Although there are known about five hundred species of sea-cucumbers many of which live along the shores, they are much less familiar to us than the starfishes and sea-urchins. They usually rest buried in the sand by day, feeding at night. Some of them attain a large size. A great orange-red species of the genus Cucumaria, which is found in the Bay of Monterey, California, is three feet long.

The people of some nations use sea-cucumbers as food. They are called "trepang" in the orient. The trade of preparing the trepang is almost entirely in the hands of the Malays, and every year large fleets set sail from Macassar and the Philippines to the south seas to catch sea-cucumbers.

Fig. 23.—A sea-cucumber, Pentacta frondosa. (After Emerton.)

Feather-stars (Crinoidea).—The feather-stars or sea-lilies or crinoids (fig. [24]), as they are variously called, differ from the other Echinoderms in having the mouth on the upper side of the central disc, and in the fact that all of the species are fixed, either permanently or for a part of their life, being attached to rocks on the sea-bottom by a longer or shorter stalk which is composed of a series of rings or segments. The central disc is small and the radiating arms are long, slender, sometimes repeatedly branched, and all the branches bear fine lateral projections called pinnulæ. Most of the feather-stars live in deep water and are thus only seen after being dredged up. They feed on small crab-like animals, and on the marine unicellular animals and plants.

Fig. 24.—A crinoid or feather-star, Pentacrinus sp. (After Brehm.)