Behavior of the Lower Organisms

2 He says that when the direction of the light is changed, the swarm spores become oriented " Nach verschiedenen Schwankungen." in the swarm spores as in Euglena. As set forth on page 139, the movement toward or from the source of light, in a field of which all parts are equally lighted, is due to the fact that in the unoriented individuals the sensitive anterior end is subjected to frequent changes in the intensity of illumination. It is first directly lighted, then shaded. These changes induce reaction. By the method of trial the organism then comes into a position such that these changes cease. Such a position is found only in orientation. All these relations evidently hold equally well for the swarm spores; for details the reader may refer to the account of the behavior of Euglena.

What happens if the field containing the organism is righted from one side, and there are at the same time variations in the intensity of light in different parts of the field? Strasburger devised certain experiments to answer this question. These experiments have become celebrated, and an immense amount of ingenuity has been expended in endeavoring to interpret them in one way or another. Strasburger's experiments involved the use of the wedge-shaped prism shown in Fig. 99. This prism was placed over the drop containing the swarm spores, in such a way that the light came obliquely from the direction of the thick end of the wedge, as in Fig. 99, Y. Now the intensity of illumination is greater on the side farthest away from the source of light, and decreases as we pass toward the source of light. Will the positive swarm spores move toward the source of light, and thus into a region of less illumination, or will they rather move into the region of greater illumination, and thus away from the source of light?

Strasburger found that the positive swarm spores move toward the source of light, and hence into the region of less illumination. It is extraordinary that this result should have occasioned the surprise and comment which have been bestowed upon it. Strasburger's previous experiment with perpendicular light (Fig. 99, X) had shown that the variations in intensity of illumination in different parts of a drop under this prism were too slight to cause reaction, the organisms remaining scattered throughout the drop. Evidently so far as the organisms were concerned these slight variations did not exist ; they were not perceived. Therefore, when the light comes from one side, the organisms react exactly as they do when such variations do not exist. They swim toward the source of light for the same reason that they do when the prism is not present. The experiment consists essentially in making the differences in the intensity in neighboring regions so slight that they are unperceived. We need not, therefore, be surprised that the organisms fail to react to them.

The experiments show, what they were designed to show, that the reason for swimming toward the source of light is not the progression into a lighter region. But they do not indicate in the least that the reactions are not due to changes in intensity of illumination. So long as turning the sensitive anterior end away from the source of light causes a greater decrease in its illumination than does movement into the slightly less illuminated region, the organism will move toward the source of light. If the difference in intensity of light in different parts of the drop were increased till the change in illumination due to progression is greater than the change due to swinging the anterior end away from the source of light, then the positive organisms would gather in the more illuminated regions. This is the condition of affairs in the experiment shown in Fig. 98.

In the swarm spores, as in Euglena, the positive reaction usually changes to a negative one when the light is much increased. We can thus distinguish an optimum intensity of light, to which the organisms may be said to be attuned. Either increase or decrease from the optimum causes the avoiding reaction. Often the organisms are positive when placed at some distance from a window, but become negative when brought nearer. There is much variation among different species, and even among different individuals of the same species, as to the amount of light that causes this change from positive to negative. Sometimes, with a given intensity of light, half the individuals of Ulothrix are found to be positive, the other half negative (Strasburger, I.e. p. 17). The same individual is seen at times to be at first positive, later negative. Some of the influences which modify the reaction to light are known. Certain swarm spores are attuned to a stronger light in the early stages of development than in the later stages. Specimens grown in shaded regions seem attuned to less intense light than those living in welllighted cultures. That is, the organisms are attuned more nearly to the light to which they are accustomed. But subjection to darkness sometimes causes negative organisms to become for a short time positive. Haematococcus is negative in a certain intensity of light, gathering at the negative side of the drop. Now the preparation is covered and left in the dark for a few minutes, then the cover is removed. At once the Haematococci leave the negative side and swim toward the light for a short distance. But this lasts only a moment. After reaching the middle of the drop, they swim back again to the negative side. An increase of temperature increases the tendency to a positive reaction to strong light ; a decrease of temperature has the opposite effect.

Lack of oxygen increases the tendency to a positive reaction. This is accounted for by the fact that the green organisms produce oxygen in the light. A change in the intensity of light does not as a rule produce its characteristic effect immediately, but requires a definite interval of time. When the fight is faint and the organisms are swimming toward it, if the light is suddenly increased to an intensity to which they are negative, the swarm spores continue to swim toward it for some time. The interval may amount to as much as half a minute. At the end of this period they turn and swim away from the light. Again, when the organisms are swimming away from a strong light, a sudden decrease in illumination causes them to become positive only after some seconds. But in some species there is no such delay in the effects of a change of illumination.

