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

Example:
Echinoidea- How do they respire and how does there circulation system work?

Taxonomy
sea urchin, echinoids, spines, urchins, Superorder, test, dangerous spines, species, fossils, tube feet, Subclass, Cretaceous period, free encyclopedia, pencil, heart urchins, spiny, class Echinoidea, humans.

Defination from the web:
Echinoidea) ... Besides sea urchins, the Echinoidea also includes three groups of "irregular" echinoids: flattened sand dollars, sea biscuits, and heart urchins. ...