Behavior of the Lower Organisms

internal Conditions ; It OCCUrS Mter Wagner (1905). 1-6, Successive positions. external conditions. A common method of movement in sea anemones is to glide about on the foot, — the lower surface of the foot sending out extensions and moving in a manner similar to that of the foot of mollusks. There are doubtless other methods of locomotion. The spontaneous contraction and change of position which plays a subordinate part in fixed forms has become the rule in medusae. They are commonly found swimming about by means of rhythmical contractions. Since there are no corresponding changes in external conditions, these contractions must be due to internal changes. The internal changes need not of course be themselves of a rhythmical character. They may take place steadily, inducing a contraction only

when a result of a certain intensity has been reached (see Loeb, 1900, p. 21). In the small medusa Gonionemus there is under natural conditions a cycle of activity that is of great interest; it has been well described by Yerkes (1902, a and b, 1903, 1904) and Perkins (1903). At times the animal is found attached by certain adhesive pads on its tentacles to Fig. 118. — Young Gonionemus resting on the bottom, with the opening of the bell upward. After Perkins (1903).

the vegetation of the bottom or to other surfaces (Fig. 118). Leaving its attachment, it swims upward to the upper surface of the water, the convex surface of the bell being upward, and the tentacles contracted (Fig. 119). Reaching the upper surface it turns over " and floats downward with bell relaxed and inverted, and tentacles extending far out horizontally in a wide snare of stinging threads which carries certain destruction to creatures even larger than the jellyfish itself (Fig. 120)" (Perkins, 1903, p. 753). Reaching the bottom, it swims again to the top and repeats the process. It may thus continue this process of " fishing," as Perkins calls it, all day long. It is chiefly in this way that it captures its food. This cycle of spontaneous activities is in some respects similar to that of the green Hydra described above, though much more complex. Both illustrate the fact that complex movements and changes of movement may occur from internal causes, without any change in the environment.

Fig . 1 1 g . — ■ Gonionemus swimming upward with contracted tentacles. After Perkins (1903). Hydra and the sea anemones tend to retain a certain position; we usually find them at rest with foot attached and head free. This usual position is often said to be due to a reaction to gravity or to contact, or to some other simple stimulus. It will be found instructive to examine the different conditions on which depends whether the animal shall or shall not retain a given position in which it finds itself. It will be found that the matter is not an entirely simple one.

Let us take first the case of Hydra. Suppose the animal to be placed on a horizontal surface with head downward and foot upward. It does not retain this position, but bends the body, placing the foot against the bottom, releases its head, and straightens upward. This is what is commonly called the "righting" reaction. In Hydra it is not due to a tendency to keep the body in a certain position with reference to gravity, for the animal may remain attached to the bottom, with head projecting upward, or to the surface film, with head projecting downward, or to a perpendicular surface, with the body transverse or oblique to the direction of gravity. There is even apparently a certain tendency to direct the head downward. Thus out of 100 green Hydras attached to a perpendicular surface, 96 had the head lower than the foot, 3 were horizontal, and 1 had the head directed upward. It is thus clear that the righting reaction of a Hydra which has been inverted on the bottom cannot be due to any unusual relation to the direction of gravity.

To what, then, is the reaction due ? Evidently there is a tendency to keep the foot in contact with a surface, for the body is bent till the foot comes in contact. But this is not all ; the reaction does not stop at this point. There is likewise a tendency to keep the head free, for it is released. But still this is not all, for now the body is straightened ; then the tentacles are spread out symmetrically in various directions. It is clear that the reaction is directed toward getting the organism into its usual position, which might perhaps be called the "normal" one; this normal position has various factors, — attachment of foot, freedom of head, comparative straightness of the body, and tentacles outspread.

This is, of course, exactly the position which is most favorable for obtaining food. Suppose now that our Hydra has reached this position, and all the conditions remain constant ; is this sufficient ? We find that it is not. If the conditions remain so constant that no food is obtained, the Hvdra becomes restless and changes the position of its body repeatedly, though still retaining its attachment by the foot. But later even this is given up, and the animal, of its own internal impulse, quite reverses the position attained through the "righting reaction." It now bends its body, attaches its head, and releases its foot, thus bringing it back into the inverted position. Is this because the irritability

Fig. 121.— Process by which Cerianthus rights of nead ail 1 foot have become itself when inverted in a tube. The figures are taken reversed, SO that the head now at intervals during the course of one hour. After , . , , , foot free? Apparently not, for no sooner has the organism taken the inverted position than it draws its foot forward and now performs the "righting reaction" again, so that it stands once more on its foot. These alternations of behavior are repeated, and we find that by this means the animal is moving from place to place, as in Fig. 117.

