The Dynamics of Living Matter

When such tubes with their inhabitants are put into an aquarium which receives light from one side only, it requires, as a rule, a day or more until the foot end of the tube is again attached to the bottom of the aquarium. As soon as this occurs, the anterior end of the tube is raised by the worm until the axis of sym- metry of the gills falls into the direction of the rays of light which enter through the win- dow into the aquarium (Fig. 22).* When the animal has once reached this position it retains it as long as the position of the aquarium and the direction of the rays of light remain unchanged. If, however, at any time the aquarium is turned 180° so that the light falls in from the opposite direction, the animal bends its tube during the next twenty-four or forty-eight hours in such a way that the axis of symmetry of its circle of gill? is again in the direction of the rays of light (see Fig. 23). When the light strikes the aquarium from above, the animals assume an erect position, like the positively heliotropic stems of plants when they grow in the open.

In these phenomena the mechanical properties of the tube play a role. When the animal is taken out of the bent tube, the latter retains its form. How does this permanent change of form of the tube come about? In my opinion through new layers being secreted on the in- side. The youngest layers of the secretion are more elastic than the old layers, and, moreover, have at first a powerful tendency to shorten. FIG. 22. — Positive heliotropism of Spirographis Spallanzani. (From nature.)

If such a secretion occurs on one side of the tube only, or more so than on the opposite side, the former must become shorter than the latter, and the result must be a curvature of the tube, that side becoming con- cave where the new secretion has occurred. On this assumption, which is based on many observations, the pro- cess of heliotropic curvature is in this case as follows: when the light strikes the circle of gills from one side only, in these elements certain chemical re- actions occur more quickly, or to a larger extent, than on the opposite side. This results in correspond- ing alterations of the sensory nerve endings, the sensory nerves, and the corresponding motor nerves, and their muscles. The sense of these changes is such as to throw the muscles connected with the nerves of the gills on the light side into a more powerful tonic or static contraction than the muscles on the opposite side of the body. The consequence is a bending of the circle of tentacles, or the head, toward the source of light, which will continue until the axis of symmetry of the circle of tentacles falls into the direction of the rays of light. When this happens, symmetrical tentacles are struck at the same angle (or in other words with equal intensity) by the rays of light, and therefore the tone (state of contraction) of the antagonistic muscles is the same. The result is that the circle of tentacles becomes fixed in this position. The bending of the head produces an increased pressure and friction of the animal against that side of the tube which is directed toward the light, and this pressure and friction lead to an increased secretion and the formation of a new layer inside the tube.

Observations on another marine worm which lives in a stony tube, Serpula uncinata (Fig. 24), add an interesting detail.* These worms occur in colonies of thousands whose tubes are in close contact. The tubes of this form differ from those of Spirographis in that they are made of calcium salts (probably carbonates), and are inflexible. Never- theless, these worms are positively heliotropic, like Spirographis, and in the ocean all the tubes of a colony are straight and parallel, and

directed upwards. If such a colony is put horizontally into an aqua- rium which receives its light from above in the direction of the arrow cd, Fig. 24, it will be observed that very soon the heads of the worms are turned upward so that the axis of symmetry is in the direction of the rays of light. Very soon the tubes begin to grow in front through the deposition of new lime salts (which are secreted by glands). But the direction of this growth is now at right angles to the longitudinal axis of the old tubes. This again shows that the primary effect of the light in the heliotropic reactions is the bending of the tip, or head, of the animal through protoplasmic or muscular contraction. The bend- ing of the tube or growth are secondary phenomena which follow the former. I believe that the phenomena of heliotropism of sessile animals and plants are essentially alike. The presence of nerves in animals is no reason for denying this identity, especially since some botanists, e.g. Hildebrandt, claim that tissues which functionally resemble nerves also exist in plants.

