Examples of surprisingly dexterous co-operation and concerted adaptive action are observed in these animals in their climbing on difficult surfaces, and in their attempts, also, to regain their normal position when placed on their backs or made to swim in reversed positions by discs of cork fastened to the extremities of their arms. Scientists have observed members of the orders Asteroidea and Ophiuroidea, in difficult positions of this kind, display an astounding sense of equilibrium and a skilfulness in gaining firm holds, suggestive of the athletic feats of monkeys, and that even when placed in very unusual positions such as never occur in nature. Thus many star-fishes let themselves drop from steep rocks and cliffs, if that happens to be the best way of getting down; but in such cases before they let their whole weight go hold fast to the last moment with one or two arms, as if it were previously necessary to calculate the leap into the depths below. To furnish the counter-test of this, and to prove that the central nerve-ring is, as assumed, the indispensable and necessary condition of this united co-operation, Preyer severed the ring in individual specimens of the class between every two arms, sparing the other parts as much as possible. In this way the nervous systems of the five rays were disconnected. As was expected, it was found that the more connections there were severed, the more difficult the animal found it when placed on its back to regain its normal position. For since the recovery of the normal position must be introduced by the groping about and the fastening of the pedicels of one or of several adjacent and half-turned arms, two arms or pairs of arms might for want of a mutual understanding act directly in opposition to one another and thus make the turning impossible. On the other hand, the central disc was able, though deprived of all arms, to accomplish the turning, if only the nerve-ring were preserved intact; and the more there remained of the nerve-ring on a single arm the better the single arm was able to do it.
But in circumstances which were wholly new, the adaptive co-operation of the arms demonstrated itself in so striking a manner that we may say they are not to be easily put out of countenance or confounded. When Professor Preyer, for example, slipped narrow rubber bands or cylinders over their rough spiny arms, they rid themselves as a rule of these unwonted fetters in a very short time, and in the most various but always well calculated ways. Generally the two nearest ones seized their poor imprisoned fellow "under the arms," bracing themselves with their rough spiny surface against the rubber sleeve, and thus finally stripping it off. (See Fig. 5; next page.) Sometimes, when the band was loosely adjusted, twisting movements of the arm in the water sufficed gradually to loosen it, until it could be finally cast off. Often the peeling off was effected by pressing against a rough surface, whereby sometimes an adjacent arm held the sleeve fast; and when no other expedient was of avail the animal cast the arm, sleeve and all, away from itself; and the latter may possibly have not gotten rid of it at all. At times the casting off of the arm occurred subsequently, after the obstacle had been entirely removed, and often even a day later, as if the impeded arm was still sensible of some obstruction which caused it to afterwards separate from its companions.
[Illustration: Fig. 5.—REMOVAL BY OPHIOMYXA OF A RUBBER SLEEVE. (After
Preyer.)
The figure represents the moment at which the band is about to be removed. An adjacent arm is braced against the lower edge of the band, forcing it off in the direction of the extremity of the ray.]
Attempts at flight and liberation from unwonted compulsory positions or narrow confinement, also deserve special attention. Many a person who has put a star-fish into a cage and fancied that he was assured of its possession, has been disappointed on finding that the animal had effected its escape through the meshes. But star-fishes have, in consequence of their abhorrence of the air, been made to creep into the narrow necks of bottles filled with water. Professor Preyer, for example, thrust two of the arms of a common star-fish species (Asterias glacialis) into a tube filled with salt water leaving the three other arms exposed to the atmosphere outside; and although it would have been impossible to force the animal into the tube without crushing it, the three arms exposed to the air were also pulled in within the space of three minutes. If the tube was placed perpendicularly in water the animal quickly crept out again. The performance seemed utterly impossible, for each single arm of the star-fish was almost as thick at its base as the greatest width of the tube, and yet three of these arms had to pass in side by side. This was made possible by the animal emptying during the passage all the numerous water-vesicles in the interior of the arms which serve to fill and to empty the distensible pedicels therein; the star-fish, after the expulsion of the water, becomes very soft in all its parts and does not harden again until it has forced itself completely through and refilled itself with water. In order to accomplish these emptyings, bendings, turnings, and rollings, thousands of muscular fibres must work in harmony within the body of the animal. This experiment was also successfully carried out with other star-fishes, but I cannot agree with the observer when he says that in so doing he brought the animals into a completely new and hitherto unexperienced position. In their haunts on rocky coasts they must assuredly often have to force their way through narrow fissures and holes; and they must find occasion to make use of the advantages of being able to evacuate water in the case also of single arms, as when they search with them in narrow apertures and snail-houses.
