Behavior of the Lower Organisms

This statement doubtless does not express the behavior completely, yet the general fact which it sets forth is on the whole clearly evident. The result of this method of action is to make the behavior regulatory, or adaptive. Through it, the bacteria, like higher organisms, avoid injurious conditions and collect in beneficial ones. There are some exceptions to this ; the adaptiveness is not perfect, as nothing is perfect under all conditions. The exceptions are perhaps not more numerous in these lowest organisms than in the highest ones.

Putting all together, the behavior of the bacteria may be summed up as follows : They swim about in a direction determined by the position of the body axis, until the movement subjects them to an unfavorable change ; thereupon they reverse and swim in some other direction. With rapid movements and much sensitiveness to unfavorable influences, this soon results in their finding and remaining in the favorable regions. In the presence of a localized region of favorable conditions

(food or oxygen, for example) the organisms do not show movement in a single direction, adapted to reaching these favorable conditions. On the contrary, they show movements in all sorts of directions; one of these is finally continued or selected by its success. We find again behavior based on the "selection from among the conditions produced by varied movements." The name Infusoria is applied to those unicellular organisms (aside from bacteria) that swim by means of cilia or flagella, as well as to a few others. The organs of locomotion are protoplasmic processes on the body surface. Where these are short and numerous, they are called cilia; where they are long and the organism bears but one or a small number, they are called flagella. The organisms bearing cilia are classed together as Ciliata ; those with flagella are the Flagellata. Figure 31 shows a number of characteristic forms of the Ciliata. Along with the infusoria we shall take up other unicellular organisms or developmental stages that swim by means of such protoplasmic processes, — for example, spermatozoa and swarm spores.

The infusoria are commonly found, as the name implies, in infusions of decaying animal and vegetable matter. One of the commonest and best known of the infusoria is Paramecium, found in water containing decaying marsh plants, or in hay infusion with which some marsh or pond water has been mixed. The behavior of Paramecium has been studied more than that of any other infusorian, so that we shall take this up first as a representative of the group. The behavior of other species will be then examined to discover how far the relations in Paramecium are typical, and to bring out differences — especially points for which Paramecium is not a favorable object of study.

Paramecium (Fig. 32) is a whitish, cigar-shaped animal, living in immense numbers in decaying vegetable infusions, and visible to the naked eye as a minute, elongated particle. The anterior part of the body is slender but blunt, the posterior part thicker, but more pointed. Thus the two ends differ, as in some bacteria, and there is a further Fig. 31. — Examples of ciliate infusoria, a, Spirostomum ambiguum Ehr., after Stein. b, Slentor roeselii Ehr., after Stein, c. Vorticella nebulifera O. F. M., after Biitschli. d, Colpidium colpoda Ehr., after Schewiakoff, from Biitschli. e, Loxophyllum meleagris O. F. M., after Biitschli. /, Stylonychia mytilus Ehr., after Engelmann.

differentiation of the lateral surfaces. One side, the oral surface, bears a broad, oblique groove, known as the oral groove, or peristome, extending from the mouth in the middle of the body forward to the anterior end. When the animal is placed with the oral surface below, the groove extends from the right behind toward the left in front (see Fig. 32). The animal is thus not bilaterally symmetrical, but slightly spiral in form. The surface opposite the oral groove is marked by the presence near it of two large contractile vacuoles ; this may be called the aboral surface. By considering the oral surface as ventral we may distinguish for convenience right and left sides. The entire body is covered with fine cilia, set in oblique rows. Those at the posterior end are a little longer than the others.

As to internal structure, we may distinguish an outer firm layer known as the ectosarc, enclosing an inner fluid portion, the endosarc. The ectosarc is covered by a thin outer cuticle; below this it is thickly set with rodlike sacs, placed perpendicular to the surface and known as trichocysts ; the contents of these may be discharged as fine threads. The endosarc contains two nuclei, the large macronucleus and the minute mirrormrleim food vacuoles; 8> Sullet'' w> mouth; Cieus dna me mmuie micronUCieUS, maCronucleus ; mi., micronucleus; o.g., oral

together with numerous masses of groove; P., pellicle; tr., trichocyst layer. f^^A w.^f „f +1-.^™ „~„l~„„J ;„ The arrows show the direction of movement food, most of them enclosed m Qf the food vacuoles, vacuoles of water. The endosarc is in continual movement, rotating lengthwise of the body, in the direction shown by the arrows in Fig. 32. Between endosarc and ectosarc, but attached to the latter, are the two contractile vacuoles, which at intervals collapse, emptying their contents to the outside. From the mouth (m) a passageway the gullet (g), leads through the ectosarc into the endosarc.