It seems clearly impossible to refer each of these acts or the whole behavior to any particular present external stimulus. Through hunger the Hydra is driven to move to another region, and these different opposite acts are the means by which another region is reached. Each step in the behavior is partly determined by the preceding step, partly by the general condition of hunger. The same behavior is often seen^ as we shall see later, under continued injurious stimulation of different kinds.

In speaking of righting reactions, it is often said that the organism is forced by the different irritabilities of diverse parts of the bodv to take a certain orientation with reference to gravity or to the surface of contact (see, for example, Loeb, 1900, 4mToHH^ p. 184). The facts just MMMiM$ brought out show that we can in Hydra consider this orientation forced only in the general sense that all things which occur may be considered forced. Man takes sometimes a sitting position, sometimes a FlG. I22.-_Pos;tion taken by Cerianthus after it

Standing one, Sometimes a renas been placed on its side on a wire mesh. After Loeb. his "physiological state" and past history, and the facts are quite parallel for Hydra. So far as objective evidence shows, the behavior is not forced in Hydra in any other sense than it is in man. The animal takes that position which seems best adapted to the requirements of its physiological processes; these requirements vary from time to time. In the sea anemone Cerianthus the conditions for retaining a certain position are somewhat more complex thanin Hydra, according to the account given by Loeb (1891). The animal is usually Fig. 123. — Cerianthus found in an upright position, occupying a mucus-

a meshwork, as a result of rehned tube in the sand. If placed head downpeatediy inverting the latter. warcj in a test-tube, it rights itself in the same way as Hydra, freeing the head, bringing the foot into contact, and straightening the body (Fig. 121). But in this animal, gravity clearly plays a part in the behavior. Loeb placed the animal on its side on a wire screen of large mesh. Thereupon it bends its foot down through the meshes, lifts up its head, and takes its usual position in line with gravity (Fig. 122). If now the screen is turned over, the animal again directs its head upward, its foot downward — as a human being under similar circumstances would do if possible. It may thus weave itself in and out through the meshes (Fig. 123).

But to be in line with gravity, with head free, is not the only requirement for Cerianthus. Loeb found that it would not remain indefinitely in this position on the wire screen, as it does in the sand. After a day or so it pulls its foot out of the wire and seeks a new abode. Only when it can get the surface of its body in contact with something, as is the case when it is embedded in the sand in its natural habitat, is it at rest. If this condition is fulfilled, the requirement of the usual position in line with gravity may be neglected. Loeb found that when the animal is placed in a test-tube, so that its body is in contact with the sides, it remains here indefinitely, even though the tube is placed in a horizontal position (Loeb, 1891, p. 54). The head is bent upward, but the body remains transverse to the direction of gravity. Similarly, the anemone Sagartia may ofttimes take a position on the surface film with head down, although usually it maintains an upright position (Torrey, 1904).

But even the usual position in line with gravity, and with sides in contact, does not satisfy Cerianthus indefinitely, if left quite undisturbed. If it secures no food, it again leaves its place and seeks another region. Thus that the animal may remain quiet in a given position a considerable number of conditions should be fulfilled, constituting altogether what we may call the "normal" state of the animal. The conditions are the following: (1) the foot should be in contact; (2) the head should be free; (3) the body should be straight; (4) the axis of the body should be in line with gravity, with the head above; (5) the general body surface should be in contact ; (6) food should be received at intervals.

If these conditions are largely unfulfilled, the animal becomes restless, moves about, and finds a new position. But no one of these conditions is an absolute requirement at all times, unless it be that of having the head free. In the wire screen (Fig. 122) the animal remains for a day or so if in the required position with reference to gravity, even though foot and body surface are not in contact. In the horizontal testtube it remains with foot and surface in contact, though the body is not straight nor in line with gravity. If all conditions are fulfilled save that of food, the animal remains for a time, then finally moves away.