To sum up, we find that the reactions to light occur in the infusoria in essentially the same way as do the reactions to most other stimuli, through the avoiding reaction ; that is, by the method of trying movements in different directions. The cause of reaction is a change in the intensity of light, — primarily that affecting the sensitive anterior end. Changes in intensity may be produced either (1) by the progression of the organism into a region of greater or less illumination, or (2) by the swinging of the sensitive anterior end toward or away from the source of light, so that it is shaded at one moment and strongly lighted the next. Usually these two classes of changes work in unison ; when they are opposed, the organism reacts in accordance with that which is stronger. When the second class of changes above mentioned is the determining factor, the organism continues to react by trial till these changes cease. This results in producing orien'ation with anterior end directed toward or away from the source of light. In strong light the effect of an increase or decrease of intensity is often the reverse of that observed in weak light.

A considerable number of infusoria have been found to react to gravity in much the same way as does Paramecium (Jensen, 1893). As a rule, when placed in vertical tubes, they rise to the upper end. The following infusoria have been found to behave in this way : Among the flagellates: Euglena, Chlamydomonas, Haematococcus, Polytoma, Chromulina; among the ciliates: Paramecium bursar ia, Urostyla. S pirostomum ambiguum takes at times a vertical position in the water a short distance above the bottom, with anterior end upward. Under these circumstances it is anchored by an invisible thread of mucus, as may be observed by passing a glass rod between it and the bottom (Fig. 82). The stationary position oriented with reference to gravity

seems to be the result of a slight activity of the cilia, tending to cause movement upward, combined with the downward pull of the thread at the posterior end. Jensen found that Colpoda cucullus, Colpidium col pod a, Ophryoglena flava, and Coleps hirtus showed no clear reaction to gravity. There is reason to suppose that reaction to gravity, where it occurs, is brought about in the same manner as in Paramecium. The details given in the account of Paramecium therefore need not be repeated here.

As a general rule the reaction to gravity is easily masked by reactions to other stimuli. It is shown in a marked way only when other effective stimuli are largely absent, and in cases of conflict with other reactions, it is usually the reaction to gravity that gives way. In some cases the action of other agents causes the reaction to gravity to become reversed, just as in Paramecium. Massart (1891 a) finds that this effect is produced in Chromulina by lowering the temperature to 5-7 degrees C.

A number of infusoria are known to react to centrifugal force in the same way as to gravity. They swim in the opposite direction from that in which the centrifugal force tends to carry them, just as Paramecium does. It is probable that in all cases centrifugal force could be substituted for gravity without essential alteration of the reactions. Schwarz (1884) found that Euglena and Chlamydomonas react to centrifugal force when it is equal to about -|- the force of gravity, and continues the reaction till the centrifugal force is about 8^- times gravity. Above this they are passively carried in the direction of action of the centrifugal force.

There is great diversity in the gross features of the behavior of different infusoria under the action of the continuous electric current. Some swim, like Paramecium, to the cathode ; some to the anode ; some take a transverse position ; some swim to one electrode in a weak current, to the other in a strong current ; some, finally, do not react at all. Yet, in spite of this great diversity, we find the fundamental effect of the current on the motor organs to be almost identically the same throughout the series. In all infusoria having cilia in different regions of the body, the cilia of the cathode region strike forward, those of the anode region backward, just as we have seen to be the case in Paramecium. How the organisms move under these conditions depends on the peculiarities of structure and of the action system of the infusorian in question. We shall review here the different types of behavior under the action of electricity, endeavoring to show how each is brought about.

We may again take up, first, the reactions to single induction shocks, studied by Roesle (1902) and Statkewitsch (1903). In all infusoria investigated the reaction to moderately strong induction shocks is essentially similar to the reaction to other stimuli. The animal usually responds to the shock by the avoiding reaction, which begins with a reversal of the cilia in that part of the body directed toward the anode. In some cases, however, the induction shock causes, like a weak mechanical stimulus, a mere movement forward (Roesle, 1902). If the shock is a powerful one, the body may contract in the anode region, or, in the case of very contractile species, such as Lacrymaria and Spirostomum, the entire body may contract. Reaction takes place most readily as a rule when the sensitive anterior end is directed toward the anode, or especially, according to Roesle, when the mouth opening is precisely directed toward the anode. When the animal is in the transverse position, it is least affected by the induction shock, and in many cases it is

less affected when the aboral side is directed toward the anode, than in the opposite position. Under the action of the constant current there are a few infusoria which do not react at all, so far as known. This is the case, for example, with Euglena viridis. Even with powerful currents it shows no reaction. The larger number of free ciliate infusoria swim under the influence of the constant current to the cathode, while a few swim to the anode or take a transverse position. A considerable number of flagellates swim to the anode, though some swim to the cathode.

