E-chīn'o-dĕrm', n.

n. [Fr. échinoderme, from Gr. ἐχῖνος, Lat. echinus, and δέρμα, skin, from δέρειν, to skin, flay.] (Zoöl.) An animal of one of the grand divisions of the Radiates. See ANIMAL KINGDOM.—The species have an exterior calcareous crust or shell made of many pieces, more or less firm, rarely coriaceous, and are often covered with movable spines, to which last characteristic the name echinoderm alludes. They are either free animals with a mouth at the center below, locomotive without swimming; or they are attached, like a flower, by a stem made of many disk-like, calcareous joints, and have the mouth upward. They are divided as follows:—

I. Free.—1. ECHINIDANS, or Echinus tribe. These have the forms of an ovoid flattened sphere or disk, the shell inflexible, and covered with stiff, movable spines; as the Echini (sea-urchins, or sea-hedgehogs).—2. ASTERIDIANS, or the star-fish tribe. These have the form of a star (often five-rayed), or of a disk; the rays or disk flexible and without true spines.—3. HOLOTHURIANS. These have an oblong, slug-like form, and are very flexible, the exterior being coriaceous.

II. Attached by a stem.—1. CRINOIDEANS, or Encrinite tribe. These somewhat resemble a star-fish inverted, and attached by a pedicel.—2. CYSTIDEANS. These resemble the Crinoids in habit, but are more echinus-like than star-shaped above, and are otherwise peculiar.

The Crinoids and Cystideans, with the exception of a few species of the former, are fossil, and abound most in the older fossiliferous rocks. The free species (Echini, Star-fish, and Holothurians) are common in existing oceans, especially within the tropics. Dana.