Behavior of the Lower Organisms

(Fig. 8). Many other special peculiarities of movement are described in the great work of Penard (1902) on these organisms. The conditions under which Amceba lives are not always the same, and as the conditions change, the behavior of Amceba changes also. Such changes in behavior are usually called reactions, while the external agents that induce them are called stimuli. One of the commonest stimuli is that due to contact with a solid object. If a solid body strikes strongly against one side or one end of a

moving Amceba, the part affected contracts and releases its hold on the substratum, and the internal currents start away from it. The Amceba changes its course and moves in another direction. We may call this a negative reaction, since it takes the animal away from the source of stimulation. This reaction can be produced experimentally by touching the animal, under the microscope, with the tip of a glass rod drawn to a minute point (Fig. 9). The animal does not, as a rule, move directly away from the side touched, but merely in some other direction than toward this side. If we touch it at the anterior edge, the part touched stops and contracts, while the current turns to one side at this point, so that the animal moves at an angle with its former course (Fig. 9). Often the course is altered only a little in this way. But if all of one side or one end is strongly stimulated, then a pseudopodium may be sent out on the side opposite, so that the animal moves almost directly away from the stimulated region (Fig. 10).

By repeatedly stimulating Amceba it is possible to drive it in any desired direction. The advancing edge is touched with the rod ; it thereupon withdraws. A new pseudopodium is sent out elsewhere. If this does not lead in the direction desired, it is touched, causing retraction, whereupon the Amceba tries a new direction. This continues Fig. q. — Negative reaction to mechanical stimulation in Amceba. An Amceba advancing in the direction shown by the arrows is stimulated with the tip of a glass rod at its anterior edge (a). Thereupon this part is contracted, the currents are changed, and a new pseudopodium sent out (&).

till a pseudopodium is sent out in the direction desired by the experimenter. The animal may now be compelled to follow a definite straight course, by stimulating any pseudopodium which tends to diverge from this course. If the posterior end of a moving Amoeba is stimulated, the animal con- & tinues to move forward, usually hastening its course a little. The posterior end is of course already contracted, and the new stimulation merely causes it to contract a little more.

The negative reaction is of course the method by which Amoeba avoids obstacles. If an Amoeba in creeping comes against a small solid bodv, the reaction is often less sharply defined than mechamcai stimulus when the entire in the Cases which we have thus far anterior end is strongly stimulated, a , -i i a -l i • i and b, successive stages. The arrow described. A typical example is shown x shows the original direction of in Fig. II. A progressing Amoeba Came motion; the arrows in a show the

in contact at the middle of its anterior In b a new taii (/') has been formed edge With the end Of a dead alga filafrom the former anterior end, uniting ,-,-,1 ,T , , with the old tail (/). to flow forward at the point of contact c, while on each side of this point the motion continued as before. In a short time, therefore, the animal had the form and position shown by the broken outline in Fig. 1 1 ; the filament projected deeply into a notch at the anterior edge. Motion continued in this manner would have divided the Amoeba into two parts. But soon motion ceased on one side (x), while it continued on the side y. The currents in x became reversed

and flowed around the end of the filament into y, as shown at B, Fig. n. Thus the animal had avoided the obstacle by reversing a part of the current and flowing in another direction. But not all mechanical stimuli cause a negative reaction. Sometimes Amoeba, on coming in contact with a solid body, turns and moves toward it, — responding thus by a positive reaction. At times an Amoeba which is moving along on the glass slip used in microscopic work comes in contact by its upper surface with the under surface of the cover-glass. Thereupon it sometimes pushes forth a pseudopodium

Fig. 12. — Amceba velata passing from the slide to the cover-glass, side view. After Penard (1002). At a the animal is creeping in the usual way, with the tentaclelike pseudopodium projecting into the water. At b the pseudopodium has reached the cover-glass and attached itself. At c the animal has released its hold on the slide, and is now attached to the cover alone. on this under surface; the pseudopodium attaches itself; the Amoeba releases its hold on the slide, and now continues its course on the under side of the cover-glass. Penard (1902) has observed this in Amceba velata, when the long, tentaclelike anterior pseudopodium of this animal comes during its feeling movement in contact with the cover-glass. The process is represented in Fig. 12. In a similar manner Amoebae frequently pass to the under side of the surface film of water, creeping on this as if it were a solid body.

