Dendrochirotacea, O. Dendrochirotida, F. Sclerodactylidae Introduction Echinoderms are radially symmetric deuterostomes whose ancestors were bilaterally symmetric. The radial symmetry is pentamerous with body parts occurring in fives or multiples thereof. They show strong affinities with the ancestral oligomerous coelomates especially in the organization of the body cavities, or coeloms. Echinoderm larvae are bilaterally symmetrical and have a coelom divided into three regions as is typical of the early coelomates.
All echinoderms are marine and benthic.
Echinoderms are characterized by possession of a unique water vascular system that in most groups functions in support of locomotory tube feet but is also often involved in gas exchange, excretion, and feeding. The body wall includes a thick connective tissue dermis in which calcareous ossicles are almost always an important component. These ossicles make up an endoskeleton which assumes different forms in different classes. Excretion in echinoderms is accomplished by simple diffusion of metabolic wastes across thin regions of the body wall. A variety of gas exchange structures is found in various echinoderms.