Comparative Physiology of the Brain and Comparative Psychology

1. LOEB, J. Untersuchungen zur physiologischen Morphologic der Thiere, I., 1891. Wurzburg, G. Hertz. 2. LOEB, J. Zur Physiologic und Psychologic der Aktinien. Pfliigers Archiv, Bd. lix., 1895. 3. PARKER, G. H. The Reactions of Metridium to Food and Other Substances. Bulletin of the Museum of Comparative Zoology at Harvard College, vol. xxix., 1896. 4. POLLOCK, W. H. On Indications of the Sense of Smell in Actinia. Jour. Linnean Soc., London, vol. xvi., 1882.

5. NAGEL, W. Experimented sinnesphysiologische Untersuch- ungen an Coelenteraten. Pfluger's Archiv, Bd. Ivii., 1894. i. The nervous system of the starfish consists, first, of a central nerve-ring around the mouth (Fig. 16), and, second, of the peripheral nerves radiating from this ring into each of the arms. It is a well-known fact that if such an ani- mal be laid on its back it soon rights itself. In species like that repre- sented in Fig. 1 6 the ambulacral feet found on the ventral surface in great numbers ex- ecute the righting. These little feet are muscular tubes, which end in a plate. By means of this plate the foot, like the sucker of the leech, can cling to solid bodies. If a starfish be laid on its back, the tube-feet of all the arms

a, central nerve-ring that surrounds the mouth, />, peripheral nerves of the arms. are stretched out at once and are moved hither and thither as if feeling for something-, and soon the tips of one or more arms turn over and touch the un- derlying surface with their ventral side (Fig. 17). The tube-feet of these arms attach themselves to this The tube-feet of the three arms at the left are pulling while the other two arms are quiet. This causes the animal to turn a somersault toward the left which brings it again into the natural position.

surface, and the animal is then able to turn a somer- sault and regain its normal position. For this result, it is essential that all five arms do not attempt simultaneously to bring the animal into the ventral position. Should the tips of all five, or even four, arms tug simultaneously, it would be impossible for the animal to turn over. In normal starfish having five arms, not more than three begin the act of turn- ing; the other two remain quiet. If we, however,

destroy the nervous connection between the arms, for instance, by making two incisions at a and 6, Fig. 18, this cooperation of the arms ceases. The normal starfish requires but a few minutes to turn over, but the specimen represented in Fig. 18 remained on its FIG. 18. THE SAME EXPERIMENT ON A STARFISH WHOSE NERVE-RING HAS BEEN SEVERED IN TWO PLACES (a AND b). The right and left arms are consequently no longer connected nervously. If such an ani- mal is laid on its back, the tube-feet of four or even all the arms in most cases tug simultaneously. This prevents the animal from righting itself.

back the whole afternoon, although the arms were struggling constantly to right it. The experiments seem to indicate that in a normal starfish the stimulus produced by the pulling of two or three arms in the same direction has an inhibitory effect on the other arms. This inhibition ceases when the nervous con- nection between the single arms is broken. Romanes found that a single arm containing only the peripheral arm-nerve rights itself when laid on its back. Hence the central nerve-ring acts only as a conductor and not as a " centre " for this reaction (i).

2. In analysing this righting reflex of the starfish, there are two possibilities to be considered. Either gravity forces the starfish to turn the ventral side toward the centre of the earth, or contact-irritability, i. e., stereotropism, forces the animal to bring the vent- ral side in contact with solid bodies. The fact that the animals leave the horizontal bottom of an aqua- rium and attach themselves to the vertical sides shows that gravity is not the cause. Preyer made an ex- periment from which he concluded that the righting of the starfish is due to their being forced to have the ventral side down. He suspended a starfish in the middle of the aquarium by fastening each of its arms by threads to a cork that floated on the surface of the aquarium. If suspended with its back down, Preyer noticed that the starfish turned over. This might suggest the idea that the righting of the star- fish is a geotropic phenomenon. I have repeated Preyer's experiment and confirmed his observation (2). At the same time, however, I made a control experi- ment which Preyer omitted. In the beginning I fastened the starfish to the cork-plate in such a way that the ventral side was turned toward the bottom. But the starfish even then turned over. This shows that the suspension makes it restless and causes it to perform all sorts of turning movements. I believe

that the ventral side of the starfish is positively ste- reotropic, or, in other words, that the starfish becomes restless if its ambulacral feet are not in contact with solid bodies. 3. Preyer accredits the starfish with possessing " intelligence." He placed one arm of an Ophiuris in a piece of rubber tubing in order to see whether the animal would be clever enough to rid itself of this impediment to its movements. He found that after a time the arm " freed itself " from the tube. I have repeated the experiment in these animals and found that the Ophiuris pays no attention to the rubber tube. The animal of course loses it after a time un- less it fits too closely, but it is always purely a matter of chance, and there is no more intelligence involved than the clothes-line displays when the clothes are blown from it by the wind. Romanes found that when one arm of a starfish is stimulated the animal moves in a direction opposite to the stimulated arm. This also looks like intelligence, for the animal seems to be able to avoid a danger. The late Professor