FlG. 24. — Positive heliotropism of Serpula uncinata. The light had originally struck the animal in the direction of the arrow at, and their tubes were parallel with the direction of the rays. When the light fell in the direction cd the tubes began to grow at right angles to their former direction. Partly diagrammatic. We have seen that the essential feature of the heliotropic reaction consists in the fact that the light automatically puts the plant or the animal (Eudendrium, Spirographis} into such a position that the axis of symmetry of the body, or organ, falls into the direction of the rays of light. In the case of positively heliotropic organs, the tip, or head, is directed toward the source of light, while it is the reverse in the case of negatively heliotropic organs. If we imagine that such a positively heliotropic organ, e.g. the polyp of Eudendrium, or a worm, like Spiro- graphis, be endowed with the power of spontaneous locomotion, and if for some internal reasons the animal were compelled to be constantly in motion (as is the case with many pelagic larvas), we should notice that these animals had no choice left in regard to the direction of their motion. The light would turn them automatically until their axis of symmetry was in the direction of the rays of light, and the animal could

then move only in this direction. If the positively heliotropic polyp of Eudendrium could be transformed into a free-swimming animal, it would be compelled to swim automatically toward the source of light. It had been known since man began to use artificial light that cer- tain animals, especially insects, show a tendency to fly or creep to the flame. The explanation generally given of this phenomenon was an- thropomorphic; it was assumed that the animals fly into the flame because they are fond of light, or that they are driven by curiosity, or that they are afraid of the dark. It seemed to me that we had no right to see in this tendency of animals to fly into flame the expres- sion of an emotion, but that this might be a purely mechanical or com- pulsory effect of the light, identical with the heliotropic curvature observed in plants. I believed that the essential effect of the light upon these animals might consist in a compulsory automatic turning of the head toward the source of light, corresponding to the turning of the head, or the tip, of a plant stem toward the light ; and that the process of moving toward the source of light was only a secondary phenomenon. It seemed to me also that if the stem of the plant could suddenly acquire the power of locomotion, it would act exactly like the animals which fly into the flame.*

I have since been able to prove directly that this deduction is cor- rect. Eudendrium furnishes us the opportunity of observing the same organism in rapid succession as a free-moving and as a sessile organ- ism. In an early stage of development the larvae of Eudendrium are ciliated pelagic organisms which swim actively. When these larvae are in an aquarium which receives its light from one side only, they swim at once toward that side and remain there as long as the direction of the rays of light remains unchanged. If the aquarium is turned, they also turn at once, and swim toward the lighted side of the aquarium. This condition does not last long, for the larva soon attaches itself, or rather adheres, to a solid body, and immediately afterward a polyp grows out from the end opposite that which is attached to the solid body. As soon as the polyp grows out, it undergoes a positively helio- tropic curvature, as described above, provided that the light continues to fail into the aquarium from one side only. It is thus possible to see the same individual behave in twenty-four hours, first, like an insect that is attracted by the light, and then like a heliotropic plant. I men- tioned before that the heliotropic curvature of the stem of Eudendrium occurs much more rapidly behind a blue than behind a red screen, if

* The first paper on the identification of the flying of animals into the light with the heliotropic curvatures <>f plants appeared in January, 1888. Sitzungsberichte der Wiirz- burger med. physik. Gesellsch., 1888. The same number contained also a preliminary notice (m the identity of geotropism in animals and plants. it occurs in the latter case at all. I have found that the ciliated larvae of Eudendrium swim rapidly toward the source of light behind a blue screen, while they react quite slowly, or not at all, behind a red screen.* We will now show that the same ideas also hold for forms which, like the insects, possess a central nervous system. f We may choose for this purpose animals like the caterpillars of Porthesia chrysorrhcea, or the winged Aphides. When the young caterpillars of Porthesia, which hibernate in a nest, are brought during winter into a warm room, they leave the nest. If a large number of these larvas are put into a test-tube which is placed upon a table with its longitudinal axis at right angles to the plane of the window, all the caterpillars move toward the window side of the tube, where they remain. If the test-tube be turned carefully by an angle of 180° in a horizontal plane, the animals will go back at once to the window side, and the quicker, the stronger the intensity of the light. They react in this way, whether the source of light is sunlight, diffused daylight, or lamplight. The representa- tives of the anthropomorphic viewpoint would say that the animals go to the source of light because it is brighter at the window side of the test-tube than at the room side. It can, however, be shown that in this case the animal has no choice, but that its head is turned mechanically toward the light by the latter, and that it is compelled to move in this position. The proof of the correctness of the mechanical, automatic, or heliotropic view lies in the fact that the animals also move toward the source of light, even if in so doing they must pass from the light into the shade. The experiment can be made in the follow- ing simple manner: Let, through the upper half of a window (ww, Fig.

