Behavior of the Lower Organisms

If the animal is at rest against a mass of vegetable matter or a bit of paper under the action of the contact stimulus, and it is then struck with the tip of a glass rod, we find that at first it may not react to the latter stimulus at all. A touch that would cause a free swimming specimen to give the avoiding reaction in a pronounced way often has no evident effect on the quiet specimen. Sometimes, however, a touch coming from behind causes the animal to move forward, still remaining in contact with the solid object ; it thus creeps a short distance over the surface of the solid. Finally, a strong blow on the anterior end causes the animal to leave the solid and give the typical avoiding reaction.

Thus we find that under the simultaneous action of the two stimuli the infusorian may either react to the more effective of the two, whichever it is, without regard to the other, or its behavior may be a sort of compromise between the usual results of both. If specimens showing the contact reaction are heated, it is found that they do not react to the heat until a higher temperature has been reached than that necessary to cause a definite reaction in free swimming specimens. Thus Putter (1900) found that at 30 degrees C. all the free specimens are strongly affected, moving about rapidly in all directions, while the attached specimens remain quiet or make only slight vibratory movements. Many of them remain attached until the temperature has reached 37 degrees, when the free specimens are dashing about wildly. At this temperature or a somewhat lower one the attached specimens become free ; they then dash about as furiously as the others. Thus the contact reaction interferes with the reaction to heat, preventing it until a much higher temperature has been reached than is necessary to cause reaction in free specimens.

On the other hand, both heat and cold interfere with this contact reaction. Paramecia much above or much below the usual temperature do not settle against solids with which they come in contact, but respond instead by a pronounced avoiding reaction. At a still higher temperature even the avoiding reaction ceases. A Paramecium coming against a solid presses the anterior end against it and continues to try to swim forward, — succeeding only in revolving on its long axis (Massart, 1901 a).

Specimens in contact with a solid react less readily to chemicals than do free specimens, so that a higher concentration is required to indue ■ the avoiding reaction. On the other hand, immersion in strong chemicals prevents the positive contact reaction ; Paramecia under such conditions coming against a solid react by the avoiding reaction. In this case, then, the effect of the chemical is tO change the method of reacting to Fig. 66. — Paramecia which have

another Stimulus — tO the Solid object, formed a ring about a bubble of co2, l-ii 1 and have then come to rest against the Certain other chemicals have the oppo- g\ass supporting rods, forming two site effect, favoring the positive contact dense groups. reaction. This is notably true of carbon dioxide. In water containing this substance the infusoria are strongly inclined to settle down against any object with which they may come in contact. They thus often form under these conditions dense masses attached to the glass rods used for holding up the cover-glass (Fig. 66), though usually they do not come to rest against smooth, hard objects.

The contact reaction may completely prevent the reaction to gravity. Paramecia placed in a tube which contains many bits of solid matter, or has its walls rough or dirty, usually do not rise to the top, but settle against the solid matter on the wall and remain. They may thus remain scattered through all parts of the tube, or may gather in any portion of it where the material inducing the reaction is found. Specimens at rest against a solid may occupy any position with reference to gravity. In similar ways the contact reaction may prevent the usual reaction to water currents.

The interference between the contact reaction and the reaction to the electric current produces a number of peculiar results. If a weak electric current is passed through a preparation containing many specimens attached to a bit of debris or to the surface of the glass, the free specimens swim at once toward the cathode, while the attached specimens do not react at all. If the current is made stronger, it produces for an instant the usual effect on the cilia of the attached specimens. The cathodic cilia strike forward, the anodic cilia backward. But this does not continue; after a moment the contact reaction resumes its sway, and the cilia have their usual positions. If the current continues, after a short time the cilia are again affected as before; then resume their original positions. This may occur many times, — the two stimuli alternating in their control of the cilia. If the current is made much stronger, the animal finally leaves the solid. It then swims directly to the cathode in the usual way. To induce this reaction in a resting specimen, it requires as a rule two or three times as intense a current as that needed for producing the same effect on free swimming animals.

