur is one of the fundamental problems of physiology. We must remember that even what we call memory, intelligence, and reasoning are composed objectively of certain physiological processes. In other words, as Liebmann has emphasized, there are objective material processes that follow the laws of intelligence, of reasoning, of logic. This is a capital fact. In searching for the laws of life processes we must remember that those just mentioned are as real as any others, and their laws must be provided for in the physics and chemistry of colloids if these are to give us the laws of life processes.

We shall attempt to analyze some of these matters farther in our general discussion of behavior, which forms the remainder of the work. We have now examined the behavior of a number of Protozoa and of a number of Metazoa. What characteristic differences do we find between the two? This question is of interest from a number of points of view. The Protozoa consist each of but a single cell, while the Metazoa are composed of many cells, which are differentiated for the performance of different functions. Does this difference in structure correspond to any fundamental difference in behavior? Le Dantec (1895) proposed to distinguish the life manifestations of the Protozoa as " elemental life ': from the life of the Metazoa, holding that the two are so different in fundamental character that it is improper to apply the same name to them ; this point of view is often met in scientific literature. The life of the Protozoa is considered "as the direct result of the diverse reactions of a small mass of a certain chemical substance in the presence of appropriate substances " {I.e., p. 26), while that of the Metazoa is " the result of the functioning of an extremely complicated machine, in which the reactions of the chemical in question serve as motor power." The former is compared to the burning of the alcohol in an alcohol motor, the latter to the functioning of the motor itself (p. 27). We are interested in the question whether this theoretically fundamental difference shows itself in any way in the phenomena to be observed. Is there any objective evidence in the behavior for the belief that the life of the Protozoa differs fundamentally from that of the Metazoa?

Again, the Metazoa possess a nervous system, while the Protozoa have none. To the specific properties of the nervous system many of the manifestations of behavior in higher animals have been attributed. This system is often considered an essential prerequisite for certain fundamental features of behavior. Do we find a striking difference in the behavior of organisms after a nervous system has been developed ? What can animals do without a nervous system? A comparison of organisms with and those without this system should give us evidence as to the real nature of the functions of the latter, and will perhaps prevent us from overestimating its importance.

We will sum up briefly in a number of paragraphs the resemblances and differences between the behavior of animals with and without a nervous system. 1. First, we find that in organisms consisting of but a single cell, and having no nervous system, the behavior is regulated by all the different classes of conditions which regulate the behavior of higher animals. In other words, unicellular organisms react to all classes of stimuli to which higher animals react.1 All classes of stimuli which may affect the nervous system or sense organs may likewise affect protoplasm without these organs. Even the naked protoplasm of Amoeba responds to all classes of stimuli to which any animal responds. The nervous system and sense organs are therefore not necessary for the reception of any particular classes of stimulations.

2. The reactions produced in unicellular organisms by stimuli are not the direct physical or chemical effects of the agents acting upon them, but are indirect reactions, produced through the release of certain forces already present in the organism. In this respect the reactions are comparable with those of higher animals. This is true for Amoeba as well as for more differentiated Protozoa. 3. In the Protozoa, as in the Metazoa, the structure of the organism plays a large part in determining the nature of the behavior. There are only certain acts which the organism can perform, and these are conditioned by its organization ; by one of these acts it must respond to any stimulus. If the behavior of the Metazoa is comparable in this respect to the action of a machine, the same comparison can be made for the behavior of the Protozoa.

4. Spontaneous action — that is, activity and changes in activity induced without external stimulation — takes place in the Protozoa as well as in the Metazoa. Both Vorticella and Hydra, as we have seen, spontaneously contract at rather regular intervals, even when the external conditions remain uniform. Continued activity is the normal state of affairs in Paramecium and most other infusoria. The idea that spontaneous activity is found only in higher animals is a totally erroneous one ; action is as spontaneous in the Protozoa as in man.

1 Considering auditory stimulation as merely a special case of mechanical stimulation. 5. In unicellular organisms, without a nervous system, certain parts of the body may be more sensitive than the remainder, forming thus a region comparable to a sense organ in a higher animal. Whether such a part may become more sensitive to one form of stimulation, while insensitive to others, as in higher organisms, seems not to have been determined. 6. Conduction occurs in organisms without a nervous system. This is, of course, seen in the fact that a stimulus limited to one part of the body may cause a contraction of the entire body, or a reversal of cilia over the entire body surface. A strongly marked case is the contraction of the stalk in Vorticella, when only the margin of the bell is stimulated.

7. Summation of stimuli occurs in Protozoa as in Metazoa. This is shown most clearly in Statkewitsch's experiments with induction shocks (p. 83). Weak induction shocks have no effect until frequently repeated. 8. In the unicellular animal, as in that composed of many cells, the reaction may change or become reversed as the intensity of the stimulus increases, though the quality of the stimulus remains the same. Such a change in reaction has sometimes been claimed as a specific property of the nervous system. The protozoans Amoeba and Stentor, as well as the metazoan Planaria, move toward sources of weak mechanical stimulation, away from sources of strong stimulation.