Fig. 32. — Paramecium, viewed from the oral surface. L, left side; R, right side. an., anus; ec, ectosarc; en., endosarc; j.v., Paramecium swims by the beating of its cilia. These are usually inclined backward, and their stroke then drives the animal forward. They may at times be directed forward ; their stroke then drives the animal backward. The direction of their effective stroke may indeed be varied in many ways, as we shall see later. The stroke of the cilia is always somewhat oblique, so that in addition to its forward or backward movement Paramecium rotates on its long axis. This rotation is over to the left (Fig. 33), both when the animal is swimming forward, and when it is swimming backward. The revolution on the long axis is not due to the oblique position of the oral groove, as might be supposed, for if the animal is cut in two, the posterior half, which has no oral groove, continues to revolve.

The cilia in the oral groove beat more effectively than those elsewhere. The result is to turn the anterior end continually away from the oral side, just as happens in a boat that is rowed on one side more strongly than on the other. As a result the animal would swim in circles, turning continually toward the aboral side, but for the fact that it rotates on its long axis. Through the rotation the forward movement and the swerving to one side are combined to produce a spiral course (Fig. 33). The swerving when the oral side is to the left is to the right; when the oral side is above, the body swerves downward ; when the oral side is to the right the body swerves to the left, etc. Hence the swerving in any given direction is compensated by an equal swerving in the opposite direction ; the resultant is a spiral path having a straight axis.

Fig. 33. — Spiral path of Paramecium. The figures 1, 2, 3, 4, etc., show the successive positions occupied. The dotted areas with small arrows show the currents of water drawn The spiral swimming is evidently the resultant of three from in front. factors, — the forward movement, the rotation on the long The spiral course plays so important a part in the behavior of Paramecium that we must analyze it farther. axis, and the swerving toward the aboral side. Each of these factors is due to a certain peculiarity in the stroke of the cilia. The first results from the fact that the cilia strike chiefly backward. The second is due to the fact that the cilia strike, not directly backward, but obliquely to the right, causing the animal to roll over to the left. The third factor — the swerving toward the aboral side — is due largely to the greater power of the stroke of the oral cilia, and the fact that they strike more nearly directly backward. It seems partly due however to a peculiarity in the stroke of the body cilia, by which on the whole they strike more strongly toward the oral groove than away from it, thus driving the body in the opposite direction.

Each of these factors may vary in effectiveness, and the result is a change in the movements. The forward course may cease completely, or be transformed into a backward course, while the rotation and the swerving continue. Or the rotation may become slower, while the continues or increases; then the spiral becomes much wider. This result is brought about by a change in the direction of the beating of the cilia to the left of the oral groove; they beat now to the left (toward the oral groove) instead of to the right (Fig. 34). The result of this is, as the figure shows, to oppose the rotation to the left, but to increase the swerving toward the aboral side. The width of the spiral, or the final

Fig. 34. — Diagrams of transverse sections of Paramecium, viewed from the posterior end, showing the change in the beat of the cilia of the left side. a, Stroke of the cilia in the usual forward movement. All the cilia strike toward the right side (r), rotating the organism to the left (I), as shown by the arrows. b, Stroke of the cilia after stimulation. The cilia of the left side strike to the left, opposing the lateral effect of the cilia of the right side. This causes the animal to cease revolving, and to swerve toward the aboral side (ab). 0, Oral groove, complete cessation of the rotation on the long axis which sometimes occurs, depends on the number and effectiveness of these cilia of the left side that beat toward the oral groove instead of away from it. A large part of the behavior of Paramecium depends, as we shall see, on the variations in the three factors which produce the spiral course.

How does Paramecium meet the conditions of the environment ? Under the answer to this question must be included certain aspects of the spiral movement, described in the foregoing paragraphs. The problem solved by the spiral path is as follows : How is an unsymmetrical organism, without eyes or other sense organs that may guide it by the position of objects at a distance, to maintain a definite course through the trackless water, where it may vary from the path to the right or to the left, or up or down, or in any intermediate direction? It is well known that man does not succeed in maintaining a course under

similar but simpler conditions. On the trackless snow-covered prairie the traveller wanders in circles, try hard as he may to maintain a straight course, — though it is possible to err only to the right or left, not up or down, as in the water. Paramecium meets this difficulty most effectively by revolution on the axis of progression, so that the wandering from the course in any given direction is exactly compensated by an equal wandering in the opposite direction. Rotation on the long axis is a device which we find very generally among the smaller water organisms for enabling an unsymmetrical animal to follow a straight course. The device is marvellously effective, since it compensates with absolute precision for any tendency or combination of tendencies to deviate from a straight course in any direction whatsoever.

The normal movements of Paramecium are adaptive in another respect. The same movements of the cilia which carry the animal through the water also bring it its food. The oral cilia cause a current of water to flow rapidly along the oral groove (Fig. 33). In the water are the bacteria upon which Paramecium feeds ; they are carried by this current directly to the mouth. In the gullet is a vibrating membrane which carries particles inward; the bacteria which reach the mouth are thus carried through the gullet to the endosarc, where they form food vacuoles and are digested.

