has resulted in the production of a thin, cake-like form (Fig. 94).

If the spherical body of a sea-urchin were to be stretched in the direction of a line joining the mouth and the center

of the dorsal surface, a form resembling a sea-cucumber (Fig. 95) would be the result. These latter organisms live among crevices of the rocks, embedded in the mud or burrowing in the sand at the bottom of the sea. In such situations they are well protected, and a highly developed skeleton, such as that of the sea-urchin, would not only be of little value, but a positive hindrance to locomotion. The skeleton, therefore, is much reduced, consisting of a few scattered calcareous plates embedded in the fleshy bodywall. Another peculiar feature is almost universally present, in the form of a circlet of tentacles surrounding the mouth, which serve either for the purpose of respiration, for locomotion, or to convey food to the mouth.

A very good imitation of the general plan of a sea-lily or crinoid (Fig. 96) could be made by attaching a serpent

star, especially one of the basket-stars, by its dorsal side to a stalk. In the crinoids the numerous branches of the

arms are comparatively short, and in the arrangement of the internal organs

there are numerous differences, but for all that the resemblance of these organisms to the other echinoderms is undoubted.

144. Haunts. The greater number of starfishes occur alongshore, slowly crawling about in search of food, or concealed in dark crevices of

the rocks, where they may often be found as the tide goes out, and we know that in gradually lessening numbers other species lead similar lives at different levels far down in the dark and gloomy depths. In these same locations the seaurchins occur, sometimes singly, but more usually associated in great numbers, several species excavating hollows in the rocks, within which they obtain protection. The brittle-stars and sea-cucumbers may also be found occasionally in open view, but more often they make their way about in search of food buried in the sand. The crinoids are usually inhabitants of deeper water, where they are found associated often in great numbers. A few species upon attaining the adult condition separate from the stalk, and are able to move about (Fig. 97), but the remaining species never shift their position.

145. The organs of defense and repair of injury.-As we have seen, the body-wall of the echinoderms is provided with a series of plates, often bearing spines which serve as organs of defense, and to protect the internal organs. The starfishes and sea-urchins also possess numerous modified spines (pedicellaria) scattered over the surface of the body, which have the form of miniature birds' beaks, fastened to slender muscular threads. During life these jaws continu. ally open and close, and it is said they clean the body of débris that settles on it; but on the other hand there are several reasons for the belief that they also act as organs of defense. Thus protected, the natural enemies of echinoderms appear to be relatively few, and are confined chiefly to some of the fishes whose teeth are especially modified for crushing them. In this

way, and owing to the action of the breakers, they suffer frequent injury, but many species exhibit to a remarkable degree the ability to regenerate lost parts. Experiments show that if all the arms of a starfish be separated from the disk the latter will within two or three months renew the arms; and a single arm with a part of the disk is able to renew the missing portions in about the same length of time.

The brittle-stars, as their name indicates, are usually excessively delicate, often dropping all of their arms upon the slightest provocation; but here again the ability is present o develop the lost portions.

to

Sea-cucumbers resent rough treatment by vigorously contracting their muscular walls and removing from the body almost the entire digestive tract, the respiratory tree,

and a portion of the locomotor system; but some species, at least, renew them again.

In some of the starfishes and brittle-stars portions of the body appear to be voluntarily detached and to develop into new individuals, and it is thought that such self-mutilation is a normal method of reproduction. M146. Locomotor system. One of the most characteristic and remarkable features of the echinoderms is the water-vascular system, a series of vessels containing water which serve in the process of locomotion. Their arrangement and mode of operation are, with slight modifications, the same throughout the group, and may be readily understood from their study in the starfish.

On the dorsal surface of a starfish, in the angle between two of the arms, is a round, slightly elevated, calcareous plate, the madreporic body (Fig. 98, m.p.), which under the microscope appears full of holes, like the "rose" of a watering-pot. This connects with a tube that passes to the opposite side of the body, where it enters a canal completely encircling the mouth. On this ring-canal a

number of sac-like reservoirs with muscular walls are attached, and from it a vessel extends along the under surface of each arm from base to tip. Each of these radial water-mains gives off numerous lateral branches that open out into small reservoirs similar to those located on the ring-canal, and a short distance beyond communicate through the wall of the body with one of the numerous

The

tube-feet, which, as we have seen, are slender tubular organs, many in number, filling the grooves on the ventral surface of each arm. This entire system of tubes and reservoirs is full of water, taken in, it is said, through the perforated plate, and, when the starfish wishes to advance, many of the little reservoirs contract, forcing water into the cavity of the feet, with which they are in communication, thus extending the extremity of the tubes a considerable distance. terminal sucker of each foot, acting upon the same principle as those on the cuttlefish, attaches firmly to some foreign object, whereupon the muscles of the foot contract, drawing the body toward the point of attachment.V This latter movement is similar to that of a boatman pulling himself to land by means of a rope fastened to the shore. When the shortening of the tube-feet has ceased, the sucking disks release their attachment, project themselves again, and this process is repeated over and over. At all times some of the feet are contracting, and a steady advance of the body is the result.

This method of locomotion also obtains in the sea-urchins and cucumbers, but in the serpent-stars the tube-feet have become modified into feelers, and the animal moves, often rapidly, by means of twisting movements of the arms. The feet have this character also in the crinoids, where the animal is generally without