Forced Movements, Tropisms, and Animal Conduct

however, tires sooner and fails to re¬ spond. If the strip is placed with its subumbrella surface upward and ex¬ tended in a straight line parallel to the current lines, the making of the current causes the tentacles at the anode end to turn through an angle of 180° and point toward the cathode. The ten¬ tacles at the cathode end become more crowded together, reminding one of the tip of a moistened paint brush, and also point more directly toward the cathode (Fig. 13). The experiment may be varied in still other ways by cutting smaller or larger pieces from the edge of the swimming bell, but the response is always the same. The tentacles, wherever pos¬ sible, and to a less extent the manubrium, bend so as to point toward the cafhode. The response depends in no way upon the con¬ nection of these organs with the swimming bell, muscles, or nerve ring, for it is obtained equally well with isolated tentacles and pieces of tentacle. Isolated tentacles when placed transversely to

Fia. 12.—Tentacles T and manu¬ brium Af of a jellyfish (Polyorchis) under influence of galvanic current are turned to the negative pole. (After Bancroft.) Fig. 13 . —Strip of jdlyfieh showing that under the influence of galvanic current tentacles on both ends point towards cathode. (After Bancroft.) the current lines curve eo as to assume a more or less complete XJ-shape, with their concave side toward the cathode. "When placed parallel to the current, the tentacles do not curve."

The latter observation shows the fact that the whole reaction is due merely to an increase in the tension of the muscles on the cathode side of the organ. Phenomena of galvanotropism can he observed also in infusorians. Thus Verworn*®® observed that when a current goes through a trough containing Paramcscia the animals will all move toward the cathode. The mech¬ anism of the reaction was discovered by Ludloff.®” The locomotion of Paramcecium is brought about by cilia. As a rule the cilia are directed backward (Fig. 14), and in their normal movement they strike powerfully backward and are retracted ^ \

by the current, the position of the ciha A A on the cathode side is reversed inasmuch ThYr as they are now turned forward. On the * anode side they continue to be directed backward (Fig. 15, a). Instead of po?nt£“towMd*i,bor“ai striking symmetrically on both sides of the animal, the cilia on the cathode side strike for¬ ward powerfully while those on the anode side strike powerfully backward. The animal is thus under the influence of a couple of forces which turn its oral pole toward the cathode side. As soon as it is in this condition the symmetrical cilia are struck at the same angle by the parallel current lines and they must assume a symmetri¬ cal position which is as in Fig. 15, h, namely the cilia are pointed forward toward the cathode at the oral end, and backward toward the anode at the aboral end. As long as the current is not too strong, the oral region, where the cilia are pointing forward, is rather small and there¬ fore the action of those cilia prevails which’are in the majority and which are pointed backward. As a result the organism moves slowly forward to the cathode.

A similar meclianism of galvanotropic conduct exists in Volvox & spherical, unicellular organism which is sur¬ rounded hy cilia on its whole surface. A definite pole of the organism is always foremost in all locomotions. This organism usually swims to the anode when in a galvanic field. Bancroft made the action of the cilia of Volvox visible with the aid of india ink and was able to show that the current made the cilia on the anode side stop, while those on the cathode side eontinue to beat.®®

Fig. 15. — a, current going from left to right through Paramascium, the poeition of cilia on the cathode side is now reversed, their free ends pointing forward. The animal when swimming is automatically turned with its oral end toward the cathode. 6, current going through ParamcBcium from aboral to oral end. Cilia symmetrical on both sides but pointing forward at oral end and backward at aboral end. Since the backward stroke is always the effective one the organism is thus carried automatically toward the anode.

Terry found that Volvox can he made to move toward the anode as well as toward the cathode. It moves to the anode after having been kept in the dark for two or three days, while after exposure to light it swims to the cathode. Volvox contains chlorophyll and the change in the sense of reaction is therefore connected with the formation of a product of chlorophyll activity. Bancroft found that when Volvox was made cathodic by exposure to sunlight, the cilia stop on the cathode side.