Not only food, but also other substances, may be brought to Paramecium by the currents due to the movements of the cilia. It is important for understand- •V ;":'•'.;;•., ing the behavior of this Fig. 35. — Paramecium approaching a region containfont Ot It, OT II tne water ing India ink (shown by the dots). The India ink is drawn {§ warmer Or Colder, OF out to the anterior end and oral groove of the animal. ..„ . , i differs in any other way, a sample of this differentiated region is pulled backward in the form of a cone, and as a result of the stronger beating of the oral cilia, passes as a stream down the oral groove to the mouth (Fig. 33). This may best be seen by bringing near the anterior end of a resting Paramecium, by means of a capillary pipette, some colored solution, such as

methyline blue, or by using in the same way water containing India ink. Or if a cloud of India ink, with a definite boundary, is produced in the water containing swimming Paramecia, a cone of the ink is seen to move out to meet the advancing animals (Fig. 35). Thus Paramecium is continually receiving " samples " of the water in front of it. Since in its spiral course the organism is successively pointed in many different directions, the samples of water it receives likewise come successively from many directions (Fig. 33). Thus the animal is given opportunity to "try" the various different conditions supplied by the neighboring environment. Paramecium does not passively wait for the environment to act upon it, as Amoeba may be said, in comparison, to do. On the contrary, it actively intervenes, determining for itself what portion of the environment shall act upon it, and in what part of its body it shall be primarily affected by the varying conditions of the surrounding water. By thus receiving samples of the environment for a certain distance in advance, it is enabled to react with reference to any new condition which it is approaching, before it has actually entered these conditions.

Let us suppose that as Paramecium swims forward in the way just described, it receives from in front a sample that acts as a stimulus, — that is perhaps injurious. The ciliary current brings to its anterior end water that is hotter or colder than usual, or that contains some strong chemical in solution, or holds large solid bodies in suspension, or the infusorian strikes with its anterior end against a solid object. What is to be done? Paramecium has a simple reaction method for meeting all such conditions. It first swims backward, at the same time necessarily / reversing the ciliary current. It thus gets rid of the stimulating agent, — itself backing out of the region where this agent is found, while it drives away the stimulus in its reversed ciliary current. It then turns to one side and swims forward in a new direction. The reaction is illustrated in Fig. 36. The animal may thus avoid the stimulating agent. If, however, the new path leads again toward the region from which the stimulus comes, the animal reacts in the same way as at first, till it finally becomes directed elsewhere. We may for convenience call this reaction, by which the animal avoids all sorts of agents, the

In the foregoing paragraph we have given only a general outline of the behavior. The avoiding reaction has certain additional features, which add greatly to its effectiveness. After getting rid of the stimulus by swimming backward a distance there must be some way of deter- Fig. 36. — Diagram of the avoiding reaction of Paramecium. A is a solid object or other source of stimulation. 1-6, successive positions occupied by the animal. (The rotation on the long axis is not shown.)

mining the new direction in which the animal is to swim forward. It is evident that some method of testing the conditions in various different directions in advance would be the most effective way of accomplishing this. The infusorian now moves in precisely such a way as to make such tests. It will be recalled that in its usual course the animal is revolving on the long axis and swerving a little toward the aboral side (Fig. 33), so that it swims in a narrow spiral. After swimming backward a certain distance in response to stimulation, the revolution on the long axis becomes slower, while the swerving toward the aboral side is increased. As a result the anterior end swings about in a large circle; the animal becomes pointed successively in many different directions, as illustrated in Figs. 37 and 38. From each of these directions it receives in its ciliary vortex a "sample" of the water from immediately in advance, as the figures show. As long as the samples contain the stimulating agent, — the hot or cold water, the chemical, or the like, — the animal holds back and continues to swing its anterior end in a circle — "trying" successively many different directions. When the sample from a certain direction no longer contains the stimulating agent, the animal simply resumes its forward course in that direction. Thus its path has been changed, so that it does not enter the region of the chemical or the hot or cold water. Mechanical obstacles are avoided in precisely the same way, save that of course the ciliary vortex does not bring samples of the stimulating agent, so that the infusorian is compelled to try starting forward repeatedly in various directions, before it finds one in which it can pass freely.

This method of behaving is perhaps as effective a plan for meeting all sorts of conditions as could be devised for so simple a creature. On getting into difficulties the animal retraces its course for a distance, then tries going ahead in various directions, till it finds one in which there is no further obstacle to its progress. In this direction it continues. Through systematically testing the surroundings, by swinging the anterior end in a circle, and through performing the entire reaction repeatedly, the infusorian is bound in time to find any existing egress from the difficulties, even though it be but a narrow and tortuous passageway.