25), direct sunlight -S1 fall upon a table, through the lower half, the diffused daylight (£)). 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 ab is in the direct sunlight, the other half in the shade. If at the beginning of the experiment the animals are in the direct sunlight at a, they promptly * These observations on the larvae of Eudendrium were made in 1895 at Woods Hole, but have not been published heretofore.

t Loeb, Der Heliotropismus der Thiere ttnd seine Uebereinstimmung mil dent Helio- tropismus der Pflanzen, Wiirzburg, 1889. Reprinted in Studies in General Physiology, Vol. I, Chicago, 1905. move toward the window, gathering at the window end c of the tube, although by so doing they go from the sunshine into the shade. This shows that the effect of light consists in turning the head of the animal, and subsequently its whole body, toward the source of light, so that the symmetrical points of the photosensitive surface of the body — in this case the eyes - - are struck by the rays at the same angle. The animals will remain at the window side of the tube at c (Fig. 25). The experiment disproves the anthropomorphic idea that the animals go to the brightest spot in space.

It can also easily be shown that in these animals, just as in plants, the more refrangible blue rays are more effective than the red rays, and that the latter act like weak light. Let us suppose that a test-tube containing the animals be placed on a table near the window (ww, Fig. 26), through which diffused light D enters; and that one half of the test-tube, namely, that near the window, be covered with blue glass ab. At the beginning of an experiment the animals are gathered at the room end of the test-tube. They behave as if the test-tube were entirely uncovered, and move toward the window side of the test-tube, where they remain. The same experiment may be repeated, only with the difference, that the window side ab of the test-tube is covered with red instead of blue glass. The animals now creep in the direction of the window to that -^5^- point in front of a where :~c~^ the light, filtered by the red glass, begins to strike them. Here they gather, migrat- ing constantly in a narrow circle at the limit between red and diffused light. The explanation of the latter experiment is as follows : As long as the animals are at the room end of the test-tube, they are struck simultaneously by the diffused daylight D which falls through the win- dow, and by the weak light R which is reflected from the walls of the room. Under these circumstances the animals are forced to turn their heads toward the stronger source of light, namely, the window, and consequently move toward it. - As soon as they reach the point where the light from the window has to pass through the red glass, before striking them, the light reflected from the walls of the room, which contains the effective blue rays, is heliotropically more effective than the light from the window, which has lost most of its heliotropi-

cally effective rays. Consequently the heads of the animals are turned automatically toward the room side of the test-tube, just as would be the case with the tip of a positively heliotropic stem under similar cir- cumstances. They move toward the room side, but cannot go far, because, as soon as the unfiltered light from the window again strikes them, the latter being stronger, the head is now turned automatically toward the window again, and they move toward the latter until they get under the red glass; and now the whole process repeats itself. It thus happens that the animals gather in front of a at the limit between the uncovered part of the test-tube and the red glass, where they keep on moving in a narrow circle. Similar results are obtained if an opaque body is substituted for the red glass.

If the whole test-tube be covered with red glass, the animals still show a slight tendency to move toward the window side of the tube; but their motions are no longer in a straight line as before, but more irregular. They finally, however, gather at the window side of the tube; but it requires much more time before they gather there than if the test-tube is covered with blue glass. Red light acts upon these animals like weak light ; this can be shown directly by experiments with daylight towards sunset, or in a comparatively dark room.

It seems, therefore, that these phenomena are indeed the same as those in positively heliotropic sessile animals and plants; and we may designate such animals whose heads are turned automatically toward the light, when the light strikes them from one side, as positively helio- tropic. It should be observed that the essential feature in these re- actions is the compulsory turning of the head by the light, which leaves the animal no choice, making all the caterpillars of Porthesia or all the plant lice of the same culture behave exactly alike, just as in the case of a magnet all the pieces of iron are compelled to behave alike. This compulsory character of heliotropic reactions seems to have been overlooked by those anthropomorphic opponents of the theory of animal heliotropism, who offer the objection that we can turn toward the win- dow voluntarily. This objection is about as absurd as if we should argue against the existence of magnetism because we can turn and move toward a magnet without being made of iron.

