Behavior of the Lower Organisms

ditions Paramecium soon becomes acclimatized ; it is change that causes reaction. To this general statement there are certain exceptions. If we place the infusoria in conditions of such intense action that they are quickly destructive, — for example, in 2 per cent potassium bichromate, or in water heated to 38 degrees C, — the animals continue to react till they die. For two or three minutes they rapidly alternate swimming backward with turning toward the aboral side and swimming forward, till death puts an end to their activity. Thus very injurious conditions may produce reaction independently of change. But as a general rule, it is some change in the conditions that causes the animal to change its behavior. The animal, having been subjected to certain conditions, becomes now subjected to others, and it is the transition from one state to another that is the cause of reaction. This is a fact of fundamental significance for understanding the behavior of lower organisms.

But it is not mere change, taken by itself, that causes reaction, but change in a certain direction. This is shown by observation of the behavior of the individuals as they pass from one set of conditions to another. If we place Paramecia on a slide in ordinary water, then introduce into the preparation, by means of a capillary pipette, a drop of ^ per cent sodium chloride, as shown in Fig. 40, we find that the animals react at the change from the water to the salt solution, so that they do not enter the latter. If, on the other hand, the animals are first mixed with ^ per cent salt solution, and a drop of water is introduced into the preparation (as in Fig. 40), they do not react at passing from the salt solution to the water. In the same way, Paramecia at a temperature of 30 degrees react at passing to a higher temperature, but not at passing to a lower temperature. Paramecia at 20 degrees, on the other hand, react at passing to a lower temperature, not at passing to a higher. To these relations we shall return.

A relation which is worthy of special emphasis is the following : The direction toward which the animal turns in the avoiding reaction does not depend on the side of the animal that is stimulated, but is determined by internal relations. The animal always turns toward the aboral side. It is true that with chemical stimuli the stimulation usually occurs on the oral side, so that the animal turns away from the side stimulated. But, as we have just seen, it turns in the same way when all parts of the body are equally affected by the stimulating agent. Furthermore, it is possible to apply mechanical stimuli to various parts of the body, and observe the resulting reaction. If with the tip of a fine glass point we touch the oral side of Paramecium, the infusorian turns directly away from the point touched. But if we touch the aboral side,

the Paramecium turns in the same manner as before, — toward the aboral side, and hence toward the point touched. This experiment is more easily performed, and the results are more striking, with certain of the Hypotricha,1 because these animals do not continually revolve on the long axis, as Paramecium does. The general effect of the avoiding reaction is to cause the animals to avoid and escape from the region in which the stimulus is acting. This may be illustrated for the different classes of stimuli in the following ways.

The effects of this reaction to chemicals may best be seen by introducing a little ^ per cent solution of sodium chloride into the water containing the animals. For this purpose water with many Paramecia is placed on a slide and covered with a long coverglass supported near its end by glass rods. A medicine dropper is drawn to a long, slender point, and with this a drop of the salt solution is introduced beneath the cover-glass, as illustrated in Fig. 40. The Paramecia are swimming about in all directions, but as soon as they come to the

region of the salt solution, the avoiding reaction is given in the way already described, and the animals swim elsewhere. Thus the drop of salt solution remains empty (Fig. 41). Fig. 41.- Slide of Paramecia four minutes ™d"^ the avoiding reaction, SO after the introduction of a drop of \ per cent that Paramecia do not enter them. alkalies, neutral salts, and organic substances, and for strong acids. In the case of acids the reaction differs in certain respects from the behavior under the influence of other chemicals ; this will be brought out later.

The reaction to heat or cold may easily be shown by placing a drop Fig. 40. — Method of introducing a chemical into a slide of infusoria. of hot or cold water on the cover-glass of a slide of Paramecia, or by touching the cover-glass with a hot wire, or a piece of ice. The animals respond by the avoiding reaction, just as when stimulated by a chemical, so that the hot or cold region remains vacant. The intensity of the reaction depends on the temperature, and very hot water causes a much more decided reaction than very cold water.

