Behavior of the Lower Organisms

Fig. 70. — Groups of individuals adhering to each other by their oral surfaces, from cultures of Paramecia undergoing conjugation, a. Two attached individuals swimming in opposite directions, b, Three individuals attached by their oral surfaces to a fourth, c. Three individuals irregularly attached, d, A conjugating pair, swimming to the left, with a third individual attached by its oral surface to the posterior part of one of these, and a fourth individual transversely attached to the third. The third and fourth were dragged about by the first pair.

the other hand, if the aboral surfaces face each other, the currents tend to separate the two Paramecia. Hence when two Paramecia come in contact it will usually be by the oral surfaces. This often happens under usual conditions, but no conjugation results, because the oral surfaces have no tendency to adhere ; the animals therefore quickly separate again. But at times when the oral surfaces are viscid, specimens which come thus in contact remain united. The succeeding internal processes fall in the field of physiology rather than that of behavior. Details concerning them will be found in text-books of zoology.

Thus nothing seems to be required for producing conjugation beyond the usual movements and the viscidity of the oral region. The present author has been unable, after careful study, to detect any differences in the methods of reacting during periods of conjugation. The groups formed on the surface of solids and the rapid movements of the organisms, described by Balbiani (1861, p. 441), as occurring at such periods, are by no means peculiar to conjugat- / / ing infusoria. They take place in the same Sf% / manner in cultures where none are conjugating. The significant part played in conjugation by the viscidity of the oral surfaces is demonstrated by the peculiar phenomena observed when specimens accidentally come in contact irregularly. This often happens where the animals are numerous. If any part of the body of one specimen comes by chance against the oral surface of another, the two stick together, without regard to their relative position.

two Paramecia together when formed in this way (Fig. 70). The individuals the oral sides face one another. occupy all sorts of irregUlar positions, and each endeavors to swim forward in his own direction. Some are pulled backward, others sidewise, against their vigorous struggles. Often one succeeds in freeing itself, and then swims away; others remain caught in such groups indefinitely. Even moribund specimens and specimens undergoing fission sometimes thus become united irregularly with others. But the regular union of individuals by the oral surfaces is more common than the formation of irregular groups, owing to the strong tendency, produced by the usual currents, for Paramecia to come together at the oral surfaces.

During conjugation the two united individuals behave in much the same way as a single specimen. They revolve on the long axis to the left as they swim through the water, and they react to stimuli by the avoiding reaction in the usual way. The direction of turning in the avoiding reaction seems determined usually by one of the components; the pair always turn toward the aboral side of this particular individual. If subjected in the transverse position to an induction shock, only the specimen next the anode responds by ejecting trichocysts (Statkewitsch,

Let us now try to form a picture of the behavior of Paramecium in its daily life under natural conditions. An individual is swimming freely in a pool, parallel with the surface and some distance below it. No other stimulus acting, it begins to respond to the changes in distribution of its internal contents due to the fact that it is not in line with gravity. It tries various new positions until its anterior end is directed upward, and continues in that direction. It thus reaches the surface film. To this it responds by the avoiding reaction, finding a new position and swimming along near the surface of the water. Now there is a strong mechanical jar, — some one throws a stone into the water, perhaps. The Paramecium starts back, tries certain new directions, and finishes by reacting to gravity in the reverse way from its former reaction; it now swims downward. But this soon brings it into water that is notably lacking in oxygen. To this change it responds as before, trying new directions till it has come near the surface again. Swimming forward here, it approaches a region where the sun has been shining strongly into the pool, heating the water. The Paramecium receives some of this heated water in the current passing from the anterior end down the oral groove. Thereupon it pauses, swings its anterior end about in a circle, and finding that the water coming from one of the directions thus tried is not heated, it proceeds forward in that direction. This course leads it perhaps into the region of a fresh plant stem which has lately been crushed and has fallen into the water. The plant juice, oozing out, alters markedly the chemical constitution of the water. The Paramecium soon receives some of this altered water in its ciliary current. Again it pauses, or if the chemical was strong, swims backward a distance. Then it again swings the anterior end around in a circle (Fig. 38) till it finds a direction from which it receives no more of this chemical; in this direction it swims forward.