Norman called my attention to the fact that when one arm of a starfish is stimulated the feet of this arm are drawn in and the arm becomes inactive. This is, however, only true of the stimulated arm ; the others remain active. Therefore, accordine to the parallelogram of forces, a movement away from the point of stimulation will take place. Intelligence plays no part in this phenomenon. surfaces is a pronounced reaction of Echinoderms and is quite common in other animals, for instance in the Actinia mesembryanthemum of the Mediterranean, and in the Coccinelli. This tendency is also present in plant-organisms -- for example, Plasmodia,-- and here Sachs has traced it back to negative geotrop- ism. I will repeat here the description which I have already given in a former publication of the phenome- non as it appears among Echinoderms (3).

No one who observes the animals on rocks or posts near the surface of the ocean when the water is quiet can fail to notice the relatively large number of Echinoderms. Many of these — for example, the Cu- cumaria cucumis, which is very common in the Bay of Naples — always live near the surface, not beyond a depth of about 30 m. It can be shown that Cucuma- ria, like Plasmodia or Coccinelli, are forced, when on vertical surfaces, to crawl upward. Cucumaria has a slender pentagonal body, 10 cm. or more in length, with radial, branchino- tentacles on its oral end.

There are five ridges on the body, and in these are situated longitudinal rows of tube-feet, by means of which the animal crawls upward, even on smooth glass walls. If placed in an aquarium, it crawls about on the bottom until it comes to a vertical side ; it then crawls upward and remains on the highest point, if possible just below the surface of the water. This position then usually becomes permanent, and the animal is converted into a sessile organism.

zontal axis in the aquarium, it will crawl upward whenever the glass is turned. This is not a compen- satory movement produced by the centrifugal force, for during the rotation of the glass the animal re- The animals are in a battery jar (a, 6, c, d). It is filled with water and rests on the bridge B B in the aquarium A A. Running water is supplied through the tube g at o. They collect at the highest point (<r, d) of the glass. mains quiet, and not until a quarter or half an hour after the rotation does it begin to migrate upward. Neither is the upward migration caused by the light falling in from above. If the animals are placed in an aquarium in which light is allowed to enter only from the side or from below, they will still crawl up- ward on the vertical sides. In a dark room they be- have just as they do in the light.

One might believe that the need of oxygen determ- ined the upward migration of the Cucumariai to the surface of the water. It can be shown, however, that this is not the case. If a large beaker filled with water be placed inverted in the aquarium, the Cucu- mariae that are under the beaker begin to creep up to the bottom of the glass. They also do so when the experiment is made in the manner represented in Fig. 19. A bridge B B is placed in the aquarium A A, the horizontal part of the bridge B B being below the surface of the water of the aquarium. The hori- zontal part has a round opening o over which the in- verted beaker abed filled with water is placed. Fresh water is supplied at a low pressure at o through a glass tube^, which has been properly bent. The Cu- cumariae nevertheless go away from o and remain at the highest point c d, or near cd on the vertical sides (Fig. 19), where they ultimately die.

Experiments on the centrifugal machine yielded no result, for the animals did not move during the rota- tion. Gravity is the only condition which can account for the phenomenon, and I imagine the influence which gravity exercises to be in a manner similar to that observed among insects — for example, in butter- flies which have just emerged from the chrysalis. The wings of the butterfly do not unfold immediately, and it runs about restlessly until it comes to a vertical surface. When this is reached, the butterfly creeps upon it and remains there for some time with its head up. After the wings are spread, other conditions cause the animal to be restless again.

cumarire are of necessity inhabitants of the surface- regions of the ocean. If a larva were carried clown to a great depth, its negative geotropism would force it to migrate upward until the highest point was reached or until death put an end to its upward journey. Certain starfish - - for instance Astcrina gibbosa, which also lives near the surface of the water — behave like Cucumaria. All the experiments I have made on Cucumaria can likewise be successfully performed on Asterina gibbosa, but with the difference that the exceptionally voracious Asterina does not remain per- manently at the highest point of the vertical surface. In two days, or sometimes even sooner, it begins to move or drops down.

