ON THE PHYSIOLOGY O F THE CENTRAL NERVOUS SYSTEhI IN THE STARFISH, ASTERIAS

TENUISPINA

A. E. HOPKINS

Parker Traveling Fellow i n Zoology, Harvard University

INTRODUCTION

The behavior of echinoderms has f o r long attracted con- siderable attention, perhaps largely because of the relatively slow movements of the animals and the high degree of autonomy exhibited by certain organs and parts.

The first of the more important modern physiological observations on the starfish were made by Romanes and Ewart ('81) and more particularly by Romanes ('85). Later, Preyer ( '86-'87) made extensive observational and experimental studies on many echinoderms and contributed very accurate information on responses to stimuli, coordina- tion in locomotion, and other phases of behavior. I n the present century, Jennings ('07) made a series of careful observations on the behavior of a starfish, with especial reference to the righting reaction. More recently, Mangold ('08a, '08b, '21) made thorough studies on the physiology of the nervous system.

That the central nervous system of the starfish, consisting of a circumoral nerve ring from which a radial nerve is given off into each arm, functions in the coordination of the activities of the animal as a whole is evident from the ob- servations of many workers. Hamann ('85) and others have shown that these nerves contain ganglion cells, so that both structurally and functionally they are probably of a truly central nervous nature.

263

264 A. E. HOPKINS

Romanes and Ewart ('81) found that section of a radial nerve at any point destroys physiological connection between the tube feet on the two sides of the wound, and that removal of the nerve ring destroys coordination between the tube feet of the different arms. The nerves are conducting pathways over which the coordinating impulses travel. Conduction, however, is not their only function, for the same observer found that an isolated arm can move as well and as rapidly as the entire animal, the radial nerve acting as directing center and center of spontaneity. Other workers, notably Preyer ( '86-'87) and Mangold ( '08 a ) , have confirmed and extended these observations. There is little or no spon- taneous motion of the tube feet of an a rm if the radial nerve be removed (Mangold, '08 a ) . It may be of interest to gain some concept of how one par t of the nervous system may dominate all other parts and cause them to coordinate with it to produce locomotion in a particular direction and also to learn if it is the radial nerve of the leading a rm which dominates or if this is a function of the nerve ring.

These experiments were made a t the Stazione Zoologica, Naples, and I here express my thanks to Prof. R. Dohrn and other officers of the station for their many courtesies. To the American Association for the Advancement of Science I owe great appreciation for the use of its table a t Naples.

EXPERIMENTS

For these experiments Asterias tenuispina was chiefly employed. Animals of this species have a varying number of arms with seven as the modal number. Usually some of the arms are regenerating following fission of the animal. When experiments on other species are recorded, it is so stated.

When a resting starfish is picked up and replaced, it will soon begin to move, due to the stimulation of handling. As has before been observed, if the animal does not have an already existing impulse to move in a determined direction, the first noticeable activity is a stretching out of all arms in

PlIYSIOLOGY O F THE CENTRAL NERVOUS SYSTEM 265

their axes, the feet taking hold and each arm attempting to progress with its distal end forward. Initially, each arm attempts to lead. Soon, however, one ray or two adjacent ones have success, and the other arms come slowly into coordination with the leaders. The direction finally taken by the animal may depend sometimes on environmental factors, as Jennings ('07) suggested and sometimes upon an innate predisposition of the animal to move with certain rays in advance (Cole, '13). The point of importance here is that initially the undominated arms move with their distal ends forward. This fact suggests the possibility that the arms may be polarized with the distal end as a sort of functional anterior.

To test this, the arms were cut off near the disc from several specimens and the arms observed periodically for several days. In a few cases the arms moved a short distance with the tip forward immediately after the operation, but in most cases they remained for a few moments still and then all proceeded with the basal ends in advance. Very often such an arm would writhe and bend, but the tube feet would continue to push it along, base forward. The arms have a tendency to bend dorsally, causing them to fall over on one side, but the feet on the distal half o r third of an arm are able to reach the surface beneath and so propel the arm around in a circle. The same was found to hold f o r isolated arms of Asterias glacialis. At first this was thought to be due to the stimulation of the wound, but later observations and other experiments indicate that such is probably not the case. The arms were kept alive for periods up to six days, and they persisted in moving with their basal ends forward. Mangold ( '08 a ) noted this tendency of isolated arms of three species.