The reaction of the flagellates has been little studied in any precise way. Owing to their minuteness it is usually very difficult to determine their exact movements. According to Verworn (1889 b), Trachelomonas and Peridinium swim to the cathode; Polytomella uvella, Cryptomonas ovata, and Chilomonas Paramecium to the anode. In stronger currents some of the individuals of Chilomonas swim to the cathode. The reason for the diversity in the reactions of different flagellates has not been determined. In the case of Trachelomonas, according to Verworn, the flagellum is strongly stimulated when directed toward the anode. The result is that it strikes strongly in such a way as to turn the organism around, — doubtless by a typical avoiding reaction similar to that described on page in for Chilomonas. On reaching a position with anterior end directed to the cathode, it is no longer effectively stimulated ; it therefore continues to move toward the cathode. In Chilomonas the orientation to the electric current is known to be brought about through the typical avoiding reaction. That is, the animal turns toward the smaller lip (Fig. 72, y), till orientation is attained (Pearl, 1900). Since in the flagellates the motor organs are all at one end, all bear the same relation to cathode or anode, so that we cannot expect any opposition in the action of the different flagella, such as we find in the cilia of different regions in Paramecium. There is thus no sign in the flagellates of that lack of coordination or of an apparent attempt to move in two directions at once, which we find in Paramecium.

Among the Ciliata, most species, under usual conditions, turn the anterior end to the cathode and move toward that electrode. But Opalina moves, usually, to the anode, and Spirostomum as a rule takes a transverse position. Certain variations in the reactions under different conditions will be brought out later. Among the organisms which pass to the cathode, the manner in which orientation takes place varies in different species. The direct effect of the current is, as in Paramecium, to cause the cilia on the cathode side to strike forward, while those on the anode side strike backward. This would result, taken by ^ ^ » 5. g ~ 3 itself, in turning the animals directly, by the shortest path, toward the cathode. But in many species, as our study of the reactions to other stimuli has shown us, there is a strong tendency to turn toward one side rather than the other, usually toward the aboral side, — that opposite the peristome. The cilia of the peristome are usually more powerful than those of the remainder of the body, so that the direction in which the animal turns depends largely upon the way these cilia strike. When the peristomal cilia strike strongly backward, the organism turns toward the opposite or aboral side, with little regard to the beat of the remainder of the cilia. These peristomal cilia are as a rule limited to one of the four quarters into which the surface of the body can be divided, as illustrated in Fig. 100. They, of course, beat backward when either the end bearing them, or the side bearing them, is directed toward the anode (1-3, Fig. 100), so that in these positions the animal turns toward the aboral side in order to reach the position of orientation, just as it does in response to other stimuli. It is only when the side bearing the peristome is directed toward the cathode that these cilia beat forward, and hence tend

Fig. ioi. — Transverse (or oblique) position and movement of Oxytricha under the action of the electric current, when the animals are in contact with the substratum. The peristome is directed toward the cathode. to turn the organism toward the oral or peristomal side (Fig. ioo, 6). Under these circumstances, another principle requires consideration. Normally the peristomal cilia strike backward. When they strike forward, they develop much less energy, — less turning power, — than when they strike backward. Therefore, when in the position shown at 6, Fig. ioo, the turning is much less rapid than in other positions, and may easily be prevented by a slight resistance. These relations will be understood by an examination of the diagram (Fig. ioo).