Positive heliotropism naturally has the same effect as negative geotropism. Asterina tenuispina, like As- terina gibbosa, lives at the surface of the sea. It is not, however, geotropically irritable ; but it is posi- tively heliotropic. I put a large number of specimens of both species in a heap in an aquarium, into which rays of light from one side only fell nearly horizon- tally. In a short time the two species had parted, the Tenuispinse crawling off on the floor toward the source of light. The Gibbosae, scattered about on the bottom of the aquarium in every direction, crawled up the vertical sides without being influ- enced at all by the light in their movements. In the ocean, where the vertical rays of daylight are chiefly concerned in the orientation of animals, positive

heliotropism must drive Asterina tenuispina to the surface of the ocean, just as Asterina gibbosa is driven there by negative geotropism. Preyer mentions briefly in his extensive work on The Movements of the Starfish the " tendency of these animals to move upwards." " The strong tendency of starfish and brittlestars to go upward cannot be traced back to lack of air, lack of food, changes in tempera- ture or current, or to a desire for light, for they climb up just the same when these conditions are eliminated. Probably some peculiarity of the bottom, or of just that part of the bottom where the animal is, makes it unsuitable for the suction of the tube-feet. The ani- mals can remain there no longer, so they move up- wards. But it is possible that parasites, which I have often found in the ambulacral furrows, may cause this upward migration, for as the stimuli produced by them come from below, they might seem to belong to the bottom."

The first sentence in this generalisation is wrong ; the light attracts Asterina tenuispina upwards. Sec- ond, the character of the bottom does not determine the phenomenon. If Asterina gibbosa be placed in a cubical box with glass sides, the animals leave the basal horizontal side and crawl up the vertical sides. If the box then be turned 90° around a horizontal axis, the side which is now basal is deserted by the animals. They crawl up and remain on the side which, while horizontal, they had left. Finally, if Preyer believed that parasites force the animals to

crawl upward, it is difficult to see why they should not drive the animals down from the vertical side. As a fact, however, Asterina gibbosa, as well as Cucu- maria cucumis, remains on the highest point of the vertical side. I believe it is much nearer the truth to ascribe the vertical upward movements of certain starfish to an action of the force of gravity. 2. PREYER, W. Ueber die Bewegung der Seesterne. Mit- theilungen aus der zoologischen Station zu Neapel, Bd. vii, S. 96.

i. We shall consider separately in this chapter two kinds of worms : first, those in which the ganglia are all crowded together in the head end - - c. g.t Planarians ; and second, those with a series of segmental ganglia - - c. g., Annelids. Sea- and fresh - water Planarians differ little structurally, yet they may show different re- actions upon losing the oral ganglion. cm. long and nearly as broad. The oral end of the body, which can be recognised by two tentacles

Fig. 20), contains the brain of the animal. This consists of two connected ganglia, from which a series of nerves, containing single ganglion-cells, go out ; among the latter, the two large longitudinal nerves running length- wise throughout the animal (it, Fig. 20) are conspicuous. In the periphery a plexus is formed ( i ). The central nervous system consists of the double ganglion in the forward end. Like all Planarians, Thysanozoon crawls on the side of the aquarium or on the surface film of the water. It differs from the fresh - water Planarians in being able to perform, in addition, genuine swimming move- ments. With the sides of its body it makes vibra- tions similar to those made by the wings of a but- terfly. If while a Thysanozoon is gliding about on the surface of the water it be divided transversely with a pair of scissors, the posterior or aboral half (b, Fig. 21) at once falls to the bottom, while the oral piece (a, Fig. 21) containing the brain creeps on undisturbed. If the division be made with a

The anterior piece a, containing the brain, shows spontaneity ; the posterior piece 6, none. sharp knife while the Planarian is crawling on a glass plate, the oral piece (a, Fig. 21) crawls on un- disturbed, while the progressive movement of the posterior piece ceases entirely. The spontaneity of the progressive movement of the Thysanozoon is then a function of the part of the body containing the brain (2). In a Thysanozoon that has been divided, both pieces live and regenerate the lacking parts. The oral piece, however, regenerates more rapidly than the aboral piece, which has to form a head. I have not investi- gated whether the latter also forms a new brain. I kept such pieces alive for four months. The spon- taneity of the posterior piece never returned ; the spontaneity of the anterior piece remained.

If we put a normal Thysanozoon on its back it soon rights itself. The question now arises whether, like the progressive movements, these righting movements are a function of the brain. This is not the case. A Thysanozoon from which the brain has been removed rights itself if laid on its back, only the reaction pro- ceeds more slowly than in the normal animal, or even in a piece of an animal if this piece contains the brain. We see here again that the nervous system only serves to bring about a quicker reaction.