It proved to be very difficult to cause these arms to move tip forward. When the distal end of an arm was squeezed with forceps o r fingers, the arm only increased its rate, but the direction remained the same. The same was true if the basal end was so treated. Under such stimulation, the arm

266 A. E. I IOPKINS

would often bend and its direction would be so altered, but the base was persistently in advance. Occasionally an arm, especially after violent stimulation, would move directly sideways f o r 15 em. or more, and slowly the motion would become more and more toward the basal direction. An arm could be caused to move temporarily with its distal end in advance by making an incision through the dorsal wall along the whole length of the arm, or by a series of short, diagonal cuts. Even when an arm is cut into several pieces, each piece, if i t is not too much damaged to move, progresses with its more basal end forward. These results appear to indicate that an arm which is not in connection with the nerve ring and which depends upon the radial nerve alone for locomotion is so polarized that the basal end acts as the functional anterior.

If the nerve ring be removed from a specimen or if each radial nerve be cut through near its base, and the motion of the tube feet observed through the glass bottom of a dish on which the animal is attached, it is seen that the feet extend toward the disc. This is especially evident in the more active specimens. Each arm acts as if it were completely isolated from the disc and attempts to move in the basal direction.

Contrasted to the above is the behavior of a single arm left attached normally to the disc after all of the other arms have been cut off near their bases. Here the radial nerve retains its normal continuity with the nerve ring. Such a mutilated animal moves with the distal end of the single arm in the lead, as was seen above to be the initial movement of each arm of the normal starfish, and acts as if it were an entire animal in responses to mechanical stimuli. The same is true of a regenerating specimen with one long arm and several very small ones, so this is not the result of moving away from stimulation of cut surfaces. Squeezing of the distal end of the arm causes it to bend a little and proceed in the new direction to which it points. Stronger stimulation may cause the animal to move f o r a short distance with the disc leading, but soon the tip of the arm is again in advance. Removal of

PHYSIOLOGY OF THE CENTRAL NERVOUS SYSTEM 267

the sense organ at the tip of the arm produces only such a temporary reversal of movement. However, if the radial nerve be severed near its base, there is a lasting reversal of the direction of movement, so that the animal moves only disc forward, being propelled by the tube feet on the arm. These facts indicate that it is the nerve ring which controls the polarity of an arm whereby it moves in the distal direction, and that the reversal noted in isolated arms is not due to the severing of any other nervous connections than the radial nerve.

The question arises as to how much of the nerve ring is necessary for the proper functioning of an arm. It is well known that if a starfish be cut into two parts through the disc, each part is capable of apparently normal movement. Such a part of A. tenuispina, consisting of two o r more arms and the adjacent portion of the disc, is well capable of movement, and it is usually the arm or arms in the midst of those present which lead. There is a very decided tendency for the direc- tion of progression to be that indicated by the direction in which one arm or two adjacent ones point, even when the sense organs have been removed. It was shown that an isolated arm with a portion of the nerve ring attached acts in essentially the same manner. However, apparently only a relatively small piece of the ring is necessary. Arms were so isolated that parts of the nerve ring of various sizes were left, and it was found that those arms which moved with the distal ends in advance were in connection with pieces of nerve ring normally extending on either side to the base of the adjacent radial nerve. Those arms with a smaller piece acted as if they were entirely separated from the nerve ring and moved in the basal direction. In A. glacialis not so much was necessary, for an arm would move in the distal direction if only that portion of the nerve ring immediately in connec- tion with the radial nerve were left.

The results above described suggest the possibility that the activity of each arm is controlled by some sort of nerve center in the nerve ring at the base of the arm. Morphologi-

268 A. E. H O P I i I N S

cally, the nerve ring is not clearly separated into ganglia and it may well be that the nervous tissue of the ring at the base of each radial nerve acts, because of its immediate connection with the radial nerve, as the controlling center for the activities of that nerve. Such a specialization may, indeed, be expected. If physiological centers of this sort exist in the nerve ring of the starfish, they are quite well connected together, for the coordination of the righting and locomotory activities of the animals is very remarkable. The leading arm mag so dominate the activities of the other arms that they work in perfect harmony with it.