Clearly, the holding of any given position depends, not on the relation of the body to any one or two sources of stimulation, but on the proper maintenance of the natural physiological processes of the organism. The animal does not always maintain a certain position with relation to gravity, nor does it always keep its body straight, nor its foot in contact, nor its body surface in contact. It does not at all times receive food. It may remain for considerable periods with one or more conditions lacking. It tends on the whole to take such a position as is most favorable to the unimpeded course of the normal physiological processes. Certain usually required conditions may be dispensed with, provided

other favorable ones are present. The behavior represents a compromise of the various needs imposed upon the animal by its physiological processes. In the sea anemone Antholoba reticulata, according to Burger (1903), the requirements for retaining a given position are extraordinary. This animal is usually found attached to the backs of crabs ; it is thus carried about, and finds much opportunity for obtaining nourishment. If removed from the crab's back, the animals attach themselves to the stony bottom and spread the tentacles. But after four or five days they release their hold on the bottom and invert themselves, directing the foot upward. Now when a crab's limb comes in contact with the foot, the latter attaches itself and folds about the limb, so that the anemone is dragged about by the crab. It now, in the course of several hours, climbs up the crab's leg to its back, where it establishes itself. The sea anemone thus by its own activity attains the extraordinary situation where it is usually found. The whole train of action is like that shown in the complicated and adaptive instincts of higher animals.

The most characteristic reaction of the ccelenterates to intense stimuli of all sorts is a contraction of the whole body. In Hydra and the sea anemones the body is thus shortened and thickened, becoming more nearly spherical. The animals thus shrink close to the substratum and present less surface than before to the stimulating agent. In the medusae the sudden contraction of course carries the animal away from the stimulating object. The first contraction is usually repeated many times, thus inaugurating a period of swimming by which the animal may be widely removed from the stimulus. Such contractions occur in response both to general stimulation and to local stimulation, if the latter is very intense.

Under most circumstances the contraction of Hydra or the sea anemone of course tends to remove the organism from any source of danger, rendering it for example less likely to be seized by a predatory animal. But the reaction takes place in the same way under circumstances in which it is of no defensive value. If the foot of the attached Hydra is strongly stimulated, the animal contracts as usual ; the contraction is then of course toward the source of stimulation, not away from it. If the entire vessel containing the animals is heated to 30 degrees, the Hydras contract, though this of course does not tend to remove them from the high temperature. It is clear that for all sorts of stimuli that are unfavorable these animals have a certain reaction which is usually

regulatory (beneficial) ; they give this reaction whatever the nature of the unfavorable stimulus, even under circumstances where it is not regulatory. This is an illustration of a characteristic general trait of behavior in lower animals ; their reactions are commonly not specific, but general in character. As we shall see later, this contraction is not the final recourse of the stimulated ccelenterate. If stimulation continues, the animal usually sets in operation other activities, which remove it from the stimulating agent.

Hydra. — In Hydra, intense stimuli restricted to a small spot on the body or a tentacle usually produce contraction at that point, sometimes spreading much or little, sometimes not at all. This reaction is produced by many sorts of stimuli. If the contraction remains precisely localized, as it sometimes does, the body or tentacle bends sharply at the point stimulated. A precisely localized chemical stimulus is produced in the following way. . , . , , 7 , A fine capillary glass rod is

certain spot on one side of a Hydra (a). Thereupon dampened and its tip IS this spot contracts, bending the Hydra toward the side dipped in SOme powdered chemical. Methylene blue or methyl green is convenient to use, since the distribution of 4he chemical in the water is easily seen by means of the color. The point of this fine rod, covered with the chemical, is brought close to the body of a Hydra. The chemical diffuses and reaches a small area on the body. Local stimulation by heat may be produced with the simple apparatus devised by Mast (1903). A glass tube is drawn out at its middle to capillary size, then bent so as to form a loop. The two ends are passed through a cork for support, and to them are attached rubber tubes. In this way water of any desired temperature may be passed through the fine tube, and this may be brought close against the body of the animal at any desired point.

When the strong chemical or the heat reaches a certain spot on the body, this spot at once contracts, so that the body makes a knee-shaped bend at this point (Fig. 124). Such a bending is produced by most strong chemicals; strong acids placed in a capillary tube, the tip of which is applied to the body, show it clearly. As a result of the bend the head of the animal becomes directed toward the chemical or the heated region, and is therefore strongly stimulated, so that the Hydra now contracts as a whole. Thus the result of the bending is to carry the most sensitive part of the animal into the injurious agent, where it is still further injured. This reaction is produced only by strong, injurious agents, and is really an incidental result of the local injury produced. The point injured remains contracted for a long time after the stimulating agent has ceased to act. The Hydra may contract completely, so that the bend disappears, but on extension the bend is still found at the injured spot. It is evident that this bending reaction is not a regulatory one, and it is apparently never shown in nature, since the conditions necessary for its production are practically never present. It is a product of the laboratory. As we shall see later, after reaction in this manner, Hydra usually sets in operation other reactions, which do act in a regulatory way.