Thus the animal swims about, continually hesitating as it reaches regions where the conditions differ, trying new directions, and changing its course frequently. Every faint influence in the water affects it, for the animal is very sensitive. Other Paramecia swim about in the same way. They do not avoid each other, but often strike together; then one or both draw back and turn in another direction. The animal may strike in the same way against stones or the sides of a glass vessel. In such cases it may be compelled to try successively many different directions before it succeeds in avoiding the obstacle, — acting like a blind man who finds a stone wall in his course.

After a time our animal comes against a decayed, softened leaf. At first it draws back slightly, then starts forward again, and places itself against the leaf. The body cilia cease their action, while the oral cilia carry a strong stream of water to the mouth. It so happens that this leaf has lately fallen into the water and has no bacteria upon it, so that the Paramecium receives no food. Nevertheless the animal "tries" it for a while. Other Paramecia may gather in the same way, but after a considerable time they one by one leave the dead leaf. Our Paramecium swims about again, being directed hither and thither by the various changes in the chemical constitution or temperature of the water, till it comes to a region containing more carbon dioxide in solution than usual. It gives no sign of perceiving this, save perhaps by swimming a little less energetically than before. The area containing carbon dioxide is small, and soon the animal comes to its outer boundary, where the water drawn to its oral groove contains no carbon dioxide. It stops, and tries different regions, by swinging its anterior end around in a circle, till it again finds a direction from which it receives carbonic acid ; in that direction it swims forward. Since it behaves in the same way whenever it comes to the outer boundary of the carbonic acid, it remains swimming back and forth within this region, and thus in time explores it very thoroughly. Finally it comes upon the source of the carbon dioxide, — a large mass of bacteria, embedded in zooglcea, that are giving off this substance. The infusorian places itself against the mass of zooglcea, suspends the activity of the body cilia, and brings a strong current of water along the oral groove to the mouth. This current removes some of the bacteria from the zooglcea and carries them to the mouth, where they are swallowed. While the animal is thus occupied, other Paramecia in their headlong course may strike against it. But now it does not react to such a shock at all ; it remains in place, engaged with its food taking. After the animal has been in this position for some time, the sun begins to shine strongly on this part of the pool, heating the water.

All the free-swimming Paramecia in this region thereupon begin to swim rapidly about, repeatedly backing and trying new directions, till a direction is found that leads to a cooler region. But our Paramecium, busy with its food-getting, does not react to the heat at all. The water becomes hotter and hotter, and after a time our infusorian moves about a little, turning over or shifting its position, but still remaining against the zooglcea. All the free swimming specimens have left this region long ago. As the water becomes still hotter, our Paramecium suddenly leaves the mass of zooglcea and now dashes about frantically under the influence of the great heat. It first swims backward, then forward, and tries one direction after another. Fortunately one of these directions soon lead it toward a cooler region. In this direction it continues and its behavior becomes more composed. It now swims about quietly, as it did at first, till it finds another mass of bacteria and resumes the process of obtaining food.

In this way the daily life of the animal continues. It constantly feels its way about, trying in a systematic way all sorts of conditions, and retiring from those that are harmful. Its behavior is in principle much like that of a blind and deaf person, or one that feels his way about in the dark. It is a continual process of proving all things and holding to that which is good. Passing in review the behavior of Paramecium, we find that the animal has a certain set of actions, by some combination of which its behavior under all sorts of conditions is made up. The number of different factors in this set of actions is small, and they are combined into a coordinated system, so that we may call the whole set taken together the action system. The action system of Paramecium is based chiefly on the spiral course, with its three factors of forward movement, revolution on the long axis, and swerving toward the aboral side. The behavior under most conditions is determined by variations in these three factors. Such variations, combined in a typical manner, produce what we have called the avoiding reaction. Other elements in the action system are the resumption of forward movement, in response to stimulation, and the coming to rest against solid objects in what we have called the positive contact reaction. Subordinate activities, playing little part in the behavior, are the contractions of the ectosarc and the discharge of trichocysts.