This harmony of movement may, however, be interfered with so that oppositely located arms pull against each other. If a moving animal be lifted up and held so that only those arms on the side away from the direction of locomotion are in contact with the bottom of the aquarium, the tube feet of these arms take hold and soon begin pulling outward so as to move the animal in the direction of the tips of the arms. Then the formerly leading arms are gently let down. The latter begin to move as before, while those on the opposite side continue in the opposite direction. Consequently, the specimen comes to a standstill, each side unable to cause motion against the efforts of the other and the controlling centers of each set of arms unable to dominate the other centers and bring them into coordination. After a few moments of active struggling, the feet take hold as they do in going up a vertical surface. When observed through the glass bottom of the aquarium, it is seen that the feet on the two physiologically separated sets of arms are attached out- wardly, away from the disc. This experiment is best per- formed by allowing the feet of the previously following set of arms to become attached and then pulling the animal a little in the direction of former locomotion. When the feet a re stretched in this manner, they immediately begin pulling in the opposite direction.

The coordination of the movements of A. glacialis is much more rapid and perfect. The arms could not be made to pull

PHYSIOLOGY OF THE CENTRAL NERVOUS SYSTEM 269

against each other more than momentarily. The direction of locomotion is very quickly altered.

The above observation, that coordination between the arms may be partially upset physiologically, is probably of con- siderable significance in regard to the nervous physiology of starfishes. It indicates that the nerve ring does not neces- sarily act as a unit. It is not always easy to produce this interference of coordination, for it appears that if the arms on one side of the animal under experimentation are more numerous or longer or more stimulated by light o r other factors, these arms will be able to gain some headway. When this once happens, the other arms slowly come into coordina- tion. I t is as if the success of the effort acts as a further stimulus which causes the opposing centers of activity to coordinate. Sometimes the strength of the already existing impulse to move in a particular direction is so great that arms opposite will not attempt to lead. Often when the leading arms are held up while those opposite take hold, the direction of movement is completely reversed, especially when the latter arms are allowed to gain a good start. In such experi- ments it would often happen that an arm which stands out at right angles to the direction of progression would become the new leader and bring into coordination with it the arms which had been opposing each other. F o r the experiments it was found better to use an animal which had about half of its arms pointing in the general direction of motion while the others pointed in the opposite direction. An active animal in this condition could often be made stationary as above described. Progression of a ‘locked’ specimen was fre- quently brought about after a number of minutes by the bending of one or two arms, so that their pull became lateral to the direction of strain. This appeared to make it easier for the initial progression to begin.

I n cases in which the two sets of arms are well balanced and pulling strongly against each other so as to prevent locomo- tion we have, apparently, the equivalent of two animals tied together and each attempting to pull the other along. From

270 A. E. HOPKINS

this point of view the two halves act quite independently. I t was noted above that part of an animal, consisting of one o r more arms the radial nerves of which retain their connec- tion to the corresponding parts of the nerve ring, moves persistently with the arms extended in the general direction of motion. It is this same tendency of the arms to move tip forward which is here demonstrated in the morphologically intact animal. The control of this tendency, as indicated by experiments on isolated arms deprived of nerve-ring connec- tions, appears to lie within the nerve ring. It may be considered, then, that the centers controlling the activity of the arms on each side of the physiologically divided animal coordinate with each other, but not with those centers of the opposite side. The arms of each side act under the domina- tion of one or of two adjacent arms of their number or, better, of the centers in the ring corresponding to these.

Asterias tenuispina has been observed by a number of investigators to undergo self-division in which the animal separates into two more o r less equal parts. Crozier ('15, '20) made extensive observations on fission in this species in Bermuda. Almost all specimens have three or four relatively long arms on one side and usually four short ones on the other which are regenerating following division. The ob- servation described above may be significant in regard to the mechanism of this process. The relatively poor physiological coordination between the arms and the probability of the animal's becoming divided physiologically into two opposing parts might readily result in a tearing of the disc. How this may be brought about in nature is not difficult to conceive. Crozier ( '20) showed that more specimens undergo fission as the weather becomes warmer, indicating that the proper physiological condition, as determined by external influences, may be an important factor. The same author indicated that each plane of fission is roughly at right angles to the previous one, and also that in a regenerating individual it is the older and stronger arms which lead in locomotion. These two processes appear to be associated, f o r it is the strong, leading

PHYSIOLOGY OF THE CENTRAL NERVOUS SYSTEM 271

arms which must become opposed to each other in order to produce division, because the fact that these always, or very commonly, lead demonstrates that they, when working together, easily dominate the activity of the smaller arms.