The different phases of this avoiding reaction are evidently due to modifications of the three factors in the spiral course. The swimming backward is due of course to a reversal of the forward stroke of the cilia. The turning toward the aboral side is an accentuation of the swerving that takes place always; it is due to the fact that the cilia at the left side of the body strike during the reaction toward the oral groove instead of away from it. Thus the cilia of both right and left sides now tend to turn the animal toward the aboral side. The difference between the usual condition and that found during the reaction is illustrated in Fig. 34. Finally, the decrease or cessation in the revolution on the long axis is due to the same factor as the increase in swerving toward the aboral side. During the reaction the cilia of the left side oppose the usual revolution on the long axis to the left (as shown in Fig. 34), through the same change which causes them to assist in turning the body toward the aboral side.

The avoiding reaction varies greatly under different conditions, though its characteristic features are maintained throughout. But its different phases vary in intensity depending on circumstances. The backward movement may be long continued, or may last but a short time ; or there may be merely a stoppage or slowing of the forward movement. The swerving toward the aboral side may be only slightly increased, while the revolution on the long axis becomes a little slower. In this case the anterior end swings about in a small circle, as in Fig. 37, so that the animal is pointed successively in a number of directions varying only a little from the original one. With a stronger stimulus the swerving toward the aboral side is more decided, while the rotation on the long axis is slower; then the anterior end swings about a larger circle, as in Fig. 38. The Paramecium thus becomes pointed successively in many directions differing much from the original one. Finally, the rotation on the long axis may com-

Fig . 37. — Paramecium swinging its anterior end about in a small circle, in a weak avoiding reaction. 1, 2, 3, 4, successive positions occupied. pletely cease, while the swerving toward the aboral side is farther increased ; then the Paramecium swings its anterior end about a circle Fig. 38. — More pronounced avoiding reaction. The anterior end swings about a larger circle. 1-5, successive positions occupied. end near the centre (Fig. 39). In this case the animal may turn directly away from the stimulating agent.

Such variations are seen when the infusoria are subjected to stimuli of different intensities. If the animals come in contact with any strong chemical, or with water that is very hot, they respond first by swimming a long way backward, thus course. But often the reaction is so violent that the anterior end swings about in two this violent reaction is the behavior when the stimulus is very weak. A weak stimulus is produced for example by -^ per cent to -^ per

cent sodium chloride, or by water only three or four degrees above the normal temperature. The Paramecium whose oral cilia bring it a Fig. 30. — Avoiding reaction when revolution on the long axis ceases completely. The anterior end swings about a circle of which the body forms one of the radii. sample of such water merely stops, or progresses more slowly, and begins to swing its anterior end about in a circle, as in Fig. 37, thus "trying" a number of different directions. As long as the oral cilia continue to bring it the weak salt solution or the warmed water, the animal holds back, and continues to swing its anterior end about in a circle. When the anterior end is finally pointed in a direction from which no more of the stimulating agent comes, the Paramecium swims forward. The reaction in this case is a very precise and delicate one; in a cursory view the animal seems to turn directly away from the region of the stimulus, — the revolution on the long axis and swinging of the anterior end in a circle being easily overlooked.

Between this delicate reaction and the violent one first described there exists every intermediate gradation, depending on the intensity of the stimulation. Paramecia react to most of the different classes of stimuli which act upon them, in the way just described. Mechanical stimuli, such as solid obstacles, or disturbances in the water; chemicals of all sorts; heat and cold ; light that is sufficiently powerful to be injurious; electric shocks, and certain disturbances induced by gravity and by centrifugal force, all cause the animal to respond by the avoiding reaction, so that it escapes if possible from the region or condition that acts as a stimulus. Certain peculiarities and special features in the action of the different classes of stimuli will be taken up separately in the following chapters.

Stimulating agents produce the same reaction when they act on the entire surface of the body as they do when they reach only the anterior end or oral groove. This is shown by dropping the animals directly into a |r per cent solution of sodium chloride, or into corresponding solutions of other chemicals ; or into hot or cold water. They at once give the avoiding reaction ; they swim backward, turn toward the aboral side, then swim forward, and this reaction may be repeated many times. If the stimulating agent is not so powerful as to be directly destructive, the reaction ceases after a time, and the Paramecia swim about within the solution as they did before in water.

This experiment shows clearly that the cause of the avoiding reaction does not lie in the difference in the intensity of the chemical on the two sides or two ends of the animal, as is sometimes held. For as we have just seen, the animal reacts in the same way when the entire surface of the body is subjected equally to the action of the chemical or the changed temperature. It is clear that the cause of the reaction is the changel from one solution or temperature to another. This is evident further' from the fact that the animal reacts as a rule when the change occurs, but ceases to react after the change is completed. To constant con-