We not only find animals whose heads bend or turn toward the light, which consequently must move toward the source of light, if they move at all, but also animals whose heads bend or turn away from the source of light. We call such animals negatively heliotropic. Such negatively heliotropic animals are, e.g. Gammarus pulex, a fresh- water Crustacean, the larvae of the house fly, when fully grown and ready to go into the pupa stage, the larvae of Limulus in a certain stage,

Copepods and other animals, under certain conditions, as we shall see later. For the negatively heliotropic animals the rule holds also, that the blue rays are more effective for heliotropic reactions than the red rays. It is easy to show that these animals move away from the source of light in the direction of the rays of light. The fact can be demonstrated nicely in the case of the fully grown larvae of the fly, by compelling them te> move on a table on which strong light, e.g. direct sunlight, falls. If a shadow is thrown on the table by means of a pencil, it will be found that the larvae move parallel with the shadow, away from the source of light. It can also be readily demonstrated that these animals are not, as the anthropomorphists would probably state, afraid of the light, or fond of darkness, but they are purely negatively heliotropic, which means that the head is bent or turned away auto- matically by the light from the source of light, exactly like the tip of a negatively heliotropic root. This can be shown by putting the larvae on a table which has been placed near the window in such a position that the half of the table which is nearer the window is struck by diffused daylight, the other half by the direct sunlight. If the animals, at the beginning of the experiment, are at the window side of the table in the shade, their heads, under the influence of the light, will be mechanically bent away from the window, and all the animals will begin to move in the direction of the rays of light. They go from the shade into the sunlight. I have modified this experiment by putting the larvae in long glass tubes, one end of which lay in the shade near the window, the other in direct sunlight.

The animals went from the shade on the room side of the tube into the end which was in the direct sunlight; here they remained permanently, notwithstanding the fact that they soon died from the effect of the sunlight (or the heat?). When the animal is struck by light on one side only, those muscles which turn the head away from the source of light contract more strongly than their antagonists ; the consequence is a bending of the head away from the light. As soon as the symmetrical points of the photosensitive surface of the animals are struck by the light at the same angle, the tension of the photosensitive surface becomes equal ; hence the animals remain in this orientation. If they move, their locomotions will there- fore occur in the direction of the rays of light, and away from the source of light.

We mentioned before that the laws of heliotropic curvature in plants can be successfully demonstrated in animals. We may add that the heliotropic motions of animals to and from the light can be demonstrated in free-moving plants. Under certain conditions, which are not yet completely analyzed, the cells of algae are transformed into smaller cells, which are provided with cilia, and move out from the algae. Strassburger showed that these swarm spores may be heliotropic and move in the direction of the rays of light to or away from the source of light. The blue rays are more effective in this case than the red rays.

When a large number of animals are tested for heliotropic reactions, we find that there are two problems to be solved : the first, to account for the variations in the degree of heliotropic sensitiveness ; the second, to account for the variation in the sense of heliotropism. As far as the first factor is concerned, we find animals that are not heliotropic at all, animals that show a slight degree of heliotropism, and others that are so pronounced in their heliotropism, that their motions, and indirectly their whole existence, is only a function of light. As far as the sense of heliotropism is concerned, we find positively and negatively helio- tropic animals. What causes these differences? We started with the assumption that the heliotropic reactions are caused by a chemical effect of light; in all such reactions time plays a role. We assume, furthermore, that if light strikes the two sides of a symmetrical organ- ism with unequal intensity, the velocity or the character of the chemical reactions in the photosensitive elements of both sides of the body is different; that in consequence of this difference the muscles, or con- tractile elements, on one side of the organism are in a higher state of tension than their antagonists. The consequence is a curvature or a bending of the head. With this assumption it becomes at once obvious what is responsible for the variation in the intensity of heliotropism. Let us consider for the time being only positively heliotropic animals. Some of these, e.g. winged plant lice or the caterpillars of Porthesia (immediately after leaving the nest), may be called most intensely posi- tively heliotropic, inasmuch as they move toward the source of light in as straight a line as their structural imperfections permit.

If they deviate from the direction of the rays of light for only a short time, possibly less than a second, the difference in the tension of the muscles on both sides of the body becomes so great that it suffices to turn their heads automatically again toward the source of light. If one side of the body alone be exposed to the light for only a fraction of a second, the light causes such a difference in the chemical substances formed on both sides of the body that the threshold for the difference in the tone of the muscles is exceeded, and the bending of the head must occur. If, however, the light does not increase the reaction velocity as much

or if the mass of photosensitive substances is less, it will require a longer one-sided exposure of the animal before the threshold for creating a difference in the tension of the muscles on both sides of the body is reached. Such animals will also move toward the source of light, but they do not move so directly in the direction of the rays of light as the strongly heliotropic forms, but much more irregularly. Finally, if the light does not accelerate the reaction velocity in the animal at all, or if the proper photosensitive substances are not present, or the proper products are not formed in the photochemical reaction, the animal will not appear in the slightest degree heliotropic.