Sea Anemones. — Intense local stimulation of the column in the sea anemones usually produces a contraction of the entire body, or a movement of tentacles on the side stimulated, in the way described later. In Sagartia (Torrey, 1904, p. 208), stimulation of the edge of the foot induces a local contraction of the foot and base of the column, with discharge of acontia — the defensive weapons of the animal. Local stimulation of the tentacles causes in the different sea anemones various reactions. Often slight local stimulation causes the tentacles to wave about ; this and similar phenomena will be described in connection with the food reactions. In most sea anemones local stimulation of the tentacles, especially if intense, causes them to shorten by contraction, or to collapse and become very slender. This is followed in many cases by a contraction of the whole body. In Aiptasia an immediate contraction of the entire body follows even a slight stimulation of the tip of one of the long tentacles.

Medusa. — In medusa?, intense stimulation of one side of the bell causes immediate contraction of that side, accompanied by a less marked contraction of the remainder of the bell. The stronger contraction on the side stimulated turns the animal away from that side, and its subsequent locomotion removes it at once from the stimulating agent. Thus the appropriate direction of movement is here determined in the simplest way — by contraction of the part stimulated. Such effects are produced by mechanical and chemical stimulations, by heat, by electricity, and apparently by light. In Hydra, as we have seen, identically

the same reaction has the opposite effect, subjecting the animal still further to the action of the stimulating agent; other reactions must supervene before the animal is removed from the stimulus. Intense stimulation of the tentacles of the medusa or of the margin of the bell induces, in Gonionemus, a direct contraction of the tentacles. When the margin or under surface of the medusa bell is locally stimulated, the manubrium behaves in a manner that is of great interest. This has been described by Romanes (1885) m tne medusa Tiar opsis indicans. If the margin or under surface of the bell is sharply stimulated with a needle, the manubrium at once bends over and applies its tip to the point stimu- Fig. 125.— The medusa Tiaropsis lated (Fig. 125). The reaction is thus

point on the margin which has been vel7 precisely localized. How does it stimulated, x, y, z, cuts made for happen that the manubrium is able to bend at once in that direction ? In answer to this question, Loeb presents a very simple explanation, which deserves attention, as it is a type of many of the recent hypotheses put forward to explain the behavior of organisms. According to Loeb, this behavior is due simply to the spreading out of the local contraction caused by the stimulus. "Every localized stimulus leads to an increase in the muscular tension on all sides, which is most intense near the stimulated spot. Now if we decompose each of the lines of increase of tension (aa'} ab', ac' ', ad', ae' Fig. 126) radiating „ „. .,,

from the Stimulated Spot, into a meridional Loeb's explanation of the localization component aa\ dd\ bb', etc., and an equa- aV^i^cToo) ^ manubriumtorial component, it is evident that the latter can have no influence on the manubrium. Only the meridional components can have an influence, and of these the one passing through the stimulated spot is the largest. This fact must necessarily cause a bending of the manubrium toward the stimulated spot" (Loeb, 1900,

This explanation represents the behavior as of the simplest character — a mere spreading of a local contraction from the point stimulated. But is this view adequate to explain the facts? In the protozoa we have found that such local action is as a rule not adequate; that the organism tests the environment; and the behavior at a given moment depends on the success or failure of a previous trial. Is there anything of this kind in the medusa, or does Loeb's simple explanation exhaust the matter?

This question is clearly answered by the experiments of Romanes. He found that if a cut is made parallel to the margin, as at x, Fig. 125, and a point lying below this cut is stimulated, the manubrium is no longer able to locate precisely the stimulated point. It bends, but no longer directly to the point stimulated. This, according to Loeb, is exactly what we should expect. The cut interrupts certain of the lines of tension, so that they no longer pull the manubrium to the precise spot. His explanation, he holds, " also shows why an incision parallel to the margin of the umbrella makes an exact localization impossible and only allows uncertain movements toward the stimulated quadrant" (1900, p. 32). It is easy to see that the manubrium, on Loeb's theory of decomposition of the lines of tension, would be pulled over in the general direction of the stimulated spot, but might not strike it exactly.