The avoiding reaction is seen under mechanical stimulation when a specimen in swimming comes against an obstacle. It may also be shown by touching the anterior end of the animal with a fine glass point. A slight disturbance in the water may be induced by injecting a fine stream of water against the animal with a pipette drawn to a capillary point ; the animal then responds by the avoiding reaction, thus swimming elsewhere. Special features in the reactions to various different classes of stimuli will be dealt with in the next chapter.

The reactions thus far described have the effect of removing the animal from the source of stimulation ; they might therefore be characterized as negative. But Paramecia are known also to collect in certain regions, giving rise to what are commonly known as positive reactions. How are these brought about ? A simple experiment throws much light on the cause of such collections. Under usual conditions the animals avoid a -^o Per cent solution of NaCl, so that when a drop of this is introduced into a slide of Paramecia, they leave it empty. But if we mix the animals with ^ per cent NaCl, then introduce into a slide of this mixture a drop of Yjj per cent NaCl, in the way shown in Fig. 40, we find that the Paramecia quickly collect in this drop, though under ordinary circumstances they avoid it. Very soon the drop of -^0 Per cent NaCl is swarming with the infusoria, as in Fig. 43, while very few remain in other parts of the preparation. The phenomena are identical with what has often been called positive chemotaxis.

Careful observation of the movements of the individuals shows, as might be expected, that the Paramecia collect in the ytj- Per cent NaCl merely because they avoid the stronger solution more decidedly. Passage from the -^ per cent solution to the -|- per cent solution causes the avoiding reaction, while passage in the reverse direction does not. The details of the behavior are as follows : The Paramecia in the -|- per cent NaCl are swimming rapidly in all directions, so that many of them are carried toward the drop. On reaching its boundary they do not react

in any way, but swim directly into it. They continue across till they reach the farther boundary, where they come in contact again with the \ per cent solution. Here the reaction occurs. The animals give the avoiding reaction, swimming backward, turning toward the aboral side, and starting forward again, etc. They of course soon come in contact again with the outlying -|- per cent NaCl, whereupon they react as before, and this continues, so that they do not leave the drop of jq per cent NaCl. The path of a single Paramecium in such a drop is like that shown in Fig. 44. Since all the infusoria that enter the drop of Yq per cent NaCl remain, it soon swarms with them.

In place of NaCl, we may use pairs of solutions of other chemicals, one stronger than the other, — taking pains of course not to employ concentrations that are decidedly injurious. With any of the ordinary inorganic salts or alkalies the animals collect in the weaker solution, through the fact that they avoid the stronger one in the way described above. The same concentration of a given chemical may play opposite roles in successive experiments, depending on whether it is associated with a weaker or a stronger solution. In the former case the Paramecia avoid it ; in the latter they gather within it. If the weaker solution surrounds a drop of the stronger, the latter is left empty, and the Paramecia remain scattered through the preparation, as in Fig. 41. If the stronger solution surrounds the weaker, the latter becomes filled with the Paramecia, as in Fig. 43, while the former is left nearly empty. Thus with the same pair of substances we get either a dense aggregation (or what is often called positive chemotaxis), or a certain area left vacant ("negative chemotaxis"), depending on the relation of the two fluids to each other.

If we use pure water in place of the weaker solution, we get the same result ; the Paramecia collect in the drop of water. This is easily shown by introducing a drop of water into a preparation of Paramecia that have been mixed with ^ per cent NaCl ; the water soon swarms with the infusoria. The culture water in which Paramecia live usually contains various salts, and is often alkaline in reaction. If a drop of distilled water is added (as in Fig. 40) to a preparation of infusoria in such culture water, the animals gather in the distilled water.

