What is an echinoderm?
The echinoderms are a group of animals consisting of over 6,500 known species that have the following features unique to themselves: (1) an internal skeleton composed of crystalline limestone bones; (2) fluid-filled, hydraulically powered organs (comprising a “water vascular system”); (3) tendons (“mutable collagenous tissue”) controlled by nerves, which can rapidly transform from rocky rigidity to puddinglike pliancy; and (4) a radial body plan that is five-sided (“pentamerous symmetry”).