Under certain circumstances Amceba seems especially disposed toward this positive reaction. Sometimes an Amoeba is left suspended in the water, not in contact with anything solid. Under such circumstances the animal is as nearly completely unstimulated as it is possible for an Amoeba to be ; it is contact only with the water, and that uniformly on all sides.- But such a condition is most unfavorable for its normal activities ; it cannot move from place to place, and has no opportunity to obtain food. Amoeba has a method of behavior by which it meets these unfavorable conditions. It usually sends out long, slender pseudopodia in all directions, as illustrated in Fig. 13. The body may become reduced to little more than a meeting point for

Fig. 13. — Amoeba proteus suspended in the water, showing the long pseudopodia extended in all directions. After Leidy (1879). greatly increases the chances of coming in contact with a solid body, and it is equally evident that contact with a solid is under the circumstances exactly what will be most advantageous to the animal. As soon as the tip of one of the pseudopodia does come in contact with something solid, the behavior changes (Fig. 14). The tip of the pseudopodium

Fig. 14. — Method by which a floating Amoeba passes to a solid. spreads out on the surface of the solid and clings to it. Currents of protoplasm begin to flow in the direction of the attached tip. The other pseudopodia are slowly withdrawn into the body, while the body itself passes to the surface of the solid. After a short time the Amoeba, which had been composed merely of a number of long arms radiating in all directions from a centre, has formed a collected flat mass, creeping alone: a surface in the usual way. This entire reaction seems a remarkable one in its adaptiveness to the peculiar circumstances under which the organism has been placed.

Positive reactions toward solid bodies are particularly common in the process of obtaining food. In our account of the food reactions we shall give examples of striking and long-continued reactions of this sort. Reactions to Chemicals. — If a strong chemical in solution diffuses against one side or end of the body, the Amoeba contracts the part affected, releasing it from the substratum, while the protoplasmic currents start in some other direction. The animal has thus changed its course. The reaction to chemicals can best be shown in the following way. The tip of a capillary glass rod is moistened, then dipped in some powdered chemical, preferably a colored one, such as methyline blue. This tip is then, under the microscope, brought close to one side of an Amoeba in an uncovered drop of water. As soon as the diffusing chemical comes in contact with one side of the body, the reaction occurs. Chemicals that are fluid may be drawn into an excessively fine capillary tube and the tip of this held near the Amoeba. Some of the variations in the reactions to chemicals are shown in Fig. 15.

Such experiments show that Amoeba is very sensitive to changes in the chemical composition of the water surrounding it, and is inclined to move away whenever it comes to a region in which the water differs even slightly from that to which it is accustomed. It has been shown to react negatively when the following substances come in contact with one side of its body: methyline blue, methyl green, sodium chloride, sodium carbonate, potassium nitrate, potassium hydroxide, acetic acid, hydrochloric acid, cane sugar, distilled water, tap water, and water

Fig. 15. — Variations in the reactions of Amceba to chemicals. The dotted area represents in each case the diffusing chemical. The arrows show the direction of the protoplasmic currents. a. A little methyl green diffuses against the anterior end of an Amceba. The latter reacts by sending out a new pseudopodium at one side of the anterior end and moving in the direction so indicated. b. A solution of NaC! diffuses against the right side of a moving Amoeba (1). The side affected contracts and wrinkles strongly, while the opposite side spreads out (2), the currents flowing as shown by the arrows.

c. A solution of NaCl diffuses against the anterior end of an advancing Amceba. A broad pseudopodium, shown by the dotted outline, pushes out from the posterior region, above the end, and the course is reversed. d. A solution of methyline blue diffuses against the anterior end of an Amceba (1). Thereupon a pseudopodium is sent out on each side of the posterior end at right angles with the original course (2). Into these the entire substance of the animal is drawn (3).

from other cultures than that in which the Amceba under experimentation lives. Reaction to Heat. — If one side of an Amceba is heated, it reacts in the same negative way as to chemicals or to a mechanical shock. The reaction to heat may be observed as follows: An Amceba creeping on the under surface of the cover-glass is chosen for the experiment. The point of a needle is heated in a flame and placed against the coverglass in front of the Amceba, or a little to one side of it. If the needle is not brought too close so as to affect the whole body instead of only

d one side, the animal responds by cs 7? 10 contracting the part affected and moving in some other direction. Reactions to Light. — Light has a peculiar effect on Amoeba. In general its functions seem better performed in the dark; strong light interferes with them seriously. Rhumbler (1898) observed that if Amoebae are suddenly subjected to light while busy feeding on Oscillaria filaments, they cease to feed, and even give out the partly ingested filaments. Harrington and Learning (1900) found that ordinary white light thrown on a moving Amoeba causes it to come to rest at once. Blue light acts in the same way, while in red light the movements are as free as in darkness. Other colors have intermediate effects. Engelmann (1879) found that sudden illumination causes an extended Pelomyxa (which is merely a very large Amoeba) to contract suddenly. It is well known that exposure to strong light is destructive to most lower organisms.