If the electric stimulus is first in action and the Paramecium then comes in contact with a solid, somewhat different results are produced. If the current is weak, often the animal, swimming toward the cathode, ceases to react to the electricity on coming against the solid ; it may then take up any position on the surface of the solid. If it comes against the surface film of the water, or the surface of the glass slide, it may cease its forward movement only for an instant, then, becoming free, it may swim again toward the cathode. If the current is a little stronger (such as to produce the maximum rapidity of movement toward the cathode, in free swimming specimens), a different effect is produced. The Paramecium stops against the surface of the solid, and places itself transversely or obliquely to the current, with the oral surface toward the cathode (Fig. 67). Here it remains, the current produced by the cilia being everywhere backward save in the oral groove, where it is forward. If the electric current is reversed, the oral cilia strike strongly backward, and the animal at once turns on its short axis till the oral surface

faces the new cathode. It remains in this position till the current is reversed anew. Thus, when in contact with a surface, Paramecia often show a transverse orientation with reference to the electric current. At times the animal while in this position moves forward along the surface with which it is in contact, transversely to the current; on reversal of the current it turns about and moves in the opposite direction. This may often be observed if the Paramecia are placed on a slide in a thin layer of water through which the electric current is passed. Many of them in swimming come against 1 the glass or the surface film of the water. Thereupon they begin to move transversely to the current, as just described. Meanwhile the free swimming specimens continue to pass toward the cathode.

With a Stronger current a Still different taken by Paramecium in contact with effect is produced. The Paramecia are a surface, when under the action of manner that is characteristic for strong currents. On coming in conT tact with the surface film or the glass, the animals at once begin to move backward (toward the anode) instead of forward. This continues as long as the contact continues. On becoming free they swim forward again. The reason for this behavior seems to be as follows : In a strong electric current, as we know, the anterior cilia tend to drive the animal backward, the posterior cilia forward (Fig. 62, b) ; the latter prevail. The contact reaction, as we have seen, causes the cilia behind the region of contact to cease movement. When swimming forward under the conditions mentioned, the Paramecia usually come in contact with the surface at the thickest part of the body, near the middle of its length. Thereupon, owing to the contact reaction, the cilia behind this spot, driving the animal forward, cease to beat, while the cilia in front, driving it backward, continue their action. Hence, the anterior cilia gain the upper hand and force the animal backward.

Why does this contact stimulus thus interfere with the reaction to other stimuli? There are two possible factors to be considered here, one physical, the other physiological. The animal seems actually to attach itself to solids, probably by a secretion of mucus. Such a secretion is very evident in many infusoria, though it has not been demonstrated in Paramecium. This attachment would, in a purely physical way, impede the movements due to other stimuli. While it is possible that this factor may play a small part in the matter, it is clear that it is not the important or essential factor. If it were, we should see the cilia

of the attached animal move in the usual manner under the influence of stimuli, though these movements would not have the usual effect. As a matter of fact, in most cases we see nothing of the kind. The cilia either do not move at all, or move in a manner different from that occurring in free specimens. The essential factor in the interference is a physiological one. When reacting to the contact stimulus, the animal is less easily affected by other stimuli, and when reacting to the other stimuli, it is less easily affected by the contact stimulus. Since the two stimuli in question require behavior of opposite character, it is indeed inevitable that one should give away to the other, or at least modify the behavior toward it ; both cannot receive the usual reaction.

Combinations of other stimuli have been less investigated than those just considered. In any combination the reaction to gravity gives way, as we have seen, to the reaction due to other factors. Paramecia swimming upward react to other stimuli without hindrance, and Paramecia at rest against a surface often show no orientation with reference to gravity. The reactions to chemical and electrical stimuli completely supplant the reactions to gravity. In a vertical tube Paramecia may form collections in any region that becomes impregnated with carbon dioxide or may avoid any region which contains a repellent chemical. If an electric current is passed through a vertical tube, the Paramecia react to it in exactly the same manner as under other conditions, swimming toward the cathode whether this is above or below. Sosnowski (1899) and Moore (1903) have shown that many different stimuli modify the reaction to gravity, changing the direction in which the animals swim. If Paramecia in the culture fluid swim upward, mixture with tap water, or with chemicals of various sorts, often causes them to swim downward. This effect soon disappears, however, and the animals return to the top. Increase of temperature to 30 degrees (Sosnowski), or decrease to 2 degrees (Moore), often has the same temporary effect. The same result is at times produced by shaking or jarring the tube containing the animals ; they go to the bottom, returning in a short time to the top. The effect of all these agents varies with different cultures of Paramecia ; in some cultures the reaction to gravity is easily changed, in others with difficulty or not at all.