If, instead of dividing the Thysanozoon completely, we leave the two parts on one side connected by a thin piece of tissue in such a way that (Fig. 22) the posterior piece can receive no direct innervations from the brain through the longitudinal nerves, conduction of stimuli would still be possible through the side nerve-plexus. Such an animal was placed after the operation on the bottom of the aquari- um ; the anterior piece im- mediately began to move, while the posterior piece attempted to attach itself to the bottom. The latter soon yielded to the tugging of the oral piece, however, and took part in its pro- gressive movements in an entirely coordinated man- ner, as though no incision had been made. After a time the oral piece turned around and crawled over the back of the posterior piece, which was dragged behind passively, and was turned on its back. It righted itself immediately and moved off actively in the same direction as the oral piece. Changes of direction originated only in the piece containing the brain and were never transmitted directly to the posterior piece. But if the oral piece continued for a time to move in the same direction and with the same rapidity, the same movement would soon take place in the posterior piece. Hence the posterior piece did not behave entirely like a piece from which the brain had been removed, for it made

progressive movements, nor yet like a normal Thy- sanozoon for it had lost its spontaneity. This becomes still more apparent from the following observation : I threw a Thysanozoon similarly operated upon into a tank of water. Both pieces performed synchronic swimming movements. The oral piece soon reached the vertical side of the aquarium and began to creep upwards. As a result of the change of direction in the anterior piece, the tissue connecting the two parts became twisted and the back of the posterior piece came in contact with the glass, while the ventral side was turned toward the water. It then made swim- ming movements and in this way followed the crawl ing movements of the oral piece. The posterior piece therefore is not simply dragged behind passively, but takes an active part in the progressive movement when the movements are continuous. This is also evident from the fact that it would often crawl along on the back of the oral piece, especially if the latter suddenly began to move more slowly.

These experiments show that a Thysanozoon from which the brain has been removed no longer moves spontaneously, nor is it possible to produce progress- ive movements in it by any external stimulus. If touched, local contractions, only, result. 2. The brain and nervous system of the fresh-water Planarians (Fig. 23, from Jijima) are so similar to those of the marine Planarians that for our purpose it is un- necessary to give a special description of them. The principal difference is probably that the two longitu-

dinal nerves contain a greater number of ganglion- cells, so that they almost form segmental aggregations. From this similarity we should infer that the brain- functions of the fresh-water Plana- rians would be analogous to those i^M'™— -• of the Polyclads. However, such is not the case. If we divide a fresh-water Planarian, for instance Planar ia torva, transversely, the posterior half, that has no brain, crawls just as well as the oral half. Spontaneity in Planaria torva is, therefore, by no means a function of the brain. Every piece of the animal that is not too small pos- sesses spontaneity. The decap- itated animals crawl with the anterior end in front like normal animals (2).

fresh-water Planarians this opera- tion does not have such a result. One is tempted to account for the difference by the fact that the fresh-water Planarians have more ganglion-cells throughout the longitudinal nerves than the Thysano- zoon. With the aid of comparative physiology it is possible to show that such a view is untenable. In the crayfish, the suboesophageal ganglion with the ventral ganglion chain represents a much more highly developed ganglion-system than the longitudinal nerves in Planar ia torva. We shall see, nevertheless, that a crayfish, which possesses these ganglia, but has lost the supracesophageal ganglion, no longer moves spontaneously. We shall see, furthermore, that a frog that has lost the cerebral hemispheres and thalamus opticus does not move spontaneously, al- though it possesses many more ganglia in the spinal cord than Planar ia torva. The same frog, however, moves spontaneously again if, in addition, the optic lobes and the pars commissuralis of the medulla oblongata be removed.

Spontaneous progressive movements are not a specific function of ganglia or of ganglion-cells ; we observe them even in the swarmspores of algae and in bacteria. Why the decapitated Thysanozoon no longer performs progressive movements, and a decap- itated fresh-water Planarian continues to move spon- taneously, we are not yet prepared to say. It is possible that the difference between fresh-water and marine Planarians is somewhat of the same character as that between Hydromedusae and Acalephae. In the latter, both parts, margin and centre, beat rhyth- mically in sea-water, while in the Hydromedusae only the margin with the nerve-ring is able to do so. But we were able to show that this difference between the two classes of Medusae is not so much due to mor- phological differences as to chemical or physical

differences. A reduction in the amount of Ca ions in the sea-water allowed the centre of a Hydromedusa to beat spontaneously. The case of marine Planarians may be similar, and further experiments may yield the result that with a change in the constitution of the sea-water the posterior half of a Thysanozoon will be able to show spontaneous locomotion. special interest. The animal is especially sensitive to changes in the intensity of light. If brought from the dark into the light suddenly, it begins to move. At first the direction of the movements seems to be influenced by the light, for the animals move away from the source of light as if they were negatively heliotropic. However, they do not collect at the point farthest from the source, as do negatively heliotropic animals, but they scatter in all directions and come to rest at last in a place where the light is comparatively weakest. From this it would seem that an increase in the intensity of light causes them to move, while a decrease in the intensity of light causes them to rest. This would account for the fact that we find them by day always under stones or in relatively dark places. I suspect that they begin to move about in the night, and that they come to rest when day returns. I have repeatedly tried the experiment of covering in the morning one-half of the dish with black paper. During the day no change takes place, but the next morning all the animals are found under the covered portion of the dish. The only possible