While the locomotor mechanism of unicellular organ¬ isms, like ParmruBcia and Volvox, is as simple as that of higher organisms, the locomotion of microorganisms possessing only one flagellum, like Euglena, is more com¬ plicated. It was generally assumed that the flagellum acted like a single oar and that it was directed forward, but this is not correct. It is shaped like a IT and its free end is directed backward; and Bancroft has emphasized that it acts by the formation of a loop which moves hke a wave from the base of the flagellum to its free tip. The same author discovered that Euglena are galvanotropic when raised in acid media. On account of the asymmetry of their locomotor apparatus they are compelled to swim in a spiral, in most cases to the cathode, exceptionally to the anode. Bancroft showed that the orientation of these organisms by the galvanic current is identical with that by light.^^

All the phenomena of galvanotropism are, therefore, reduced to changes in the tension of associated muscles or contractile elements, as a consequence of which the motion of the organism toward one pole is facilitated, while the motion toward the opposite pole is rendered difficult. Galvanotropism is, therefore, a form of forced motions produced by the galvanic current instead of by injury to the brain. There Remains then the question of how a galvanic current can bring about those changes which result in the auelectrotonic and catelectrotonic condition mentioned at the beginning. Currents can pass through tissues only in the form of ions whose progress is blocked by mem¬ branes which are more permeable for certain salts than for others. Those salts which go through the membrane carry the current through the tissue elements, those

which do not go through will increase in concentration at the surface of the membrane. It is the latter which cause the electrotonic effects; according to Loeb and Budgett by secondary chemical reactions at the boun¬ dary. Nernst has pointed out that a stationary condition must arise at the surface of the membrane due to the fact that the increase in concentration of ions by the electric current gives rise to a current of diffusion of salt in the opposite direction away from the membrane. ‘ ‘ The aver¬ age change of concentration at the membrane depends, therefore, upon the antagonistic effects of the current and of the diffusion.”®^* This must be kept in mind since otherwise the effect of the constant current should increase constantly with its duration, which is not the case, on account of the establishment of a condition of equilibrium between the increase of the concentration of ions at the boundary with the duration of the current and the diminution of this concentration by the diffusion of the ions in the opposite direction due to osmotic pressure.

The fact that certain animals go to the light had, of course, been known for hundreds of years, but this was explained in an anthropomorphic way. Thus Lubbock, and G-raber,^®*, had taken it for granted that certain animals went to the light or away from it on account of fondness for either light or darkness, and their experi¬ ments were calculated to demonstrate this fondness. Animals were distributed in a box, one-half of which was covered with common window glass, the other with an opaque body ox with colored glass, and after a while the number of animals in each half was counted. When the majority of animals were found in the dark part the animals were believed to have a preference for darkness, when in the light part they were believed to be fond of light. The same method was used to decide whether animals preferred blue to red or vice versa. The writer attacked the problem from the physical viewpoint, assum¬ ing that the animals are “fond” neither of light nor of “darkness,” but that they are oriented by the light in a similar way as plants are; being compelled to bend or—as in the case of motile algae—move automatically either to a source of light or away from it.^®®>

In the ease of unequal illumination of the two eyes the tension of the symmetrical muscles in an animal becomes unequal. In this condition the equal impulses of locomo¬ tion will result in an unequal contraction of the muscles on both sides of the animal. As a consequence the animal will turn automatically until its plane of symmetry is in the direction of the rays of light. As soon as this happens the illumination of both eyes and the tension of sym¬ metrical muscles become equal again and the animal will now move in a straight line—either to or from the source of light. What appeared to the older authors as the expression of fondness for light or for darkness was according to the writer’s theory the expression of an influence of hght upon the relative tension of symmetrical muscles. y '

Animals which are compelled to turn and move to the source of hght the writer called positively heliotropic, those which are compelled to turn and move in the oppo¬ site direction, he called negatively heliotropic. The designation hehotropism (or phototropism) was chosen to indicate that these reactions of animals are of the same nature as the turning of plants to the hght; and the writer was indeed able to show that sessile animals bend to the hght as do plants which are raised near a win¬ dow;^® while motile animals move to (or from) a source of hght as do the motile swarmspores of algae or motile algae themselves.

