Sea Urchin
one of the Echinoids, or Echinidæ, an order of Echinodermata.
They have the body covered with a calcareous crust or shell, of an extremely porous structure, 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, while new plates are also added around the superior orifice. The shell is pierced with rows of holes for the ambulacra, and is externally covered in a living state with a membrane, sometimes very delicate, sometimes thick and spongy, and unites the bases of all the spines. The spines differ very much in the different genera and species. and species .
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. By means of the spines, some can walk even on dry ground; others employ them in burying themselves in the sand. The mouth of the Echinidx is sitof the Echinidæ is situated at the lower orifice of the shell, and is generally furnished with five flat calcareous teeth moved by a very complex apparatus of bony sockets and muscles. Their food is supposed to consist of small crustaceans and mollusks. They abound in all seas.