Behavior of the Lower Organisms

result is that all the specimens which swim in any direction but that toward the cooler water are quickly stopped and turned, while all that pass toward the cooler water continue in that direction. Since all the specimens in the heated region are moving very rapidly and turning at very brief intervals, in a short time all will have become directed toward the cool water. Hence soon after the water has been heated at one end of the trough, a stream of Oxytrichas will be seen passing

toward the cool water. The animals are all "oriented" in a common direction, but the orientation has taken place by exclusion — through the fact that movement in any other direction is at once stopped. If one end is cooled to 10 degrees C. or below, while the other is left at the usual temperature, the Oxytrichas same way in the hence they leave it, as they before left the heated region. The reaction in the case of cold is much less striking and less complete than that produced by heat. This is because the cold has the effect not only of producing the avoiding reaction, but also that of making the movements slower, and of finally benumbing the animals, so that they cease to move. Thus it takes much longer for the animals to pass out of a cold region than out of a warm region, and many of them do not succeed in escaping before the cold has stopped their movements.

The reaction of Oxytricha is essentially similar to that of Paramecium. But in Oxytricha the method of reaction is much more evident, because the movements are slower, and there is usually no revolution on the long axis. In many other infusoria the reaction to heat and cold has been shown to take place in the same manner as in Oxytricha. In some species the individuals show this type of behavior, yet with slight modifications that are such as to make the reaction quite ineffective, so that the animals do not escape from the heated region, and are finally killed. This may be observed in Bursaria truncaiella. If one end of a trough containing specimens of Bursaria is heated, the animals respond with the avoiding reaction, as Oxytricha does. They begin to swim back-

Fig. 85. — Bursaria swimming backward in a circle when heated. Ventral view. ward, and at the same time to circle to the right (Fig. 85). But they do not alternate this with movement forward, as Paramecium and Oxytricha do, and they do not revolve on the long axis. Bursaria simply continues the reaction once begun, and this of course has little tendency to remove the organisms from the heated region. They circle about till they die. Among different infusoria all gradations may be found, from the ineffective reaction of Bursaria through the moderately rapid but effective behavior of Oxytricha to the quick movements of Paramecium, which can be followed only with much difficulty.

Mendelssohn (1902) has determined the optimum temperature for a considerable number of infusoria. He finds the following values : Paramecium aiirelia, 24-28 degrees; P. bursaria, 23-25; Pleuronema, 25-27; Col pod a, 25-31; Spirostomum teres, 24-33; Coleps, 28-31; Stentor, 25-28; Chlorogonium, 23-30. As a rule the organism is stimulated by temperatures both above and below the optimum, so that it seeks the optimum region. But in rare cases a higher temperature acts as a stimulus, while a lower temperature does not. This is true, according to Mendelssohn, in Pleuronema.

If the entire vessel containing the infusoria is heated, or if the animals are dropped into heated water, the avoiding reaction is produced, just as when the heat is applied from one side. The animals swim backward and turn to one side. It is thus evident that there need not be differences of temperature in different parts of the body in order to produce the avoiding reaction. In the experiment just mentioned the animal "tries" swimming in many different directions, but of course does not find a direction that takes it away from the heated region.

Like Paramecium, most colorless infusoria do not react at all to light of ordinary intensity. But many species of infusoria are colored, and these commonly react in a decided manner even to the light supplied by the natural conditions of existence. Some react positively; they gather in lighted regions or swim toward the source of light. Others are negative, avoiding light regions and swimming away from the source of light. We shall take up as examples the behavior of a negative organism, Stentor cceruleus, and of a positive organism, Euglena viridis.

The blue Stentor is a trumpet-shaped organism, with a circle of large adoral cilia or membranelke surrounding the large end or peristome. This circle leads to the mouth, lying at one side of the disklike peristome. The remainder of the body is covered with finer cilia.1 The animal is colored a deep blue. Stentor is often attached to solid objects by its pointed end or foot, but it is likewise found at times swimming freely. We shall have occasion to study the general features of the behavior of Stentor, particularly when attached, in a later section (Chapter X). Here we need to recall only the facts that in response to strong stimulation it may contract, becoming shorter and thicker, and that when free swimming it has an avoiding reaction similar to that of Paramecium. When stimulated, it stops or swims backward, turns toward the right aboral side, and continues forward in the new direction (Fig. 76). This is the reaction produced by mechanical stimulation, by heat, and by chemical stimulation acting either on the anterior end or on the body as a whole. The results of localized stimulation have shown clearly that the anterior end or peristome is more sensitive than the remainder of the body surface.

