Echinoidea /(?)/

Ech·i·noi·de·a

Echinoidea

n. pl.
  1. The class Echinodermata which includes the sea urchins. They have a calcareous shell, usually more or less spheroidal or disk-shaped, composed of many united plates, and covered with movable spines. See Spatangoid, Clypeastroid. (Zoöl.)