(′ek·ə·nə′dər·məd·ə)

(invertebrate zoology) A phylum of exclusively marine coelomate animals distinguished from all others by an internal skeleton composed of calcite plates, and a water-vascular system to serve the needs of locomotion, respiration, nutrition, or perception.

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A unique group of exclusively marine animals with a peculiar body architecture. They are headless with a fivefold radial symmetry. The body wall contains the endoskeleton, made of numerous independent calcareous plates which frequently support spines. The plates may be tightly interlocked or loosely associated. The spines may protrude through the outer epithelium, and are often used for defense. The skeletal plates of the body wall, together with their associated muscles and connective tissue, form a tough and sometimes rigid test which encloses the large coelom. A unique water-vascular system is involved in locomotion, respiration, food gathering, and sensory perception. This shows outside the body as rows of fluid-filled tube feet in conspicuous double lines or ambulacra. Within the body wall lie the ducts and fluid reservoirs necessary to protract and retract the tube feet by hydrostatic pressure. The nervous system arises from the embryonic ectoderm and consists of a ring around the mouth with connecting nerve cords associated with each ambulacrum. There may also be diffuse nerve plexuses lying below the outer epithelium. The coelom houses the alimentary canal and associated organs and in most groups the reproductive organs.

The larvae are usually planktonic with a bilateral symmetry, but the adults are usually sedentary and benthic. They inhabit all seas and oceans, ranging from the shores to the ocean depths.

The phylum comprises about 6000 existing species and many fossils, providing a good fossil record. Echinoderms first appeared in the Early Cambrian and have been evolving over 600 million years. During this vast time several divergent patterns have arisen. The surviving groups show few resemblances to the original stock. The existing representatives fall into three subphyla: Crinozoa (class Crinoidea: sea lilies and feather stars); Asterozoa (class Asteroidea: starfishes, and class Ophiuroidea: brittle stars), and Echinozoa (class Echinoidea: sea urchins, sand dollars and heart urchins); and class Holothuroidea: sea cucumbers). The fourth subphylum, Homalozoa, has no living representatives. Following is the outline of classification for the phylum.

     Phylum Echinodermata

          Subphylum: Homalozoa

          Class: Ctenocystoidea

                    Stylophora

                    Homostelea

                    Homoiostelea

          Subphylum: Crinozoa

          Class: Eocrinoidea

                    Crinoidea

               Subclass: Inadunata

                              Camerata

                              Flexibilia

                              Articulata

          Class: Rhombifera

                    Diploporita

                    Parablastoidea

                    Edrioblastoidea

                    Paracrinoidea

                    Coronoidea

                    Blastoidea

          Subphylum: Echinozoa

          Class: Helicoplacoidea

                    Camptostromatoidea

                    Edrioasteroidea

                    Echinoidea

     Subclass: Perischoechinoidea

          Order: Bothriocidaroida

                    Echinocystitoida

     Subclass: Cidaroidea

          Order: Cidaroida

     Subclass: Euechinoidea

          Superorder: Echinothuriacea

          Order: Echinothurioida

          Superorder: Diadematacea

          Order: Diadematoida

                    Micropygoida

                    Pedinoida

          Superorder: Echinacea

          Order: Hemicidaroida

                    Salenioida

                    Phymosomatoida

                    Temnopleuroida

                    Echinoida

          Superorder: Eognathostomata

          Order: Pygasteroida

                    Holectypoida

          Superorder: Neognathostomata

          Order: Cassiduloida

                    Oligopygoida

                    Clypeasteroida

                    Neolampadoida

          Superorder: Atelostomata

          Order: Disasteroida

                    Holasteroida

                    Spatangoida

          Class: Ophiocistioidea

          Holothuroidea

Subphylum: Asterozoa

          Class: Asteroidea

          Order: Platyasterida

                    Trichasteropsida

                    Paxillosida

                    Notomyotida

                    Valvatida

                    Forcipulatida

          Class: Ophiuroidea

          Order: Stenurida

                    Oegophiurida

                    Phrynophiurida

                    Ophiurida

Echinoderms evolved very rapidly near the beginning of the Paleozoic Era, and Lower Cambrian deposits contain such divergent branches of the phylum as Homalozoa, Helicoplacoidea, Edrioasteroidea, and Eocrinoidea. These are primitive sorts of echinoderms. Cystoids, crinoids, and blastoids, as well as all recognized main groups of asterozoans and echinozoans (except holothurians), appear in Ordovician strata. During the Paleozoic, numerous well-marked evolutionary trends are discernible in nearly all echinoderm groups, including free-moving forms (especially echinoids) as well as crinozoans. Many small classes of echinoderms became extinct during the Paleozoic, and the surviving groups, especially the crinoids, lost many members at the great Late Permian mass extinction. All groups of modern echinoderms have their origin in early Paleozoic stocks, and the lines of their phylogeny are mostly indicated by the fossil record. Echinoids predominate in Mesozoic and Cenozoic echinoderms.

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