1 For a figure of another species of Stentor, resembling in essentials the present one, see Fig. 31, b. The blue Stentor tends to gather in shaded regions, and when subjected to light coming from one side it moves away from the source of light. Thus, if a glass vessel containing Stentors is placed near a window, the animals swim away from the source of light, and are soon found to be collected on the side opposite the window. How is this result brought about? Just what is the cause of the reaction to light, and what is the behavior of the Stentors in reaching the shaded regions ?

In arranging experiments which shall answer these questions, let us first try the effects of sudden strong changes in the intensity of the light affecting the animals. This may be done by placing a flat-bottomed glass vessel containing many Stentors in a shallow layer of water on the stage of the microscope in a dark room. From beneath, strong light is sent directly upward through the opening of the diaphragm by means of the substage mirror, while all other light is completely excluded. In this way a circular area in the middle of the field is strongly illuminated, while the remainder of the vessel containing the Stentors is in darkness.1 The Stentors in the darkness

Fig. 86. — Reaction of Stentor at passing from a dark to a light region (1-4). about in all directions, but as soon as one comes to the lighted area it at once responds by the avoiding reaction — it swims backward and turns toward the right aboral side (Fig. 86, 1-4). Thus its course is changed and it does not enter the lighted area. Since every Stentor reacts in this way, the lighted area2 remains empty. Usually the avoiding reaction occurs as soon as the anterior end of the Stentor has reached the lighted region. In other cases the entire Stentor passes completely into the lighted area, then reacts in the usual manner, thus passing back into the dark.

1 By using a projection lantern as the source of light the field of the microscope is projected on the ceiling, or, by the use of a mirror to reflect the light at right angles, on the ordinary projection screen. When thus projected, the behavior of the Stentors is observable with the greatest ease. 2 The light is passed first through a thick layer of ice water, in order to remove the heat as far as possible. The fact that the reactions are not due to heat is shown in the following manner. Specimens of Paramecium, an organism which is more sensitive to heat than Stentor, but is not sensitive to light, are mingled with the Stentors. The Paramecia pass into the lighted region without hesitation, showing that this region is not heated sufficiently to affect them ; the heat then cannot affect the Stentors.

Thus an area righted from below acts in the same manner as a region containing a strong chemical. The animals keep out of both by the avoiding reaction. We may now arrange the conditions so that the light shall come from one side, while at the same time differences in illumination shall exist in different regions. This may be done by placing the glass vessel containing the Stentors near a source of light which falls obliquely from one side, then shading a portion of the vessel with a screen. We may first so place the screen that the vessel is divided into right and left halves, at equal distances from the source of light, but one shaded, the other illuminated (Fig. 87). The Stentors are at the beginning scattered throughout the dish and are moving in all directions. Stentors in the illuminated half whose path lies in the proper direction pass into

Fig. 87. — Reaction to light in Stentor. The light the shaded region without comes from the left, as indicated by the arrows. 5-5 is a reaction Since nearlv all screen shading one half the vessel, so that the line x-y is ; . ' the boundary of the shadow. At b, 1-4, is shown 'the keep in motion for a long reaction of a Stentor on reaching this boundary line, tirnp after an interval (The dotted outline a, 1-4, shows the reaction that would occur if the light caused increased activity in the cilia of nearly all Will have passed

Stentors in the shaded half respond by the avoiding reaction as soon as they come to the boundary of the lighted area. That is, they swim backward and turn toward the right aboral side (Fig. 87, b). Thus they remain within the shaded area, and after a short time most of the Stentors in the vessel are to be found in the shaded half. It is evident that the Stentors do not simply turn and swim parallel with the light rays from the source of light. If this were the method of reaction, a Stentor coming to the boundary x-y, Fig. 87, would turn and swim directly toward the side y. This it does not do. The direction of turning depends upon the position of the right aboral side ; the

animal may even turn toward the source of light. The essential point is the swimming back into the shaded region, without reference to the direction from which the light comes. Similar phenomena are observed if the side of the vessel next to the source of light is shaded, the shadow of the screen reaching to the middle Fig. 88. — Reaction of Stentor to light when one half the vessel next the source of light is shaded by a screen S-S (as indicated in Fig. 89). On reaching the line x-y, where it would pass into the light, the animal responds as shown at c, 1-5.