Reactions to chemicals often interfere with the reaction to the electric current. If through a preparation of Paramecia that are gathered in an area containing carbon dioxide, as in Fig. 68, A, an electric current is passed, the animals swim to the cathode side of the area, then stop. All gather in this region, seeming to make vain efforts to cross the invisible boundary (Fig. 68, B). Observation of individuals shows that as soon as they reach the boundary of the area of carbon dioxide, they give the avoiding reaction, in the usual way, and pass back into the area. Here they become oriented again by the electric current, and pass again to the boundary, where they react as before. Thus the reaction to the electric current prevails until a region of a sudden change in chemical character is reached; the reaction to this then supplants the reaction to the current. If the current is reversed, the animals gather in the same way at the opposite side of the area of carbon dioxide (Fig. 68, C). If the current is made very powerful and is long continued,

Fig. 68. — Interference of chemicals with the reaction to the electric current. At A Paramecia have gathered in an area containing C02. At B an electric current is passed through the preparation with cathode at the left; the animals gather at the left edge of the area of C02- At C the current has been reversed; the animals are therefore gathered at the right edge of the area. the Paramecia are one by one caused to cross the boundary of the acid area and to swim to the cathode. If a drop of some repellent chemical — as sodium chloride or an alkali — is introduced into a preparation (Fig. 41), the Paramecia of course leave this vacant. If the electric current is passed through the preparation, the Paramecia swim toward the cathode; coming to the boundary of the drop, they swim around it, leaving it empty, and thus reach the cathode. In this case the path followed is a resultant of the operation of the two stimuli, — the orientation due to the electric current and the avoiding reaction produced by the chemical.

If the entire region next the cathode is occupied by a repellent chemical, the Paramecia may be forced by a strong and long-continued current to enter it till they are destroyed. A very peculiar interaction of chemicals and the electric current is seen when Paramecia are placed in physiological salt solution (0.7 per cent) and the current is passed through the vessel. The strong chemical causes the animals to swim backward ; the current orients them in the usual way; the result is that they swim backward to the anode. This phenomenon is to be observed in solutions of various chemicals, as acids, potassium iodide, sodium carbonate, etc. It will probably be found to occur in any solution that causes the animals to swim backward for a considerable time. It should be investigated further. As soon as the Paramecia have become accustomed to the chemical, so that they no longer swim backward within it, they react to the current in the usual way, swimming to the cathode.

Thus we find that under the action of more than one stimulus Paramecium may behave in any of the ways which we mentioned in our first paragraph as conceivable. It may react to the first stimulus without regard to the second, or to the second without regard to the first, depending on which is the more effective. Such results are often produced when both the stimuli are sufficiently strong to cause reaction if acting alone. Which stimulus shall produce its characteristic effect sometimes depends on which comes into action first. Thus, Paramecia in contact may not react to the electric current or to heat; while free Paramecia subjected to the same strength of current or degree of heat do not show the positive contact reaction. This condition of affairs seems to occur throughout the animal series; in higher animals we express the same phenomenon subjectively by saying that attention to one thing prevents attending to others.

In some cases the behavior shown is a resultant of the action of the two stimuli. Examples of this are seen in the movement along a surface under the simultaneous action of the contact reaction and a mechanical shock, or in swimming around a chemical in solution, under the influence of the electric current ; or in swimming backward to the anode when in solutions of strong chemicals. Finally, the effect of one stimulus is sometimes merely to change the method of reaction to another. Thus heat and strong chemicals cause the animal to respond to contact by the avoiding reaction in place of the positive contact reaction ; carbon dioxide has the contrary effect. The modifications of the reaction to gravity above mentioned are examples of the same thing. Cases of this character have much theoretical interest. We shall return to them in considering the variability and modifiability of the reactions of Paramecium.

We have seen in the last section that the behavior of Paramecium under a given stimulus may be determined by the simultaneous presence of other stimuli. The behavior depends not only on the stimulus to which it is primarily reacting, but also upon other external conditions. May the nature of the behavior also depend upon internal conditions? In other words, may the same animal under the same external conditions behave differently at different times? May Paramecium, like higher animals, become modified by the stimuli which it has received, or by its own reactions, so as to react for the future in a manner different from its reactions in the past?

