Phylum Echinodermata

Phylum Echinodermata

Echinoderms ("spiny-skinned" animals) are exclusively marine animals possessing a unique water-vascular system, a network of fluid-filled canals connected to numerous external tube feet used for locomotion, feeding and gas exchange, along with an internal skeleton of calcareous ossicles usually bearing spines that project through the skin.

A distinctive and much-examined feature of the phylum is the change in body symmetry that occurs during development: echinoderm larvae are bilaterally symmetrical, swimming and feeding as free-living, ciliated larval forms, but as the larva metamorphoses into the adult, it becomes radially symmetrical (typically pentaradial, i.e. five-part radial symmetry, as seen in the five arms of a typical starfish). This larva-to-adult shift from bilateral to radial symmetry is considered strong embryological evidence that adult radial symmetry in echinoderms is a secondarily evolved condition, not the phylum's original, ancestral body plan — which is consistent with echinoderms being placed, on other embryological grounds (enterocoely, discussed in the Diversity of Living World chapter), among the otherwise bilaterally organised deuterostomes.

Two classes are covered in further detail:

• Class Echinoidea (sea urchins and sand dollars) — echinoderms with a rigid, globular or flattened internal skeleton (test) made of tightly fused calcareous plates, bearing numerous long, movable spines, and lacking distinct arms. A characteristic feature of the class is Aristotle's lantern, a complex, five-part chewing/grinding apparatus of calcareous plates and muscles situated around the mouth on the underside of the animal, used to scrape algae and other food from hard surfaces — named for its lantern-like structure as originally described by Aristotle. …