of the vessel, so that the side farthest from the source of light is illuminated (Figs. 88 and 89). Under such circumstances the Stentors gather in the shaded area, next to the window. A specimen in the shaded area which swims toward the lighted side is of course moving when it comes to the boundary line in the same direction as the rays of light. It nevertheless responds by the avoiding reaction, — stopturning toward the right aboral side, and swimming back to the shadow. This often happens when the animal has completely passed the boundary t, 0 c-j • t tu r»- • tk and is entirely within the

experiment shown in Fig. 88. The arrows show the lighted area (Fig. 88, b). In ened area it now swims of course directly toward the source of light. All together, then, our experiments thus far have shown that the cause of the avoiding reaction is the change from darkness to light. At every such change, Stentor responds by the avoiding reaction ; that is, it tries swimming in other directions until it is no longer subjected to the light. Let us now arrange the conditions in such a way that all parts of the

vessel are equally illuminated and the light comes from one side. This may be done by placing the Stentors in a glass vessel with plane sides, at one side of the source of light, as a window or an electric lamp. Movement from one part of the vessel to another cannot cause a change from darkness to light, for all parts are equally lighted.1 Yet the Stentors Fig. 90. — Method of observing the reaction of Stentor to light. A and B are two electric lights, which can be extinguished or illuminated separately.

usually, after a short interval, turn and swim away from the source of light, after a time reaching that side of the vessel farthest from the lamp or window. If the animals are observed as they turn, it is found that the turning is brought about through the avoiding reaction. A short time after the light is directed upon them, they swim more slowly or cease the forward movement, and begin to swerve more strongly toward the right aboral side, thus swinging the anterior end about in a circle. The direction of movement thus becomes changed ; in the new direction the animal swims forward. If its anterior end is still not directed away from the source of light, the avoiding reaction is repeated; the animal continues to try new directions till the anterior end is directed away from the lighted side. In that direction it continues to move, so that it finally comes to the side opposite the window or lamp.2

1 There is of course an infinitesimal difference in the illumination of different parts of the vessel, due to the fact that one part is nearer the source of light than another. The experiment succeeds equally well when the sun is employed as the source of light, in which case the difference of illumination in different regions is practically infinitely minute. The reaction cannot be therefore conceived as due to these differences. Experiments show that the differences in illumination necessary to produce reaction are much greater than those obtaining in different parts of a vessel thus lighted from one side.

2 The reaction may be obtained by focussing the Braus-Driiner binocular microscope on a shallow vessel of Stentors swimming about at random in a diffuse light, then allowing a strong light from an electric lamp or a brightly lighted window to fall upon them from one side. In order to have the reaction repeated many times, so as to give opportunity for careful study, the vessel containing the Stentors may be placed between two electric lights, as in Fig. 00. One of these lights can be extinguished at the same instant that the other is brought into action ; by repeating this process the direction of the light rays is repeatedly reversed. At each reversal the Stentors react in the way described in the text.

Why does the animal react in this way, even when the vessel is not divided into regions of light and darkness, but is lighted from one side ? The essential problem is, Why does a specimen swimming transversely or obliquely to the direction of the light rays give the avoiding reaction and continue this until the anterior end is directed away from the source of light ? To understand this, certain facts need to be recalled. We know that the anterior end is much more sensitive than the remainder of the body. We know that an increase in illumination causes the avoiding reaction. We know that this is true even when the anterior end alone is subjected to such a change. Now, Stentor swims in a spiral of some width, so that its anterior end swings always in a circle, and is pointed successively in many different directions. If the animal is swimming transversely or obliquely to the direction of the light rays, the anterior end in one phase of the spiral path is directed more nearly toward the source of light, in another phase more nearly away from it, so as to be partly shaded, — as is illustrated for Euglena in Fig. 94. The result is, of course, that the sensitive anterior end is subjected to repeated changes in intensity of illumination ; at one instant it is shaded, at the next the light shines directly upon it. As we know from other experiments, the change from light to darkness produces no reaction, while the changes from darkness to light produce the avoiding reaction. Every time, therefore, that the anterior end swings into the light, the avoiding reaction is caused; the animal therefore swings its anterior end in a large circle, trying many directions. Every time it swings its anterior end away from the source of light into the shadow of its body, on the other hand, no reaction is produced; the position thus reached is therefore retained. This process continues, the animal trying new directions every time its anterior end swings toward the light, until in a short time the anterior end must inevitably become directed away from the light.

