One of the most interesting features in connection with the echinoderms is undoubtedly the structure and function of the apparatus for locomotion. Examine a live sea urchin, or the common five-rayed starfish, in a rock pool or aquarium, and it will be seen to possess a large number of soft, flexible, and protrusible processes, each of which terminates in a little sucking-disc that enables the animal to obtain a good ‘foot-hold;’ and, having fixed itself on one side by means of a number of these little ‘feet,’ it is enabled, by the contraction of certain muscles, to pull itself along.
The little feet we are examining are really tubes filled with water, and capable of being inflated by the injection of water into them from within the body of the animal. Each one communicates with a water tube, several of which (usually five) radiate from a circular canal of water that surrounds the mouth. This circular canal does not communicate with the mouth, but with a tube, known as the ‘stone canal’ because of the carbonate of lime deposited within its walls, that opens at the surface of the body on the opposite side, and is guarded at the orifice by one or more perforated plates through which water gains admission. Thus the animal can fill its ‘water system’ direct from the sea, and, by the contraction of muscles that surround the main canals, force this water into the little ‘tube-feet,’ causing them to protrude and present their sucking-discs to any solid object over which it desires to creep. We may observe, however, that some of the little protrusible tubes have no sucking-discs, and probably serve the purpose of feelers only; also, that while these tube-feet are the principal means of locomotion in certain species, in others the movements of the body are performed almost exclusively by the five or more rays that extend from the centre of the animal, and which are readily curved into any desired position by the action of well-developed muscles.
All the echinoderms come within the domain of the marine naturalist, for no members of the sub-kingdom are inhabitants of fresh water; and it is interesting to observe that, unlike the animals previously described, none of them live in colonies.
A general examination of the various starfishes to be found in our seas will show that they may be divided into three distinct groups. One of these contains the pretty Feather Stars, which are distinguished by their long and slender ‘arms,’ usually ten or more in number, each of which bears a number of pinnules that give it quite a feathered appearance. The second includes the Brittle Stars, possessing five slender arms that are jointed to the small, flattened, central disc, and which are so named on account of the readiness with which the animal falls to pieces when alarmed or disturbed; and the third is formed by the remaining five-rayed stars, the arms of which, instead of being jointed to, are continuous with, the centre of the body.
All these starfishes have a leathery skin, supported and hardened by a framework of calcareous plates, and presenting a number of hard ridges or spines. In addition to the system of water tubes already mentioned as characteristic of the echinoderms, they also possess a second circular vessel round the mouth, from which a number of vessels are distributed to the walls of the digestive tube. These, however, are bloodvessels, and are directly concerned with the nutrition of the body. Some, also, have imperfectly developed eyes at the ends of the arms or rays.
Contrary to what one would expect after watching the somewhat sluggish movements of starfishes, they are really very voracious creatures, attacking and devouring molluscs and small crustaceans, sometimes even protruding their stomachs to surround their prey when too large to be passed completely through the mouth; and they are also valuable as scavengers, since they greedily devour dead fishes and other decomposible animal matter.
Feather Stars differ from other starfishes in that they are stalked or rooted during one portion of their early life. At first they are little free-swimming creatures, feeding on foraminifers and other minute organisms that float about in the sea. Then they settle down and become rooted to the bottom, usually in deep water, at which stage they are like little stalked flowers, and closely resemble the fossil encrinites or stone lilies so common in some of our rock beds, and to which they are, indeed, very closely allied. After a period of this sedentary existence, during which they have to subsist on whatever food happens to come within their reach, they become free again, lose their stalks, and creep about by means of their arms to hunt for their prey.
Fig. 105.—Larva of the Feather Star