The action system thus includes only a small number of definite movements. By one or another of these, or by some combination of them, we may expect the organism to respond to any stimulus which acts upon it. We cannot expect each kind of stimulation to have a specific effect, different from that produced by other stimuli, for all any stimulus can do is to set in operation certain features of the action system. Many different stimuli acting on this one organism therefore necessarily produce the same effect. Different organisms have different action systems, so that the same agent acting on different organisms may produce entirely different effects. The nature of the behavior under given conditions depends as much (or more) on the action system of the animal as on the nature of the conditions. In studying the behavior of any organism the most important step is therefore to work out its action system, — the characteristic set of movements by which its behavior under all sorts of conditions is brought about.

The most important features of the action system of Paramecium are those shown in what we have called the avoiding reaction. This, as we have seen, consists essentially in reversing, stopping, or slowing up the forward motion, then swerving more than usual toward the aboral side, while at the same time the rate of revolution on the long axis is decreased. By this combination of movements Paramecium responds to most effective stimuli that act upon it. By it are produced both negative and positive reactions.

Examination has shown us that the cause for this reaction is some change in the conditions; usually some change in the relation of the animal to the environmental conditions. Such changes are brought about chiefly by the movements of the animal. In certain cases they are due to the direction of movement, carrying the animal into environmental conditions which stimulate it ; in other cases they are due to the axial position taken by the animal, this resulting in internal or external disturbances which act as stimuli.

These stimuli produce, as we have seen, not a single, simple, definitely directed movement, comparable to the typical reflex act. On the contrary, stimulation is followed by varied movements, made up of several simultaneous or successive factors, each of which may vary, as we have seen in detail, more or less independently of the others. These movements produce varied effects, as follows: (i) They place the animal successively and in a systematic way in many different axial positions (see Fig. 38); (2) they cause it to move successively and systematically in many different directions; (3) they subject it successively to many different environmental conditions, — of temperature, light, chemicals, mechanical stimuli, etc. Now, it is evident that in this way the animal is practically certain to reach finally a position, direction of movement, or environmental condition, that removes the cause of stimulation, since the latter was due to something wrong in one of these respects. The reaction then ceases, since its cause has ceased; the animal therefore retains the axial position, direction of motion, or environmental condition thus reached. The method of reaction is then of such a character as to bring about whatever is required for putting an end to the stimulation, — whether this requirement is one of orientation, of general direction of locomotion, or of the retention of certain environmental conditions.

Thus the behavior and reactions of Paramecium consist on the whole in performing movements which subject the organism to varied conditions (using this word in the widest sense), with rejection of certain of these conditions, and retention of others. It may be characterized briefly as a selection from among the varied conditions brought about by varied movements. The fundamental question for this method of behavior is, Why does the organism reject certain conditions and retain others? We find that the animal rejects, on the whole, such things as are injurious to it, and accepts those that are beneficial. There are perhaps some exceptions to this, but these are rare and only noticeable because exceptional ; in a general view the relation of rejection and acceptance to injury and benefit is evident. It results in keeping the animals from entering temperatures that are above or below those favorable for the life processes, in causing them to avoid injurious chemicals of all sorts, in saving them from mechanical injuries, and in keeping them in regions containing food and oxygen. Clearly, the animal rejects injurious things, and accepts those that are beneficial.

How does this happen? We meet here the same question that we find in higher organisms and man. How does it happen that in man the response to heat and cold beyond the optimum is by drawing back, just as it is in Paramecium? How does it happen that in both cases there is a tendency to reject things injurious and retain things beneficial? We shall attempt in a later chapter to bring out the relations involved in this problem, in such a way as to make it possibly a little more intelligible; here we shall content ourselves with pointing out the identity of the problem in the infusorian and in man.

The infusoria form a large and varied group of organisms. In the present chapter we shall try to show how far the behavior of Paramecium is typical for the group, and to bring out important differences found in the behavior of other species. Certain features of behavior are better illustrated in other infusoria than in Paramecium ; these we shall treat in detail. This is notably true of the reactions to light, and to a less degree of the reactions to certain other stimuli. Certain infusoria are much more favorable for a study of the modifiability of reactions than . Paramecium, so that we shall examine these relations with care.