The correctness of this view can, I believe, be demonstrated, by exposing animals which in strong light are intensely positively helio- tropic to weak light. If in the strong light they move in as straight a line toward the source of light as the imperfections of their locomo- tions permit, a low intensity of light can be found where they still go toward the source of light, but where their progressive motion follows the direction of the rays of light much less accurately. In the weaker light the acceleration of the photochemical reactions is less than in strong light, hence the time during which an animal can deviate from the direction of the rays, exposing only one side of its body to the source of light, becomes longer. The same result can be obtained by putting these animals behind a red screen. This explains, also, the influence of temperature upon the precision with which the heliotropic animals follow the direction of the rays of light. Within certain limits the precision with which such animals move in the direction of the rays increases with the temperature.

If it be true that the immediate effect of the light in causing the heliotropic reactions is of a chemical nature, we should expect that it must be possible by the use of chemicals to control the precision and sense of the heliotropic reactions. I have recently found facts * which prove the correctness of this supposition. It may be of importance that these chemicals are such as may be formed by the organism itself. The experiments were made on fresh-water Crustaceans, Gammams pulex, Daphnia, and Cyclops. If Gammarus are left to themselves, they may be found in any part of the aquarium clinging to solid bodies ; but if they are disturbed by transferring them from one vessel to another, or by merely stirring the water in the vessel in which they are, they become, transitorily at least, negatively heliotropic. It is possible, however, to make them at once intensely positively heliotropic, by adding certain chemicals to the water, e.g. esters. If the negatively heliotropic Gammarus are in a glass jar containing 50 c.c. of tap water, they become

* Loeb, University of California Publications, Physiology, Vol. 2, p. I, 1904. positively heliotropic if i or 2 c.c. of a grammolecular solution of an ester, e.g. ethylacetate, is added. Ethylbutyrate and methylacetate act similarly, only they seem to be more toxic. The transformation is rapid but not instantaneous, and by giving smaller dose= of ethylace- tate the latent period may be prolonged. A second means of making them positively heliotropic if' through

n the addition of acids. If, instead of an ester, i to 2 c.c. of solution of an acid, e.g. hydrochloric, oxalic, or acetic acid, is added, the animals also become positively heliotropic. It seems to me of im- portance that CO2 is especially active in this respect. If CO2 is allowed to bubble through the vessel in which the Gammarus are, or if from 5 to 10 c.c. of soda water (or even beer!) is added to 50 c.c. of water, they become also positively heliotropic. Boracic acid seems to be ineffective.

Of other substances which act in a similar way, alcohol, paraldehyde, ammonium salts, and to a slight extent K-salts, may be mentioned. But much higher concentrations of these substances are needed than of the acids or esters. The positive heliotropism which is produced in this way is only transitory. In a colony of Cyclops some individuals, as a rule, are outspokenly positively heliotropic, others are rather indifferent to light, and a few may gather at the room side of a glass dish. If, however, i to 2 c.c.

of — HC1, or another not too weak acid is added, or CO2 is admitted, the animals all gather in a narrow region at the window side of the vessel. If the water is rendered weakly alkaline, they become less outspokenly positively or even, in part at least, negatively heliotropic. I have not been able to obtain this latter effect of alkalies in Gammarus. We see, therefore, that acid, especially CO2, not only makes negatively heliotropic Cyclops positively heliotropic, but increases the intensity of the positive heliotropism in those that were already positive at the beginning of the experiment.

In Daphnia it can be shown that when they are only weakly posi- tively heliotropic, e.g. in weak light, the addition of acid in the above- mentioned concentration makes them intensely positively heliotropic. I may perhaps call attention to the fact that acids, especially CO2, are formed in organisms; that, moreover, esters are formed in the stems of many plants. It may be that these substances play a role in the production and variation of heliotropism in plants and animals.

In my first publications on animal heliotropism I had already men- tioned the fact that chemical changes in certain animals apparently produce also changes in the degree and sense of heliotropism. I had found that the caterpillars of Porthesia chrysorrhosa are outspokenly heliotropic only as long as they are not fed. After having begun to eat, their heliotropic sensitiveness diminishes or disappears completely; and in later stages of their growth and development their heliotropism becomes very weak, even if they are caused to starve again. In ants the intensity and the sense of heliotropism seem to be connected with the development of their sexual products. At the time of sexual ma- turity the males and females are markedly heliotropic;* while in the workers not a trace of heliotropism is demonstrable.