In correspondence with the fact that light interferes with its activities, we find that Amoeba moves away from a source of strong light. If the sun is allowed to shine on it from one side, it moves, as Davenport (1897) shows, in the opposite direction. It thus moves in a general way in the same direction as the rays of light (Fig. 16). It is a peculiar fact that experiments so far have not shown a negative reaction to occur when light is thrown from directly above or below on one side or end of an Amoeba. The fact that the whole body contracts when illuminated, as shown by the work of Engel-

thrown upon it. it changed its mann (1879) on Pelomyxa, would lead us course, occupying successively the to expect that when a portion of the body is illuminated, this would contract, producing thus a negative reaction. But this Fig. 16. — Reaction of Amoeba to light, after Davenport (1897). The Amoeba was first moving in the direction indicated by the arrow x. Light coming from the direction positions 1, 2, 3, 4. The direction of the light was successively changed as indicated by the arrows b, c, d; the numbers 5-14 show the successive positions occupied by the animal. It will be observed that in every case as soon as the direction of the light is changed, the Amoeba changes its course in a corresponding way, so as to retreat steadily from the source of light.

has not been demonstrated. The experimental difficulties are great, and this may account for the lack of positive results. If future work substantiates the fact that fight falling obliquely on one side causes a reaction, while light falling from above or below on one side causes none, this would seem to indicate that the direction of the rays in passing through the body has something to do with determining the direction of locomotion. But in the myxomycete plasmodium, which resembles Amoeba in its movements and in many other respects, light falling from above or below on a part of the body does produce a negative reaction, — the withdrawal of the part affected. Probably further experimentation will show the same thing to be true in Amoeba. Reaction to Electricity. — Electric currents probably form no part of the normal environment of Amoeba, yet the animal reacts in a very definite way when a continuous current is passed through the water containing it. That side of the body which is directed toward the positive pole or anode contracts as if the animal were strongly stimulated here. Then a pseudopodium starts out somewhere on the side directed

Fig. 17. — Reaction of Amoeba to the electric current. The arrows show the direction of the protoplasmic currents; at 1 the direction of movement before the current acts is shown. 2, 3, 4, successive positions after the current is passed through the preparation. toward the negative pole or cathode, and the Amoeba creeps in that direction (Fig. 17). The reaction takes place throughout as if the Amoeba were strongly stimulated on the anode side. If the electric current is made very strong, the anode side contracts still more powerfully, and the Amoeba bursts open on the opposite side. The current is thus very injurious.

In the water in which Amoeba lives are found many other minute animals and plants. Upon these Amoeba preys, taking indifferently an animal or a vegetable diet. Its behavior while engaged in obtaining food is very remarkable for so simple an animal. Spherical cysts of Euglena are a common food with Amceba proleus. These cysts are smooth and spherical, easily rolling when touched, so that they present considerable difficulties to an Amoeba attempting to

ingest them. One or two concrete cases will illustrate the behavior of Amoeba when presented with the problem of obtaining such an object as food. A spherical Euglena cyst lay in the path of an advancing Amceba proteus. The latter came against the cyst and pushed it ahead a short distance. The cyst did not cling to the protoplasm, but rolled away as soon as it was touched, and this rolling away continued as long as the animal moved forward. Now that part of the Amceba that was immediately behind the cyst stopped moving, so that the cyst was no longer pushed forward. At the same time a pseudopodium was sent out on each side of the cyst (Fig. 18), so that the latter was enclosed in a little bay. Meanwhile, a thin sheet of protoplasm passed from the upper surface of the Amceba over the cyst (Fig. 18, 2). The two lateral pseudopodia became bent together at their free ends; the cyst was thus held so that it could not roll away. The pseudopodia and the overlying sheet of protoplasm fused at their Fig. 18. free ends, so that the

cyst was completely enclosed, together with a quantity of water was then carried away .by the animal. Amceba does not always succeed in obtaining its food so easily as in the case described. Often the cyst rolls away so lightly that the animal fails to grasp and enclose it. In such a case Amceba may continue its efforts a long time. Thus, in a case observed by the author, an Amoeba proteus was moving toward a Euglena cyst (Fig. 19). When the anterior edge of the Amceba came in contact with it, the cyst rolled forward a little and slipped to the left. The Amoeba followed. When it reached the cyst again, the latter was again pushed forward and to the left. The Amceba continued to follow. This process was continued till the two had traversed about one-fourth the circumference of a circle. Then (at 3) the cyst when pushed forward rolled to the left, quite out of contact with the animal. The latter then continued straight forward, with broad anterior edge, in a direction which would have taken it away from the food. But a small pseudopodium on the left side came in contact with