We win first discuss positively heliotropio motile ani¬ mals. The positively heliotropie caterpillars of Porthesia chrysorrhaea or the winged plant lice of Cineraria ®®* or the newly hatched larvae of the barnacle ^®® were used by the writer in his earliest experiments and they may serve as examples. The larvae of Porthesia must be used after hibernation before they have taken food. When about 50 or 100 of such larvae are put into a test tube and

the latter is placed at right angles against a window, all the animals begin to move to the window in as straight a line as the imperfections of their locomotion and col¬ lisions permit. As soon as they reach the window side of the test tube they remain there peimanently, unless the test tube is turned around. If we turn the test tube around an angle of 180° the animals go at once to the window again. They react in this way whether the source of light is sunlight, diffused daylight, or lamp light; and this can be repeated indefinitely. The animals are slaves of the light. These experiments are typical for posi¬ tively heliotropic motile animals.

When the animals have reached the window end of the test tubes they remain there, since the light prevents them from going back. But in staying there they may assume any kind of orientation, thus proving that the light orients them only as long as they are in motion. The light affects the tension of the muscles and we shall see later that when, the animals are not moving, the change in the tension of the muscles manifests itself by changes in the . position of the legs, which is noticeable in organisms with comparatively large appendages.

That these animals do not go to the light because they prefer light to darkness but because the light orients them is proved by the fact that they will go from light into the shade if by so doing they remain oriented with their heads toward the source of light.®®’^ Let direct sunlight 8 fall upon a table through the upper half of a window (W, Fig. 16), the diffused daylight D through the lower half. A test tube ac is placed on the table in such a way that its long axis is at right angles with the plane of the window; and one-half ah is in the direct sunlight, the other half in the shade. If at the beginning of the

experiment the positively heliotropic animals are in the direct sunhght at a, they promptly move toward the win¬ dow, gathering at the window end c of the tube, although by so doing they go from the sunshine into the shade. This experiment shows also that it is not the intensity gradient of hght in the dish which makes positively helio¬ tropic animals move to the light, hut that difference in intensity on both sides of the animal which is caused by the screening effect of the animaPs own body. The same holds true for chemotropism.

animals only. In the case of unequal illumination of the two eyes or of the two sides of the body of a negatively heliotropic animal the tension in the muscles turning the animal to the source of light is diminished. The impulses for locomotion which are equal for the muscles of both sides of the body will, therefore, result in turning the head of the animal away from the source of light. As soon as the plane of symmetry of the animal goes again through the source of light, the symmetrical photosen¬ sitive elements of the head receive again equal illumina¬ tion, and the animal will now continue to move in a straight line away from the source of light. The fully grown larvae of the housefly when they are ready to pupate show this negative heliotropism.

Negatively hehotropic animals, e.g., the fully grown larvae of the blowfly, can be made to move from weak light to strong light, e.g., from the shade into direct sunlight, if in so doing the illumination on the two sides of the body remains equal.*®^ This was shown by the writer by an arrangement similar in principle to the one de¬ scribed above. Thus the idea that the intensity gradient of light determines the direction of motion was disproved also for negatively heliotropic animals.

Thus far we have shown only that a heliotropio animal is oriented in such a way to a source of light that its plane of symmetry goes through the source of light. This does not yet explain why a positively heliotropio animal cannot go away from the source of light, since in going to or going away from the source of light both sides of the animal receive equal illumination. The fact that a posi¬ tively heliotropic animal cannot go away from the light finds its explanation by observations of Holmes and G-arrey,”'^ showing that when light falls from behind

and above on a positively beliotropic animal its progres¬ sive motions are stopped, and in some cases a tendency to turn a somersault backward may even arise. The case is similar to that of galvanotropism when the current goes through an animal lengthwise (see previous chapter). We must conclude from the observations of Holmes and Garrey, which will he discussed farther on, that if the head of a positively heliotropic animal is turned to a source of light its forward motions are facilitated and the backward motions rendered diflScuit; while in the case of a negatively heliotropic animal it is just the reverse. If the animal now moves to the right or to the left the illumination of the two eyes or of the two sides of the body becomes different again, causing a forced movement, whereby the plane of symmetry of the moving animal is caused to go through the source of light again; with the head toward the source of light when the animal is posi¬ tively heliotropic or away from it when it is negatively heliotropic.