In Paramecium, as we have seen, the same condition of affairs is exemplified to a certain degree, so that the organism turns toward the oral side in all positions save from d to /, Fig. 63. In the Hypotricha (Oxytricha and Stylonychia) this condition is most typically exemplified. A large share of the body cilia are absent or have taken the function of legs, while the peristomal cilia are very powerful. In almost all cases these organisms become oriented to the electric current by turning toward the aboral (right) side. It is only when the peristomal cilia are squarely facing the cathode (Fig. 100, 6) that the animal may turn toward the oral (left) side. In this position the peristomal cilia beat forward, and all the cilia of the body aid in turning the organism toward the oral side. On reaching a position with anterior end directed to the cathode the peristomal cilia are directed forward, but their beating has become so weak as to be almost without effect. The animal, therefore, retains this position. When specimens of the Hypotricha are in contact with a surface, as is usually the case, the forward beat of the peristomal cilia is often so weak and ineffective in the transverse or oblique position (Fig. 100, 6) that it does not turn the animal against the resistance offered by the attachment of the ventral cilia. Such specimens, therefore, remain in the transverse or oblique position, the anterior end usually slightly inclined toward the cathode, as in Fig. 101. In this position they run forward. When the current is reversed, so that the anode lies next the peristome, the powerful peristomal cilia strike backward. The animals, therefore, turn toward the aboral (right) side till they have again become nearly transverse to the current. They then move forward in the direction so indicated. Similar phenomena are at times to be observed in other ciliates, not belonging to the Hypotricha. This is true, as we have seen, even for Paramecium.

Fig. 102. — Diagrams of the reaction of Colpidium to the electric current when in various positions. Based on the descriptions and figures given by Pearl (1900). Thus we can distinguish two factors in the turning produced by the electric current. The first is a tendency to turn directly toward the cathode, the second a tendency to turn toward a structurally defined side, — usually the aboral side. The conflict of these tendencies when the animal is in b c

and their mutual reenforcement in other positions, often give rise to peculiar and complicated phenomena. Thus, in Colpidium, as described by Pearl (1900), we have the following different methods of reacting to the electric current. (It should be premised that Colpidium tends under ordinary conditions to turn toward the aboral side.) (1) When the anterior end is directed approximately toward the anode, or in any position in which the aboral side is nearest the cathode,

Colpidium turns toward the aboral side (Fig. 102, a, b), till the anterior end is directed toward the cathode. Both the factors mentioned above cooperate to produce this result. (2) When the animal is nearly transverse, or is oblique, with the oral side next to the cathode, it usually swims slowly forward, and at >lpid 1 reacts to the electric current when transverse the Same time gradually turns with the oral side to the cathode. Constructed from toward the OYO.I side till it bedata given bv Pearl (iqoo). • , i /-rp- „ j\

The two tendencies mentioned above oppose each other in this case, and the first one overcomes the second. (3) But in other cases when the animal is in the position described in the last paragraph (Fig. 103, a) it reacts in another way. It moves forward, slowly turning toward the oral side (Fig. 103, a-b), then turns on its long axis (b-c) (as happens in ordinary locomotion). This brings the aboral side next to the cathode (c). Now the animal turns suddenly toward the aboral side till the anterior end is directed toward the cathode (Fig. 103, d). In this case, then, the two tendencies mentioned above oppose each other till the revolution on the long axis occurs, then they reenforce each other.

(4) If Colpidium is squarely transverse, with oral side to the cathode (Fig. 104, 1), or especially if the anterior end is a little inclined toward the anode, the organism often starts transversely to the current. Suddenly it jerks its body a little toward the aboral side (Fig. 104,1-2), then moves forward again. Again it jerks toward the aboral side (3), again moves forward, and repeats this behavior until the anterior end is directed toward the anode. Then it turns steadily toward the aboral side till the anterior end is directed toward the cathode (Fig. 104, 4-5). In this behavior the two tendencies mentioned oppose each other, as in case 2, but the second one prevails over the first.

Various combinations of these different reaction types may occur, making the behavior of Colpidium under the electric current very complicated. Similarly varied beof reaction to the electric current havior is often observed in other infusoria, in Colpidium. After Pearl through the action of similar causes. infusoria as Stentor, where the peristomal cilia form a circle surrounding the anterior end, there is no reason for such a conflict of tendencies. The peristomal cilia are divided by an electric current coming from one side, so that the animal turns directly away from the side on which these cilia strike backward (Fig. 105). If the anterior end is directed toward the anode at the beginning, the animal doubtless turns as usual toward the right aboral side. In other positions the usual method of turning seems to have no effect on the reactions. In Vorticella and other infusorians resembling Stentor in the distribution of the cilia, the orientation to the current would doubtless take place in the same direct manner, though this has never been determined.

In Spirostomum and Opalina, the conflict of the two tendencies mentioned above leads to certain very remarkable and complex results. Under usual conditions Spirostomum takes a transverse position in the electric current, while Opalina swims to the anode. The gross features of the behavior thus differ markedly from those shown by most other infusoria. Fig. 105. — Re- action of Stentor when transverse to the current. It turns directly toward the cathode, all the cilia concurring to produce this effect.