9. In the Protozoa, as in the Metazoa, the reaction may change while the stimulus remains the same. That is, the animal may respond at first by a certain reaction ; later, while the stimulus remains the same, by other reactions. This has been shown in detail in the account of Stentor (Chapter X). The change may consist in either a cessation of the reaction, or in a complete alteration of its character. These changes are, as a rule, by no means due to fatigue, but are regulatory in character. The behavior thus depends on the past history of the organism. For such modifications of behavior a nervous system is then unnecessary.

10. In the Protozoa, as in the Metazoa, the reactions are not invariable reflexes, depending only on the external stimulus and the anatomical structure of the organism. The reaction to a given stimulus depends upon the physiological condition of the organism. In Stentor we could distinguish at least five different conditions, each with its characteristic reaction to the given stimulus. 11. In unicellular as well as multicellular animals we find two chief general classes of reactions, which may be designated positive and negative. The positive reaction tends to retain the organism in contact with the stimulus, the negative to remove it from the stimulus. In many classes of stimuli we can distinguish an optimum condition. A change

leading from the optimum produces a negative reaction, while a change leading toward the optimum produces no reaction, or a positive one. The optimum from this standpoint usually corresponds, in a broad way, to the optimum for the general interests of the organism. These relations hold equally for Protozoa and Metazoa. 12. In both the Protozoa and the Metazoa that we have studied, the behavior is based to a considerable degree on the selection of certain conditions through the production under stimulation of varied movements (see Chapter XII). This shows itself in two characteristic types. In the one case the organism when subjected to a change leading away from the optimum responds by a movement that subjects it successively to many different conditions, finally remaining in that one which is nearest the optimum. This form of reaction is strongly developed in Paramecium. In the second type, which may be considered a development of the first, the organism first responds by one reaction, then by another, continuing at intervals to change its response until one of the reactions frees it from the stimulation. This way of behaving is well seen in Stentor. Both methods of reaction may be expressed as follows : When the organism is subjected to an irritating condition, it tries many different conditions or many different ways of ridding itself of this condition, till one is found which is successful.

All together, there is no evidence of the existence of differences of fundamental character between the behavior of the Protozoa and that of the lower Metazoa. The study of behavior lends no support to the view that the life activities are of an essentially different character in the Protozoa and the Metazoa. The behavior of the Protozoa appears to b>e no more and no less machinelike than that of the Metazoa ; similar principles govern both. Further, the possession of a nervous system brings with it no observable essential changes in the nature of behavior. We have found no important additional features in the behavior when tl^e nervous system is added. In the lower Metazoa, experiment has shown the nervous system to have two chief functions, — the maintenance of tonus, and the bringing of the parts of the body into relation with each other by serving for conduction. But both these functions are performed in the Protozoa without a nervous system. The body of Paramecium maintains marked tonus, and the different parts of the body work together. A comparison of the behavior of the Protozoa with that of the lower Metazoa lends powerful support to that view of the functions of the nervous system which is so ably maintained by Loeb in his brilliant work on "The Comparative Physiology of the Brain and Comparative Psychology. " According to this view we do not find in the nervous system specific qualities

not found elsewhere in protoplasmic structures. The qualities of the nervous system are the general qualities of protoplasm. Certain of these general qualities have become much accentuated in the protoplasm of the nervous system, while in the remainder of the protoplasm of the metazoan body they are less strongly marked, being partially obscured by differentiations in other directions. Most if not all of the fundamental activities which have been considered peculiar to the nervous system may be demonstrated, as we have seen, in the Protozoa, yet in them no nervous system exists.

These facts show the necessity of guarding against overrating the importance of the nervous system. It is doubtful if the nervous system is to be considered the exclusive seat of anything; its properties are accentuations of the general properties of protoplasm. Dogmatic statements as to the part necessarily played by the nervous system in given cases must be looked upon with suspicion unless supported by positive experimental results. If acts objectively identical with "reflex actions" and still more complex types of behavior may exist in the Protozoa without the intervention of a nervous system, it is not impossible that they may occur in the same manner in Metazoa, as Loeb has maintained. Where a nervous system exists, we are not justified in dogmatically referring all phenomena of behavior to it, for other protoplasm exists too, and may still retain some of the characteristics which it had in the Protozoa. In an animal possessing a nervous system we cannot tell without experimentation whether a given reflex action or other reaction depends on the nervous system or not. The possibility always remains open that the remainder of the protoplasm may perform the act in question by its own capabilities, as it does in the Protozoa. In any animal, we are justified in attributing exclusively to the nervous system only those properties which rigid analytical experimentation shows it alone to possess.