It is difficult to obtain evidence on this question for Paramecium, because the animal moves about so rapidly that it is hardly possible to follow a given individual and determine whether its reactions do or do not change. Much more is known in regard to this matter, as we shall see later, for the fixed infusorian Stentor. But a number of significant facts have been brought out for Paramecium. First we have the fact that the presence of a certain agent or condition may alter the method of reaction to another. Paramecia in heated water react to solids by the avoiding reaction in place of the positive contact reaction; Paramecia in a solution of carbon dioxide, on the other hand, are much more likely to respond by the positive contact reactions. Many conditions — heat, cold, chemicals, mechanical shock, etc. — alter, as we have seen, the reaction to gravity, causing the animals to swim downward instead of upward. Such phenomena indicate that the first agents alter in some way the physiological condition of the animals, so that they now react to the second agent in a changed manner. This conclusion is impressed upon the observer by the behavior of the organisms. Specimens in heated water are swimming about violently, so that we should not expect them to come to rest against solids. Those in carbon dioxide move slowly and seem in a condition predisposing to repose, so that coming to rest against solids is the ' reaction that might be anticipated. The interference between the two stimuli is not purely physical. There is nothing in the physical action of heat or a mechanical jar to make the animals move downward, as happens when the agents reverse the action to gravity. Indeed, in the latter case one can plainly see that the downward movement is an active one. The only explanation possible for such cases is that the animals have become changed in some way by the first stimulus, so that they now react in an altered manner to the second stimulus. Further we find that there are great differences in the reactions of different individual Paramecia, and especially of Paramecia from different cultures.

In studying the reactions to chemicals, one often finds that a few individuals swim directly into the given solution, while the majority give the avoiding reaction on coming in contact with it, and hence remain outside (Jennings, 1899 c, p. 373). While in a certain case individuals from one culture were repelled by^g- per cent lithium chloride, those from another culture were found to be quite indifferent to a solution of the same chemical sixteen times as strong, swimming readily into a drop of J- per cent lithium chloride (Jennings, 1899 c, p. 374). When placed in a vertical tube, Paramecia from certain cultures gather at the top; from other cultures at the bottom; while in other cases they remain scattered throughout the tube (Sosnowski, 1899). Corresponding variations are found in the reaction to water currents. Similar differences are to be observed with regard to the positive contact reaction (Putter, 1900, p. 253). Infusoria in certain cultures are strongly inclined to attach themselves to solids, forming dense masses on the surface; in other cultures such masses are never formed. In fresh cultures the animals are usually much inclined to attach themselves in this way ; in old cultures they are not. Even in a culture where most of the animals attach themselves, there are always a number of specimens which remain persistently free. Variations are to be observed at times in the reactions to electricity (Jennings, 1904 h). One sometimes observes that while most of the specimens in a preparation are reacting to the electric current in a precise way, a few specimens do not react at all, swimming about at random. Sometimes single specimens will be seen swimming toward the anode, while all the rest swim toward the cathode. This is most often observed after the current has been reversed several times.

Whether the variations mentioned in the last paragraph are due to changes which have occurred during the life-time of the animals, or whether they are permanent differences between different individuals we do not know. In either case they are of importance, since they give much opportunity for the action of natural selection. This is a point to which we shall return later. We know, however, that sometimes the behavior of the same individual varies, and in some cases we can form an idea of the nature of the change which has occurred. If a Paramecium is subjected to a strong induction shock, it fails for some time thereafter to react to weak shocks, though at the beginning it reacted to these (Statkewitsch, 1903). This result is probably due to a change in the animal such as we commonly call fatigue. To be explained possibly in a similar way is the following occasional observation. A specimen in the continuous electric current is swimming toward the cathode ; on reversal of the current it retains its orientation and continues to swim forward, — now of course toward the anode. This lasts usually but a short time.

Paramecia which have been living at the usual temperatures show a temperature optimum of about 24 to 28 degrees ; if they are kept for some hours at a temperature from 36 to 38 degrees, the optimum rises to 30 or 32 degrees (Mendelssohn, 1902). A change in the individuals induced in this way is commonly spoken of as acclimatization. Similar changes could doubtless be induced in the reactions to chemicals and to other stimuli ; this has not yet been done.