We can imagine, for example, a resting animal which is stimulated to activity. In the initial movements in which all arms attempt to progress outward in their axes the animal may become separated into two non-coordinating parts. The opposition of the parts may then tear the animal slowly into two. It is necessary to note that self-division in the starfish appears to be quite a different process from the autotomy of a single arm. I n the latter, as is well known, the arm is dropped from the disc along a very sharp line which is deter- mined by structural peculiarity, for Crozier (%) observed that even in preserved specimens arms can readily be removed a t this place. The writer has observed a number of starfishes of this species in Bermuda in the process of division, and no sharp line or constriction was visible. During a period of several hours, the two parts would move farther and farther apart, the disc stretching and tearing until finally the two parts with frayed edges would be com- pletely separate. Such a process is readily accounted for on the basis of the physiological separation of the animal into two parts as described above.

I f this explanation is correct, it is to be expected that a similar morphological separation of the nerve ring would cause the arms of the two parts to act in opposition to each other and eventually tear the animal into two. This was tested and found to occur. A number of specimens were anaesthetized and the nerve ring cut through at two opposite points so that the arms on the two sides were roughly equal in size. Immediately upon recovery from the anaesthetic, all of the arms moved in the distal direction determined by their natural polarization, and slowly those arms which remained connected together through the nerve ring coordinated with each other and attempted to move in the same general direc- tion, distal ends leading. The animal was thus separated

272 A. E. HOPI'iINS

into two functionally independent halves, each of which attempted to produce locomotion in the direction determined roughly by the axis of its most centrally located arm. This condition persisted until the animal had pulled itself into two parts by tearing the disc-a process requiring usually between twenty-f our and forty-eight hours under laboratory conditions.

It was hoped that this might be demonstrated in the labora- tory by the purely physiological method above described, but the attempts were unsuccessful. I n many cases specimens have been physiologically divided so that the parts pulled against one another for as long as an hour and sometimes longer. In such cases the disc would show considerable stretching, but finally, due to changing stimuli perhaps, the movements would become coordinated. Under natural con- ditions, when the animal is much more active and vigorous, it is probable that a similar condition would very quickly rupture the more delicate organs in the disc at the places of greatest stretching and in this manner break the nerve ring at these points. This species would not live well in the laboratory and quickly became inactive, dying after a few days. The essential factor in fission, however, namely, the physiological separation of an animal into two opposing parts, has been well shown.

DISCUSSION

The results of the experiments described above appear to indicate that the nerve-ring portion of the central nervous system of the starfish, Asterias tenuispina, may consist of a series of physiologically separable though normally coordi- nating units. That the coordination of the movements of the tube feet of an arm is not a simple matter was pointed out by Mangold ( '08 a ) . An arm during locomotion may be bent entirely around, but a t every place on it the feet act so as to produce movement in the same direction. All feet extend in the direction of movement. In this respect, an arm is able t o function somewhat independently, but the direction of

PHYSIOLOGY OF THE CENTRAL NERVOUS SYSTEM 273

locomotion of the arm as a whole is determined outside of the arm itself. The separation of an animal into two physiologi- cally independent and opposing parts, so far as locomotion is concerned, demonstrates that the nerve ring, as that part of the nervous system which coordinates the activities of the various arms, does not necessarily function as a single unit. It appears probable that the unit of activity here is that portion of the nerve ring adjoining the radial nerve. These centers may be thought of as intimately associated with, and, perhaps, directly continuous with, each other. Apparently, the nerve ring is not merely a pathway of connections between the arms, but it is also directive in function, as shown by the difference in direction of locomotion of isolated arms with and without connections with portions of the nerve ring and also by ofher observed facts.

Mangold ( '21) was unable to discover any directive activity of the nerve ring in the righting reactions of the starfish. He concluded that the ganglion-cell mass at the base of the radial nerve where it joins the nerve ring functions for coordination with other arms, but is not directive. Isolated arms would right themselves in essentially the same manner as when retaining their normal nervous connections. The high degree of autonomy exhibited by isolated arms renders it difficult to observe a directive function unless there is such a reversal of activity as that above described.