The same results may be obtained with water of differing temperatures. This is done by surrounding an area of water at the normal temperature with water at a temperature considerably higher or lower. The Paramecia may be placed on a slide in the usual way, with a coverglass supported by glass rods. This slide is then placed on a bottle or other vessel containing water heated to forty-five or fifty degrees. As soon as the Paramecia begin to move about more rapidly in conse-

quence of the heat, a drop of cold water is placed on the upper surface of the cover-glass. At once a dense collection of Paramecia is formed beneath it (Fig. 42). Observation of the movements of the individuals shows that this collection is formed in the same way as the collections produced in chemicals (Figs. 43, 44, etc.). The Paramecia at a distance fig. 42. - a slide of Paramecia is from the cooled region do not turn heated to 40 or 45 degrees, then a drop of ancJ SWim directly toward it. But the cold water (represented by the outline a) . . ....

is placed on the upper surface of the Paramecia are swimming rapidly in cover-glass. The animals collect beneath q]\ directions, and manv enter every instant the region beneath the drop. They do not react on entering, but on reaching the opposite side, where they would pass out again into the heated water, they give the avoiding reaction. This is repeated every time they come to the other boundary of the drop, so that the path of an individual within

Fig. 43. — Collection of Paramecia in a drop of ^5 per cent acetic acid. 44. Every Paramecium that enters the cooled region therefore remains, and soon a dense swarm is formed. A collection may be formed in the same way by resting the slide of Paramecia on a piece of ice and placing a drop of warmed water on the upper surface ; the Paramecia now collect in the warmed region. But the collection is never so pronounced as in the experiment last described, because the Paramecia when cooled move less rapidly.

Thus the Paramecia collect in certain regions because they give the avoiding reaction when passing from certain conditions to others, while when passing in the reverse direction they do not. Paramecia at the normal temperature give the reaction at passing both to hotter and to colder water; they therefore tend to gather in water at the usual temperature. This temperature at which they gather may be spoken of as the optimum. Passage away from the optimum induces the avoiding reaction ; passage toward the optimum does not.

In the case of the chemicals thus far considered, the animals give the reaction at passing from the weaker to the stronger solution, not at passing in the opposite direction, so that they collect in the weaker solution. The optimum for these substances is thus zero, and this naturally results in the tendency of the animals to collect in distilled water. But there are certain chemicals of which the optimum is a certain positive concentration, so that Paramecia give the avoiding reaction at passing to weaker solutions or to water containing none of the substance in question. This is the case with acids and with oxygen. If a drop of very weak acid is introduced into a slide of Paramecia (Fig. 43) that are in ordinary water, the animals quickly gather in the drop. This may be shown by the use of about y^-g to Jjy per cent of the ordinary laboratory solutions of hydrochloric or sulphuric acid, or of -^ to ^5 per cent Fig. 44. — Path followed by a acetic acid. In a short time^the drop is sin*=le Paramecium in a drop of

Observation shows that the method of collecting in the acid is the same as in the cases before described. The rapid movements of the animals in all directions are what carry them into the drop. They do not react in any way at the moment of entering it, but swim across. At the point where they would pass out into the surrounding water they respond by the avoiding reaction ; hence they return to the acid. This is repeated each time that they come to the boundary. Hence all that enter the acid remain till it is crowded. The path of a single Paramecium within a drop of acid is shown in Fig. 44.

In the formation of all these collections the natural roving movements play an essential part. These movements cause any given specimen in the course of a short time to cross almost any given area in the preparation, and hence bring the animals to the introduced drop. The animals do not turn and swim in radial lines toward the drop of acid. If a ring is marked on the upper surface of the cover-glass, as many Paramecia will be found to pass beneath this ring before a drop of acid is placed beneath it as after. But in the latter case all that pass beneath the ring remain, and the collection results. If we wait, before introducing the acid, till all have become nearly quiet, no collection is produced.

We may sum up the usual behavior of Paramecium under the various stimuli of the environment in the following way. The natural condition of the animal is movement. In constant external conditions (unless destructive) the movements are not changed, — that is, there 1 In all these experiments it is assumed, of course, that the preparation contains the infusoria in very large numbers. With scattered specimens only, the results are slow and not striking.

is no reaction, — even though these conditions do not represent the optimum. But as its movements carry the animal from one region to another, the environmental conditions affecting it are of course changed, and some of these changes in condition act as stimuli, causing the animal to change its movements. If the environmental change leads toward the optimum, there is no reaction, but the existing behavior is continued. To a change leading away from the optimum (in either a plus or minus direction), Paramecium responds by the "avoiding reaction." This consists essentially in a return to a previous position, through a backward movement, then in "trying" different directions of movement till one is found which leads toward the optimum. Ex- pressed in a purely objective way, the animal performs movements which subject it successively to many different environmental conditions. As soon as one of the conditions thus reached is of such a character as to remove the cause of stimulation, the avoiding reaction ceases and the infusorian continues in the condition now existing. This method of reacting causes the animals to collect in certain regions (as near the optimum as possible), and to avoid other regions. Thus are produced the so-called positive and negative reactions. The behavior may be characterized briefly as a selection from the environmental conditions resulting from varied movements.

