sea-urchin

Class Echinoidea

[ sea-urchin ]

Photo by : Nikita

Any of various echinoderms of the class Echinoidea, having a soft body enclosed in a round, symmetrical, calcareous shell covered with long spines.

[American Heritage Dictionary]

Unlike sea stars or brittle stars, these creatures do not have arms or rays. The skeleton, called a test, consists of raws of radially arranged plates immovably joined to each other. Movable spines, each with a concvave base, fit on correspondingly convex bumps on each plate. Muscle fibers attached to each spine enable it to swing about in any direction.

[ National Audubon Society Field Guide of North American Seashore Creatures. ]