ECHINODERMA, fig. 2.) Thus the arms are sharply marked off from the disc-like body, to which digestive and generative organs are confined. Brittle-stars proper have long wriggling arms, fringed with prickles, with which they can progress more than two yards a minute. If seized, they break off their arms, which continue breaking into smaller pieces; but the body soon grows new ones.
Sand-stars have shorter, stouter arms, with prickles closely pressed to the sides; they progress by a rowing movement. Sand-stars and brittle-stars abound in the shallow waters of all seas, espe cially in the tropics. By constantly sweeping their arms over the sea-floor they gather in minute animals as food. They eat the bait of fishermen, and their fish as well if they find any already dead ; but they are themselves a food of many fishes, notably cod. Basket-fish or medusa-heads are ophiuroids whose arms branch several times, their ends curling and interlacing round some marine object. They live in deeper water and are often brought up clinging to fishermen's lines.
ECHINODERMA, fig. 4.) The stalk is not really absent, but reduced to a knob, from which spring numerous hooked tendrils (cirri) ; by these the animal clings to some object on the sea-bottom. From the body stretch five arms, each bifid (in other species they may fork more than once) and fringed with small branches (pin nules) giving a feathery appearance. Along these arms and pin nules, grooves pass from the central upturned mouth (ECHINO DERMA, fig. 7), and the minute hair-like cilia that line them con stantly sweep towards the mouth a stream of water, from the microscopic organisms in which the crinoid extracts nourishment. Antedon crawls by pulling and pushing with its arms, and swims, or rather treads water, by raising and depressing alternate branches. The ancestors of the feather-stars in the Jurassic period still had a stem in the adult. Since that period feather-stars have increased in number and variety, so that to-day they occur in all salt seas at all depths, often in enormous quantities, and are classified in 98 genera. (F. A. B.) BIBLIOGRAPHY.—For special works with bibl., see ECHINODERMA.