Some details of the behavior under the different classes of stimuli will be given in the next chapter. On the character of the movements and reactions of Paramecium : Jennings 1904 h, 1899, 1 90 1. Special Features of the Reactions to a Number of Different Classes of Stimuli In the preceding chapter the general method of the reactions of Paramecium to most classes of stimuli has been described. In the present chapter certain important details and special peculiarities of the behavior under the different classes of stimuli will be described.

When Paramecium strikes in its forward course against a solid object, it responds usually by the avoiding reaction, as described in the preceding chapter. In such cases the stimulus affects the anterior end of the animal. But if mechanical stimuli affect other parts of the body, will this alter the nature of the reaction? This question may be answered by drawing a glass rod to an extremely fine point and touching various parts of the body with this point under the microscope. The first discovery that we make by this method of experimentation is that the anterior end is much more sensitive than the remainder of the body surface. If the anterior end is touched very lightly, the animal responds by a strong avoiding reaction, while the same or a more powerful stimulus on other parts of the body produces no reaction at all. There is some evidence drawn from other sources1 that the region immediately about the mouth is likewise very sensitive.

A second fact brought out by these experiments is that a stimulus on the posterior part of the body produces a different reaction from a stimulus in front. If we touch the anterior end, or any point on the anterior portion of the body back nearly to the middle, the typical avoiding reaction is produced. But if we touch the middle or the posterior part of the body of a resting specimen, the animal, if it reacts at all, merely moves forward. On the other hand, as we have seen in the preceding chapter, the direction in which the animal turns in the avoiding reaction does not depend on the side of the body stimulated. The animal turns toward the aboral side as well when that side is touched, as when the oral side receives the stimulus.

The reactions which we have thus far described have the effect of removing the animal from the object with which it comes in contact, so that they may be called negative reactions. But under certain conditions, not very precisely definable, Paramecium does not avoid the object which it strikes against. On the contrary it stops and remains in contact with the object. This seems most likely to happen when the animal is swimming slowly, so that it does not strike the object violently. But this does not explain all cases; many individuals seem much inclined to come to rest against solids, while others do not. Often all the individuals in a culture are thus inclined to come to rest, while in another culture all remain free swimming, and give the avoiding reaction whenever they

Fro. 45- — Paramecium at rest against a cotton fibre, showing the motionless cilia in contact with the fibre. ming specimen, it is often seen to react as follows. When it first strikes against an object it responds with a weak avoiding reaction, — swimming backward a short distance, turning a little toward the aboral side, then swimming forward again. Its path carries it against the object again, whereupon it stops and comes to rest against the surface.

The objects against which Paramecium strikes under normal conditions are usually pieces of decaying vegetable matter or bits of bacterial zooglcea. Remaining in contact with these helps it to obtain food. The cilia that come in contact with the solid cease moving, and become stiff and set, seeming to hold the Paramecium against the object (Fig. 45). Often it is only the cilia of the anterior end that are thus in contact and immovable; in other cases cilia of the general surface of the body show the same condition. Meanwhile, the cilia of the oral groove continue in active motion, so that a rapid current passes from the anterior end down the groove to the mouth (Fig. 46). This cur-

FiG. 46. — Paramecium at rest with anterior end against a mass of bacterial zooglcea (a), showing the currents produced by the cilia. rent of course carries many of the bacteria found in the zooglcea or on the decaying plant tissue ; these serve as food for the animal. The cilia of the remainder of the body usually strike only weakly and ineffectively, so that the currents about the Paramecium are almost all due to the movements of the oral cilia. The body cilia directly behind those in contact with the solid are usually quite at rest.