We found that Paramecium has a certain set of ways of acting, — of "habits," one might call them, — of which its behavior under most conditions is made up. These are few in number and combined into a connected system, which we have called the "action system." The action system of Paramecium is typical of what we find throughout the infusoria, including both the flagellates and the ciliates. But it becomes modified among different species, in accordance with their varying structure and the conditions under which they live. Practically all the infusoria agree with Paramecium in swimming in a spiral when passing freely through the water, and in the fact that when stimulated they turn toward a certain side, defined by the structure of the organism. But some species instead of swimming freely usually creep along surfaces, while others are attached by one end to solid objects, remaining in the same spot indefinitely. These different methods of life necessitate changes in the action system. We shall take up briefly a number of species, bringing out the essential features of the action system.

The free swimming flagellates move in a spiral, keeping a certain side of the body always toward the outside of the spiral,1 just as Paramecium does. By means of the flagella they draw a cone of water from in front to the anterior end of the body, as happens in Paramecium. Among the flagellates the behavior has been most precisely studied in Chilomonas and Euglena (Jennings, 1900, 1900 a and b). Chilomonas. — Chilomonas is an unsymmetrical organism, of an irregularly oblong form. The body is compressed sideways and bears an oblique notch at the broader anterior end (Fig. 72). Of the two anterior angles which He on either side of the notch, one (x) is larger and lies more to the right than the other (y). From the notch arise two long flagella, by the aid of which the animal swims. Chilomonas often occurs in uncounted millions in water containing decaying vegetation.

In swimming, Chilomonas revolves on its long axis, at the same time swerving toward the smaller of the two angles at the anterior end (Fig. 72, y). The path followed thus becomes a spiral (Fig. 73). The animal often comes to rest against solid objects; it is then attached by one of the two flagella, while the other is free. To most effective stimuli Chilomonas responds by an avoiding reaction similar to that of Paramecium. Its forward movement becomes slower, ceases, or is transformed into a movement backward. Then the animal turns more strongly toward the side which bears the smaller angle, and finally starts forward again. Thus the path is altered. The reaction consists essentially in pointing the anterior end successively in many directions, toward one of which the animal finally swims. The different factors in the reaction vary with the intensity of the stimulation, just as they do in Paramecium. The reaction may be repeated, as in the animal last named, until it finally carries the organism away from the stimulating region. Thus it is clear that in Chilomonas, as in

Fig. 72. — Chilomonas, side view. c. v., contractile vacuole; ft, flagella; g, gullet; nu, nucleus; x, dorsal or upper lip; y, ventral or lower lip. Paramecium, the method running through the behavior is that of the selection of certain conditions through the production of varied movements. When stimulated the animal "tries" many different directions till one is found in which stimulation ceases. This reaction is known to be produced in Chilomonas by heat, by the drying up of the water containing the animals, by mechanical stimulation, by various chemicals, by passage from water containing certain chemicals (acid) to water containing none, and by the electric current. We shall take up certain details of the reactions of Chilomonas in the sections which deal with the different classes of stimuli. Euglena. — Euglena viridis (Fig. 74), like Chilomonas, swims in a spiral. The larger lip (Fig. 74, x) is always toward the outer side of the spiral (Fig. 94). When stimulated by coming in contact with a weak chemical, by a mechanical shock, or by a change in the intensity of light, Eu- glena responds by an avoiding reaction similar to that of Paramecium and Chilomonas. The Fig. 74. — Eu- forwar(j motion becomes

Kent, c v., reservoir slower, ceases, or (more of the contractile vacurareiy) Js transformed swerves more strongly than usual toward the larger lip. Thus the spiral becomes wider and the organism becomes pointed successively in many directions (see Fig. 91). In one of these directions it finally swims forward, repeating the reaction if again stimulated. We shall have occasion to describe in detail the reactions of Euglena to light (Chapter VIII).

