Classification.—The Echinodermata are divided into five classes, viz., the Asteroidea or starfishes, "free Echinoderms with star-shaped or pentagonal body, in which a central disc and usually five arms are more or less readily distinguishable, the arms being hollow and each containing a prolongation of the body-cavity and contained organs"; the Ophiuroidea, or brittle-stars, "star-shaped free Echinoderms, with a central disc and five arms, which are more sharply marked off from the disc than in the Asteroidea and which contain no spacious prolongations of the body-cavity"; the Echinoidea, or sea-urchins, "free Echinoderms with globular, heart-shaped, or disc-shaped body enclosed in a shell or corona of close-fitting, firmly united calcareous plates"; the Holothuroidea, or sea-cucumbers, "free Echinoderms with elongated cylindrical or five-sided body, ... with a circlet of large oral tentacles"; and the Crinoidea, or feather-stars, "temporarily or permanently stalked Echinoderms with star-shaped body, consisting of a central disc, and a series of five bifurcate or more completely branched arms, bordered with pinnules."

Starfishes (Asteroidea).—The starfishes feed on other marine animals, especially shell-fish and crabs. They are also reputed to destroy young fish. By means of their sucking-tubes, or tube-feet with sucker tips, they can seize and hold their prey firmly. They do much injury to oyster-beds by attacking and devouring the oysters. When attacking prey too large to be taken into the mouth the starfish everts its stomach over the prey and devours it. The stomach is afterward drawn back into the body-cavity by special muscles.

Starfishes vary much in size, color and general appearance, although all are readily recognizable as starfishes (fig. [22]). The number of arms or rays varies from five to thirty or more in different species; some have the interradial spaces filled out nearly to the tips of the rays, making the animal simply a pentagonal disc. In size starfishes vary from a fraction of an inch in diameter to three feet; in color they are yellow or red or brown or purple.

Fig. 22.—A group of Echinoderms; the upper one, a starfish, Asterina mineata, the one at the right a starfish, Asterias ocracia, at the left a brittle-star, species unknown, and at bottom two sea-urchins, Strongylocentrotus franciscanus. (From living specimens in a tide-pool on the Bay of Monterey, California.)

Brittle-stars (Ophiuroidea).—The brittle-stars, or serpent-stars (fig. [22]) as they are also called, resemble the starfishes in external appearance, that is, they are flat and composed of a central disc with radiating arms (always five in number, although each arm may be several times branched). The central disc is always sharply distinguished from the arms, and the arms are usually slender and more or less cylindrical. The distinguishing difference between the brittle-stars and the starfishes is that the body-cavity and the stomach which extend out into the arms in the starfishes are in the brittle-stars limited to the central disc, or to the disc and bases of the arms. The tube-feet also have no suckers at the tips. More than 700 species of brittle-stars are known. They feed on marine shell-fish, crabs and worms.

Sea-urchins (Echinoidea).—The sea-urchins (figs. [20], [21] and [22]) of which more than 300 species are known, have no arms or rays, and they are usually not flat like the starfishes but globular, with poles more or less flattened. As has been noted in the examination of the body-wall or "shell," the radiate character of the body is shown by the five radiating zones of tube-feet. The mouth, with its five strong "teeth," is on the ventral surface, and the anal opening and madreporic opening are on the dorsal surface. The calcareous plates (seen distinctly in a specimen from which the spines have been removed) which constitute the firm part of the body-wall, are more or less pentagonal in shape and are usually firmly united at the edges. The spines which are so characteristic of the sea-urchins vary much in size and number and firmness, but are present in some form on all of them.

While most of the sea-urchins live near the shore, being very common in tide-pools, some live only on the bottom of the ocean at great depths. Their food consists of small marine animals and of bits of organic matter which they collect from the sand and débris of the ocean floor. Many of the sea-urchins are gregarious, living together in great numbers. Some have the habit of boring into the rocks of the shore between tide-lines. I have seen thousands of small beautifully colored purple sea-urchins lying each in a spherical pit or hole in hard conglomerate rock on the California coast. How they are enabled to bore these holes is not yet known. There is great variety in size and color among the sea-urchins. The colors are brown, olive, purple red, greenish blue, etc.

A few kinds of sea-urchins have a flexible shell or test. The Challenger expedition dredged up from sea-bottom some sea-urchins, and when placed on the ship's deck "the test moved and shrank from touch when handled, and felt like a starfish." The cake-urchins or sand-dollars are sea-urchins having a very flat body with short spines. They lie buried in the sand, and are often very brightly colored. Their hollow bleached tests with the spines all rubbed off are common on the sands of both the Atlantic and Pacific coasts.

Sea-cucumbers (Holothuroidea).—The sea-cucumbers (fig. [23]) show at first glance little resemblance to the other radiate animals. The body is an elongate, sub-cylindrical sac, resembling a thick worm or sausage or cucumber in shape. At one end it bears a group of branched tentacles which are set in a ring around the mouth-opening. The body-wall is muscular and leathery, but contains many small separated calcareous spicules. There are usually five longitudinal rows of tube-feet. In some species, however, tube feet are wholly wanting; in others they are scattered over the surface.