But Wallengren has shown that the effect of the current is in these infusoria of essentially the same character as in others. Let us examine briefly the facts as set forth by Wallengren (1902 and 1903). Spirostomum (Fig. 106) is a very long, slender infusorian, easily bent in any direction, and very contractile. The peristomal cilia are very large and numerous, extending from the anterior end along one side to a point behind the middle. Whether striking forward or backward, the beating of these A cilia is decidedlv more effec- / tive than that of the cilia on the opposite side of the body. It is to this fact, taken in connection with the slenderness and suppleness of the body, that most of the peculiarities in the reaction of Spirostomum to the electric current are due.

In a very weak current, such as does not cause contraction of the body, Spirostomum swims to the cathode. The cilia on the anodic part of the body strike backward, those in the cathodic region forward, just as happens in Paramecium. As a result, the animal takes a position with Fig. 106. • ■ Diagrams illustrating reaction of anterior end directed to the Spirostomum to the electric current. A, B, D, and same manner as does Paramecium, — usually turning to the aboral side, but in certain cases toward the oral side. When the anterior end is directed toward the cathode, the cilia on the cathodic half of the body are partly directed forward, but with the weak current most of them still strike most strongly backward. Those of the anode half of course strike backward, so that the general result is to drive the animal forward to the cathode. Sometimes Spirostomum under these conditions comes against the bottom or other solid object ; it may then nearly or quite cease to move forward. The facts thus far are quite parallel to those observed in Paramecium.

As the electric current is made stronger, the cilia on the cathodic half of the body strike more powerfully forward, and at a certain strength their effect, tending to drive the animal backward, becomes about equal to that of the anodic cilia, tending to drive it forward. The result is that the animals move neither forward nor backward, or only very slowly in one direction or the other. They thus sink to the bottom before much progress has been made. Now, if in this position the anterior end is directed toward the cathode (Fig. 106, A), of course the cilia of the anterior (cathodic) half of the body tend to push the animal backward, while those of the opposite half tend to push it forward. This push in opposite directions bends the supple body near its middle. Moreover, in the cathodic half the peristomal cilia have a more powerful forward stroke than do the ordinary cilia on the opposite side, hence the anterior half of the body tends to bend toward the peristomal or oral side. The general result is that the animal is bent into the position shown in Fig. 106, B. The bending of the anterior part of the body toward the oral side continues, until this part of the body becomes transverse to the current (Fig. 106, C). The body may now become completely straightened (Fig. 106, D), or it may not. But in either case the peristome is now turned toward the anode. The powerful peristomal cilia therefore strike backward, causing the anterior end to swing toward the aboral side, directing it again toward the cathode, as indicated by the arrow in D. On becoming directed toward the cathode, the original condition (Fig. 106, A) is restored. The animal therefore again takes the positions B, C, and D. It thus continues to squirm from side to side. But during its movements Spirostomum, like Paramecium, frequently revolves on its long axis. This often happens when in the position shown in Fig.

106, C, so that the animal becomes placed transversely to the current, with peristome to the cathode (Fig. 106, E). In this position the peristomal cilia are directed forward and have therefore comparatively little motor effect. If at the same time the animal comes in contact with the bottom, the contact reaction may overcome for a time this slight motor effect, so that the animal lies nearly quiet, in the transverse position. If now the current is reversed, so that the peristome is at the anode (Fig. 106, D), the animal at once swings again toward the aboral side. Even if the current is not reversed, the animal usually does not remain long in the position shown at E. The peristomal cilia being more effective than the opposing ones, gradually swing the anterior half toward the oral side. Soon a bending takes place again, as in B, and the organism is forced to squirm about from side to side, as before.

Thus Spirostomum finds in a strong current no position of equilibrium, because the peristomal cilia have always a more powerful effect than the opposing ones, and because the opposed action of the cilia on the anodic and cathodic halves of the body soon bends the slender body. It thus squirms about from one side of the transverse position to the other, taking many shapes besides those figured. It remains quiet only for certain periods in the transverse position with peristome to the cathode, when it is in contact with a surface: this is a result of the interference of the contact reaction with the reaction to the electric current. Under the action of the current alone, the reaction of Spirostomum does not tend to bring it to a position where it is not effectively stimulated, for no such position exists. In this respect the electric stimulus shows again a marked contrast with other stimuli.