ECHINIDIE, a family of echinodermata, the species of which are popularly known as sea-Urchins, sea-eggs, etc. They have the body covered with a calcareous crust or shell, of an extremely porous structure (and thus differing very widely from the shells of mol lusks), in polygonal plates nicely adapted to each other, and increasing by additions to the edges of each plate, so that the shell may enlarge with the enlargement of the animal. whilst new plates are also added around the superior orifice. The shell is pierced with rows of holes for the ambulacra (q.v.), and is externally covered in a living state with a membrane—sometimes very delicate, sometimes thick and spongy—which communicates by many delicate processes with the interior, and unites the bases of all the spines. The spines differ very much in the different genera and species, in their length, strength, number, and arrangement; they are attached to tubercles on the surface of the shell, by cup-like bases capable of working upon the tubercles, in the manner of a ball-and-socket joint; and they are moved by means of the connecting membrane so as to be employed in locomotion. In some species, they seem to be the principal organs of locomotion; in others, the ambulacra are so. By means of the spines, some, in which they are few and
strong, can walk even on dry ground; others, in which they are minute and very numerous, employ them in burying themselves in the sand. The mouth of the E. is situ ated at the lower orifice of the shell, and is generally furnished with five flat calcareonS teeth, moved by a very complex apparatus of bony sockets and muscles—' a very power ful mill " for grinding down their food,-which is supposed to consist of small crustaceans and mollusks. The intestine is long and spiral; the vent, in the E. of most regular form, is at the upper end of the shell, exactly opposite the mouth; in others, in which there is a departure from the characteristic orbicular form, it is more or less lateral. The E. abound in all seas, and seem to have abounded still more in former geological periods. " Of all the radiata, they are most perfectly preserved in a fossil state," and the knowledge of their habits and organization is necessary to the geologist, "in order to understand the relations and associations of the numerous species which abound in many of the earth's strata."—Forbes.