Fig. 73. — Spiral path of Chilomonas. a, b, c, d, successive positions occupied. To most very intense stimuli Euglena responds by contracting into a sphere and beginning to encyst. The behavior of most other flagellates is not known in detail, since the organisms are usually very minute and their precise movements can be followed only with much difficulty. Cryptomonas ovata is known to respond to stimuli in essentially the same way as Euglena (Jennings, 1904 a), — the swerving being toward the more convex surface. The flagellate swarm spores of various algae react in much the same way, as is shown by the descriptions of Naegeli (i860) and Strasburger (1878), though the precise details have not been worked out as they have for Chilomonas, Euglena, and Cryptomonas. Naegeli (/. c.,p. 101) describes the behavior of the flagellate swarm spores on coming against a mechanical obstacle, as follows: They swim backward, turn to one side, then swim forward in the changed direction. This is exactly what Chilomonas does, as we have seen. Similar observations have been made on flagellates by various investigators, but only in the species we have named has the side toward which the organism turns been determined.

In many free swimming ciliates the action system is known to be essentially similar to that of Paramecium. All swim in spirals, swerving toward a certain side, and react to stimuli by backing and swerving more than usual toward a structurally defined side. Loxodes rostrum Fig. 75. — Reaction of Loxophyllum meleagris. 1-4, successive positions. Fig. 76. — Methods of reaction to strong stimuli in Stentor. The individual at 1 is stimulated; it thereupon swims backward (2, 3), turns toward the right aboral side (3, 4), and swims forward (5).

in reacting turns toward the aboral side. Loxophyllum meleagris reacts as a rule by turning toward the oral side (Fig. 75). Stentor polymorphic, Stentor caruleus, and Stentor rceselii (Fig. 31, b), when free swimming, react by turning toward the right aboral side (Fig. 76). Bnrsaria truncatella reacts to most stimuli by swimming backward and turning toward the right side (Fig. 77). Spirostomum amhiguum and Spirostomum tenue swim backward and turn toward the aboral side. Opalina ranarum turns toward the more convex (right) side, Nycto-

therus toward the aboral side (Fig. 78). Many of these organisms show an additional reaction to strong stimuli, consisting in a marked contraction of the body. This is particularly noticeable in Spirostomum and Stentor. Many of the Ciliata do not as a rule swim freely through the water, but creep along surfaces, keeping one side against Fig. 77. — Reaction of Bursaria, ventral view, the Surface. This is true at i-5, successive positions occupied. tjmes Qf mQst Qf tJle organisms

mentioned in the foregoing paragraph. It is much more usual in certain other ciliates, belonging to the group of Hypotricha (Fig. 31, /; Fig. 81). In these animals the cilia of one side of the body are specially modified for creeping, while the opposite side bears either few and weak cilia or none at all. The Hypotricha are usually found running about on the bottom, or on the surface of objects in the water. In addition to their creeping movements, they produce by means of strong peristomal cilia a vortex leading back to the mouth. These animals of course do not revolve on the long axis as they progress, and the corresponding feature is likewise lacking in the reactions to stimuli. On coming in contact with an obstacle, or when otherwise stimulated, they stop or , FlG- j}-~ N-vct°-

move backward a distance, then turn toward a certhe right shows the tain structurally marked side, keeping in contact with direction of turning in the substratum and not revolving on the long axis, tion, while the three This renders it much easier to observe the precise interjor arrows indicate method of reacting than in Paramecium, where the the cilia. After Dale rapid revolution on the long axis is very confusing, to01)- As examples of the creeping infusoria, the following may be mentioned : — Stylonychia (Fig. 31, /), Oxytricha, and other Hypotricha react to most stimuli by moving backward and turning to the right (Fig. 79). These organisms are particularly favorable for the study of the reaction method. The body is flat, and the right and left sides are very easily

79. — Reaction of Oxytricha, ventral 4, successive positions. distinguished, so that the direction of turning after stimulation can be determined with the greatest ease. In many respects the Hypotricha are among the most favorable objects to be found among unicellular animals for studying behavior. Microthorax sulcatus usually creeps along the bottom, and reacts to most stimuli by turning suddenly toward the convex ("dorsal") edge. The turning may or may not be preceded by a start backward.

Colpidium colpoda (Fig. 31, d) usually moves forward with one side against the substratum, following a curve with its oral edge on the concave side of the curve. When stimulated mechanically or chemically, it turns toward the aboral side and continues its course (Fig. 80). In some cases the reaction to strong stimulation takes on special features. For example, in Pleuronema chrysalis, in Halteria grandinella, and in various Hypotricha, there are powerful bristle-like cirri,