But undoubtedly new for these animals was the position in which they were fastened to a board by five long pins with broad heads, which Preyer drove in close to the central disc between the rays, so that the star-fish, as it seemed, was fastened to its resting-place in a way that admitted of no escape. Nevertheless, the star-fish found a means of freeing itself with ease and elegance from this constrained imprisonment in a great variety of ways, even when the exterior parts of their bodies were girded in by a much greater number of pins. Ordinarily they began by shoving one of their rays, accompanied by a backward bending movement of its two companions, far out between the two encompassing pins, and then drew with the greatest care first the one and then the other adjacent ray through the same narrow avenue of escape, whereupon then the two remaining rays, the one slightly overlapping the other, were enabled to follow with perfect ease. (See Fig. 6.) A practised knot-untier who had studied the position could not have given them better advice. But if no agreement of plan and purpose existed in this case between the separate rays, if each ray sought to free itself of its own accord, a successful extrication from the difficulty could hardly have been foreseen; and we must infer from this great unanimity of action in times of danger.
[Illustration: Fig. 6.—EXTRICATION OF STAR-FISH IMPRISONED
INTERRADIALLY BY TACKS. (After Preyer.)
1. Original encompassment. 2. First stage of extrication. 3. Second stage. 4. Third and last stage. The smaller figures indicate the successive positions of the same rays.]
Preyer thinks that at times the concurrence of all the rays in matters of concerted action might have to be effected by first obtaining the concurrence and assent of any individual ray that might be hostilely disposed; he holds it as not improbable that profound dissensions may arise between the united brothers, and refers to the fact that perhaps the voluntary section of a star-fish into a three-rayed and a two-rayed portion,—which frequently takes place,—may have to be regarded as the violent dissolution of a community of fellow animals formerly living in harmony, but now lapsed into a state of conflict. We shall pass this view by, however, to point out in a few words Preyer's general inferences with regard to the mutual relation of the five communal souls. Progression and flight in a direction once taken and unimpeded by obstacles,—an observation often made and easily verified,—the acrobatic performances, and lastly the intelligent behavior, so to say, of imprisoned and fettered star-fishes, prove that generally, and especially in moments of peril, strength-giving unanimity prevails.
But Preyer is nevertheless of opinion that it is not therefore necessary to assume the existence of a permanent central government, a central soul, holding simultaneous sway over the five radial souls, and in which is lodged, especially in times of battle, full executive power. He employs the simile of five hunting-dogs yoked together in the form of a ring, of like age, like power, and the same training, who hunt a hare in concert, or stand simultaneously and mechanically before a partridge; when thrown into the water make for the shore all in the same direction, and when equally tired fall simultaneously asleep. "Like the Siamese twins," he says, "these yolked-together dogs will have upon the whole apparently but one will, although they often obey only necessity in this and not their own impulses." Preyer arrives in this at the same conclusion that I pronounced in 1876 in the work I have mentioned, where I compared the concerted actions and movements of star-fishes and sea-urchins to the walking and dancing of human twin-monsters, who in spite of a difference of mental individuality, often very far reaching, nevertheless bring about perfect harmony in their external movements. In this I had especially in mind the so-called "two-headed nightingale," two girls closely united in growth, who often violently quarreled but sang and danced so harmoniously with one another that for the time being the sorrowful fate of the indissoluble union of two so different natures was completely forgotten. In the majority of their relations the five or more associates united in the star-fish are much better off than unfortunate human beings like those just described, and especially in this one particular that they do not have to die with one another, but are able to break loose with impunity from a companion whom death threatens, when they observe that he has suffered a wound or loss, simply expelling him from the community.