In this position the anterior end is no longer subjected to changes in illumination, for the axis of the course coincides with the axis of the light rays, and the body maintains a constant angle with the axis of the course. The amount of light received by the anterior end therefore remains constant. Hence there is no further cause for reaction, and the organism retains the position with anterior end directed away from the source of Hght. Attached specimens of Stentor do not become oriented with reference to the light. They may occupy any position with reference to the direction from which the light comes, even though the light shines directly on the anterior end. We have seen previously that contact interferes with many of the reactions of organisms. But if the animals are

subjected to a sudden, powerful increase in the intensity of the light falling upon them, they often contract (Mast, 1906), and later bend in various directions, till they have become accustomed to the light. To sum up, the orientation of the free Stentor in line with the light rays, with its anterior end directed away from the source of light, is due to the fact that an increase of illumination at the sensitive anterior end induces the avoiding reaction. As a necessary result the oriented Stentor swimming in a spiral path tries new directions of movement until it finds one where such changes of illumination no longer occur. Such a direction is found only in orientation with the anterior end directed away from the source of light. From a knowledge of the spiral course and the fact that increase of illumination at the anterior end causes the avoiding reaction, this result could be predicted. The reaction to light, like that to most other stimuli, is based on the method of trial of differently directed movements, till one puts an end to the stimulation.

Euglena is not closely related to Stentor; it is a flagellate, while Stentor is a ciliate. If we find similar principles governing the reaction to light in these widely separated organisms, it is probable that these principles are valid for the infusoria in general. Euglena viridis (Fig. 74) is a fish-shaped green organism, often found abundantly in the water of stagnant roadside pools, giving them a green color. At the anterior end is a notch from which there extends a single long flagellum, by the lashing of which Euglena swims. Within the body are chlorophyll masses, giving the organism its green color. Near the anterior end, close to the side bearing the larger lip of the notch, — the "dorsal" side, — is a red pigment spot, usually known as the eye spot. As we have seen previously, the "action system" of Euglena resembles in essentials that of Paramecium. It swims in a spiral (Fig. 94), and to most stimuli it responds by an avoiding reaction which consists in stopping or backing, then turning more strongly than usual toward the "dorsal" side.

If the light is not too strong, Euglenae gather in lighted areas, and when the light comes from one side, they swim toward the source of light. Thus in the culture jar the organisms are usually found on the side next the window or other source of light. In very powerful light, such as the direct rays of the sun, however, Euglena swims away from the source of light. How is this behavior brought about ? Let us first study the effect of changes in the intensity of the light. The Euglenae are placed on a slide in a thin layer of water, and are ex-

amined with the microscope in the neighborhood of a window. Soon all the Euglenae are seen swimming toward the window. Now the Fig. 91. — Diagram of the reaction of Euglena when the light is decreased. The organism is swimming forward at 1 ; when it reaches 2 it is shaded. It thereupon swerves toward the dorsal side, at the same time continuing to revolve on the long axis, so that its anterior end describes a circle, the Eu- glena occupying successively the positions 2-6. From any of these it may start forward in the directions indicated by the arrows.

light is decreased by placing the hand or a screen between them and the window. At once all give the avoiding reaction; that is, they stop or swim backward an instant, then swerve strongly toward the dorsal side, so that the anterior end swings about a circle (Fig. 91). If the light is decreased strongly, the anterior end describes a wide circle or may even turn through an angle of 180 degrees, so that the direction of movement is reversed. If only a little of the light is cut off, the anterior end describes only a narrow circle. The organisms soon resume the forward movement, but now the axis of the spiral path coincides with one of the directions indicated by the anterior end in swinging about

Fig. 92. — Change of direction in the spiral path of the Euglena, as a result of a slightly marked reaction. At a the illumination is decreased, causing the organism to swerve toward the dorsal side, thus widening the spiral path. At b the ordinary swimming in a narrow spiral is resumed; since at this point the organism was necessarily more inclined to the axis of the spiral than before the reaction, the new course lies at an angle to the previous one.