Paramecia that have long been deprived of food behave in a somewhat different manner from normal individuals (Moore, 1903; Wallengren, 1902 a). But the changes in behavior are apparently due to actual structural changes in the organism, due to lack of food, and rendering it impossible for the animal to move so strongly and rapidly (Wallengren, 1902 a). Paramecia kept in distilled water are found to be much more sensitive to most stimuli than usual (Jennings, 1897; Wallengren, 1902 a) ; owing apparently to lack of sodium salts in the body. This condition may perhaps be called that of salt hunger. If a small quantity of some sodium salt is added to the distilled water, the Paramecia return to the usual condition (Wallengren, 1902 a).

Certain changes in the behavior of individuals can hardly be classified as due either to fatigue, acclimatization, or hunger. If a bit of filter paper is placed in a preparation of Paramecia, the following behavior may often be observed. An individual swims against it, gives the avoiding reaction in a slightly marked way, swimming backward a little; then it swims forward again, jerks back a shorter distance, then settles against the paper and remains. After remaining a few seconds, it may move to another position, still remaining in contact with the paper. Then it may leave the paper and go on its way. All this may happen without the slightest evident change in the outer conditions. So far as can be seen, the Paramecium first responds to the solid by the avoiding reaction, later by the positive contact reaction, and still later suspends the contact reaction, all without any change in external conditions. The changes inducing the change in reaction must then be within the animal.

Again, as we have seen, jarring Paramecia which have collected at the top of a tube often causes them to swim to the bottom of the tube (Sosnowski, Moore). The jarring itself lasts but a moment, while the Paramecia continue for some time after to swim downward. The shock must therefore have changed the physiological condition of the animals, so that they now show a change of reaction to gravity, or possibly a lack of reaction to gravity.1 1 It is possible that the shock merely causes them to swim rapidly in any direction that is open to them. Since they are already at the top of the tube, the only direction open to them is that leading downward.

All together, it is clear that there are differences in behavior due to differences in the internal or physiological condition of the animal, — differences shown even in a single individual at different times. Some of the different physiological conditions may be characterized as fatigue, as acclimatization, as hunger, or the like. In other cases they cannot be definitely characterized. We clearly have slight beginnings of the modification of behavior through the previous experiences of the organism. The analysis of this matter will be carried farther for the behavior of unicellular organisms in the account of Stentor (Chapter X).

At intervals certain extraordinary episodes connected with the processes of reproduction interrupt the usual life of Paramecium. The behavior at such times seems not to differ in any notable manner from the usual behavior. We shall therefore describe it only briefly. Fission. — At times the animal begins to divide into two by a transverse constriction at about the middle. During the early stages of the process the two halves act in unison. The currents of water are driven by the cilia in the same direction over both, and the two halves react to any stimulus as a single animal. If subjected to induction shocks the half at the anode responds by contraction of the ectosarc and discharge of trichocysts, while the cathode half does not. As the constriction separating the two halves becomes very deep, so that they are connected only by a slender strand, they begin to behave more independently. The anterior half at times changes its direction of movement, while the posterior half tries to continue straight forward. The connecting strand is strained and bent or twisted. Soon it breaks, and the two individuals are separated.

Conjugation. — In conjugation two individuals become united by their oral surfaces (Fig. 69), and a complicated process of interchange of nuclei occurs. The union of two specimens seems brought about chiefly by the usual movements and reactions of the animals, taken in connection with a physical change of the body substance in the region of the oral groove. Here the surface becomes viscid, so that if another Paramecium comes in contact with this region, the two stick together. Often two individuals may be seen at rest close together on the surface of a bit of bacterial zooglcea. One drags its posterior end across the oral groove of the other, whereupon the two stick together (Fig. 70, a).

Each tries to continue its course, so that they pull in opposite directions. One may drag the other along with it, or the two may finally pull apart. There is of course a tendency for objects to be brought against the oral groove, owing to the strong current of water that passes along this region ; it is through this fact that Paramecium gets its food (compare Fig. 46). This tendency operates on other Paramecia in the neighborhood as well as on inanimate objects. If two Paramecia are close together with oral grooves facing each other (Fig. 71), this tendency is reciprocal; each tends to draw the other to its own oral surface. On