■ Amoeba ingesting a Euglena cyst. sive stages in the process. the cyst, whereupon the Amoeba turned and again followed the rolling ball. At times the animal sent out two pseudopodia, one on each side the cyst (as at 4), as if trying to enclose the latter, but the spherical cyst rolled so easily that this did not succeed. At other times a single, long, slender pseudopodium was sent out, only its tip remaining in contact with the cyst (Fig. 19, 5); then the body was brought up from the rear, and the food pushed farther. Thus the chase continued until the rolling cyst and the following Amceba had described almost a complete

Fig. 19. — Amoeba following a rolling Euglena cyst. The figures 1-9 show successive circle, returning nearly to the point where the Amceba had first come in contact with the cyst. At this point the cyst rolled to the right as it was pushed forward (7). The Amceba followed (8, 9). This new path was continued for some time. The direction in which the ball was rolling would soon have brought it against an obstacle, so that it seemed probable that the Amceba would finally secure it. But at this point, after the chase had lasted ten or fifteen minutes, a ciliate infusorian whisked the ball away in its ciliary vortex.

Such behavior makes a striking impression on the observer who sees it for the first time. The Amoeba conducts itself in its efforts to obtain food in much the same way as animals far higher in the scale. In cultures containing many Amcebae and many Euglena cysts it is not at all rare to find specimens thus engaged in following a rolling ball of food. Sometimes the chase is finally successful; sometimes it Is not. Many of the cysts are attached to the substratum. Amceba often attempts to take such cysts as food, sending pseudopodia on each side of and above them, in the usual way, then covering them completely with its body. But it finally gives up the attempt and passes on.

Sometimes when a single pseudopodium comes in contact with a cyst, this pseudopodium alone reacts, stretching out and pushing the cyst ahead of it and keeping in contact with it as long as possible. Meanwhile the remainder of the Amceba moves in some other direction (Fig. 20). Finally the pseudopodium is pulled by the rest of the body away Fig. 20. — A single pseudopodium (.v) reacts positively to a Euglena cyst, its protoplasm flowing in the direction of the cyst and pushing it forward, while the remainder of the Amceba moves in another direction. 1-4, successive forms taken. At 4 the reacting pseudopodium is pulled away from the cyst, whereupon it contracts.

from the cyst. Again, two pseudopodia on opposite sides of the body may each come in contact with a cyst. Each then stretches out, pulling a portion of the body with it, and follows its cyst. Soon the body comes to form two halves connected only by a narrow isthmus. Finally one half succeeds in pulling the other away from its attachment to the bottom. The latter, half then contracts* and the entire Amceba follows the victorious pseudopodium. Amcebae frequently prey upon each other. Sometimes the prey is contracted and does not move; then there is no difficulty in ingesting it. Such a case has been described and figured by Leidy (1879, p. 94, and PI. 7, Figs. 12-19). But the victim does not always conduct itself so passively as in this case, and sometimes finally escapes from its pursuer. This may be illustrated by a case observed by the present writer (Fig. 21).

I had attempted to cut an Amoeba in two with the tip of a fine glass rod. The posterior third of the animal, in the form of a wrinkled ball, remained attached to the rest of the body by only a slender cord, — the remains of the ectosarc. The Amoeba began to creep away, dragging with it this ball. This Amoeba may be called a, while the ball will be designated b (see Fig. 21). A larger Amoeba (c) approached, moving at right angles to the path of the first specimen. Its path accidentally brought it in contact with the ball b, which was dragging past its front. Amoeba c thereupon turned, followed Amoeba a, and began to engulf the ball b. A cavity was formed in the anterior part of Amoeba c, reaching back nearly or quite to its middle, and much more than sufficient to contain the ball b. Amoeba a now turned into a new path ; Amoeba c followed (Fig. 21, at 4). After the pursuit had lasted for some time the ball b had become completely enveloped by Amoeba c. The cord connecting the ball with Amoeba a broke, and the latter went on its way, disappearing from our account. Now the anterior opening of the cavity in Amoeba c became partly closed, leaving only a slender canal (5). The ball b was thus completely enclosed, together with a quantity of water. There was no adhesion between the protoplasm of b and c; on the contrary, as the sequel will show clearly, both remained independent, c merely enclosing b.