It is significant that a group of coordinating centers which usually act under the domination of one or of two adjacent of their number may become so divided that two non-coordi- nating groups result. This is due largely to the relative slowness with which one center may dominate the others. That this is the case is indicated by the fact that it is much easier to cause such a physiological separation in an animal with seven or more arms than in a specimen with only three or four. Coordinating impulses traveling out from the lead- ing arm, or from its center in the nerve ring, appear to progress slowly around the nerve ring, bringing the most adjacent arms first into coordination. Preyer ( '86-'87)

274 A. E. HOPKINS

noted such a slow transmission of impulses causing closing of the ambulacral grooves when one arm was properly stimu- lated. The two opposing activities appear to be unable to coordinate with each other, or one counteracts the influence of the other. It may be that the impulses from one part to the other mutually inhibit each other at the two points in the nerve ring which mark the place of physiological separation.

The method by which this separation was brought about if of considerable interest. The activity of the controlling centers was rendered powerless or futile, and this allowed the opposite centers to become free of domination and begin to move in the direction determined by their normal polariza- tion. The success of the effort appears to be an added stimu- lus to continue the movement and to bring adjacent parts into close coordination, while the failure of the effort weakens existing coordination and allows other centers to start ac- tivity in a new direction. If the former activity be rendered powerless f o r too long, the new activity assumes control of the animal and the direction is completely reversed; if the leading centers are restrained for a very short time, motion proceeds as before, following a short pause in which coordina- tion is being properly reestablished. I f , however, the pre- viously submissive centers are allowed to gain a coordinated state of activity comparable to that still existing in the pre- viously directing centers, these two groups become physio- logically separated from each other and act in opposite directions. Apparently, this is a consequence of the tendency of the arms, under the direction of the nerve ring, to move independently with their distal ends leading.

It appears probable that fission in the starfish is produced in this manner. The fact that all species of starfish do not divide indicates that coordination in these animals is usually more perfect. The condition is Asterias tenuispina appears to render this species especially favorable f o r observations on nervous physiology.

PHYSIOLOGY OF THE CENTRAL NERVOUS SYSTEM 275

SUMMARY

Isolated arms of the starfish without connection with the nerve ring tend to move with their more basal ends leading, as Mangold ( '08 a ) noted, while those retaining connection with a part of the nerve ring tend to move with the distal ends in advance. This indicates a possible directive function of the nerve ring. It appears probable that each arm may be controlled by a center in the nerve ring at the point of con- tinuity with the radial nerve.

Coordination of the arms may be so interfered with physiologically that two sets of arms result. The members of each group coordinate with each other, but the two groups are physiologically separate and pull against one another.

I t is probable that fission in the starfish is brought about in this manner. Section of the nerve ring at two opposite points causes the animal to divide in the plane determined by the wounds.

BIBLIOGRAPHY

COLE, L. J. 1913 Direction of locomotion in the starfish (Asterias forbesi). Jour. Exp. Zool., vol. 14, pp. 1-32.

CROZIER, W. J. On the number of rays in Asterias tenuispina Lamk. at Bermuda.

1920 11. On the temporal relations of asexual propagation and gametic reproduction in Coscin- asterias tenuispina: with a note on the direction of progression and on the signifirance of the madrepores. Biol. Bull., vol. 39, pp. 116-129.

HAMANN, 0. 1884 Beitrage zur Histologie der Echinodermen. Hef t 2, Die Asteriden. Jena.

JENNINGS, H. S. 1907 Behavior of the starfish, Asterias forreri de Loriol. Univ. Gal. Publ. Zool., vol. 4, pp. 53-185.

MANGOLD, E. 1908 a Studien zur Physiologie der Echinodermen. I. Die Fiissehen der Seesterne und die Koordination ihrer Bewegungen. Arch. ges. Physiol., Bd. 122, S. 315-360.

1908 b Studien zur Physiologie der Echinodermen. 11. Ueber das Nervensystem der Seesterne und iiber den Tonus. Arch. ges. Physiol.,

1921 Der Umdrehreflex bei Seesternen und Schlangensternen. Arch. ges. Physiol., Bd. 189, S. 73-98.

PREYER, W. 1886-1887 Ueber die Bewegungen der Seesterne. Mitt. Zool. Stat . Neapel, Bd. 7, S. 27-127, 191-233.

RONANES, G. J. 1885 Jelly-fish, star-fish and sea-urchins. London. ROMANES, G. J., AND J. C. EWART

1913 Amer. Nat., vol. 49, pp. 28-36.

Notes on the problems of adaptation

Bd. 123, S. 1-39.

1881 Observations on the locomotor system Phil. Trans. R. S., London, vol. 172, pp. 829-885. of Echinodermata.