2. Dieeot Peoof of the Muscle Tensioh Thbokt of Hbliotbopism ih Motile Animals The fact that light causes forced movements, like those described in the case of brain lesions and of galvano¬ tropism, has been proved by many observers, and espe¬ cially clearly by Hohnes and Garrey. Holmes worked on the positively heliotropic water scorpion Ranatra?^^ When this animal is iUmninated from the right side, the legs on the right side of the body are bent and those on the left side extended (Fig. 17). This effect is identical with the one observed in Palcemonetes, when a galvanic current goes sidewise through the animal. Hence Ranatra

ff^vanotropic effects observed in the position of the head of Amblystoma when the current goes forward or back¬ ward through the animal. observations of Holmes explain, as already mentioned, why a positively heliotropic anim g ) cannot move away from the light and why aLgatively Eeliotropm animal cannot move to a source of Hght. t£ Regressive motiom of the negatively heliotropic animal will be stopped when the Hght strikes it in front; while these motions of the positively heliotropie animal will be facilitated when the light is in front and will be rendered impossible when the light is behind.

The writer had observed long ago that when the con¬ vexity of one eye is cut off in the housefly it will no longer go in a straight line but will make circus movements, the normal eye being directed toward the center of the circle.^®® It was shown by Parker that blackening of one Fig. 18 . —The lower figure represents the position of Ranatra when the lii^lit is behind the body. The upper figure represents the position assumed when the light is in front of the animal. (After Holmes.)

eye of the positively heliotropic butterfly Vanessa antiopa calls forth circus movements, with the unblackened eye toward the center of the circle.®®® Holmes, Radl,*^'' Axenfeld, Garrey, and many other authors have since made similar observations. In the positively heliotropic Ranatra, Holmes described the effect of blackening one eye as follows: If one eye of Ranatra is blackened over or destroyed tke insect in most cases no longer walks in a straight line but performs more or less decided eircus movements toward the normal side. Under tbe stimulus of light the insect assumes a peculiar attitude; the body leans over toward the normal side and the head is tilted over in the same

This is the combination of oircns movements with roll¬ ing movements familiar to those who have experimented on the brain of fish, where a destruction of onfe side of the midbrain calls forth rolling motions as well as circus motions toward the same side. Holmes’s observations Fig. 19,—Robber fly under normal conditions seen from above. (After Garrey.) were extended by Garrey’s experiments on a large num¬ ber of insects. Garrey found that the robber fly {Procta- canthus) (Fig. 19), which is positively heliotropic, is an unusually good object for the demonstration that the heliotropic reactions of animals are of the type of forced movements. When one eye of this fly is blackened the legs on the side of the unblackened eye are flexed and the

legs on the side of the blackened eye are more extended than normally and spread farther apart.®' The body may tilt as far toward the side of the nnblackened eye as to press the legs to the table (Fig. 20). There is sometimes a cfSeyV ^ motion toward left possible, toward right impossible. tendency on the part of the body of the animal to become slightly concave toward the side of the nnblackened eye. G-arrey fonnd also that the same changes take place when one eye receives a stronger illumination than the

® rigs. 19 to 22 and 27 ■were drawn from photographs kindly given to the writer for this pti^ose by Professor Garrey. The draughtemnn •was imfortunately not familiar with the anatomy of insects, which acco'unta for shor^mmgs in the drawings, which, however, have no bearing on the prob¬ lem for which the drawings are intended. r™" other. Bringing one eye into the bright beam of light directed through the objective of the optical system of the string galvanometer, -while the other eye is illuminated only by the subdued light of the optical room, promptly produced the same changes in the position of the legs and body 'which were observed when one eye was black¬ ened, the more weakly illuminated eye acting like the blackened eye in the former experiment. When the illu-

Fio. 21.—Position of robber fly when the lower halves of both eyes are blackened. Head tilted up. (After Garrey.) mination on one side of such animals is stronger than on the other the legs on the more strongly illuminated side of the animal are bent, those on the opposite side are ex¬ tended ; and the head has a tendency to bend toward the light. When an impulse to move originates in the animal, it can turn easily to the light but with difficulty in the opposite direction. As soon as its head is turned to the source of light and both eyes receive the same illumination the difference in tension of the legs on the two sides of the body disappears and now the animal moves or is carried in a straight direction toward the light. By these experiments the proof of the writer’s muscle tension theory of hehotropiism is made eomplete.’^’’’’