Echinodermata

Echinodermata (Gr., 'thorny-skinned'), a well-defined division of Invertebrates, including seven classes: Holothuroidea (q.v.) or Sea-cucumbers, Sea-urchins (q.v.) or Echinoidea, Starfishes (q.v.) or Asteroidea, Brittle-stars (q.v.) or Ophiuroidea, Feather-stars or Crinoidea (q.v.), and the wholly extinct Cystoidea (q.v.) and Blastoidea. The larva is bilateral, but the adults are more or less radially symmetrical. Limy depositions, forming skeletal framework, are developed to a variable, but usually predominant, degree in the middle layer or mesoderm. There is in all a special development of a water-vascular (locomotor or respiratory) system. A metamorphosis occurs in the life-history. They are all marine, barring a few Holothurians in brackish water. They appear in the Lower Silurian strata. On the genealogical tree the Echinoderms form a very distinct offshoot from the main stem. The larva of the remarkable worm Balanoglossus has a striking resemblance to the typical Echinoderm larva. The sea-urchins and sea-encumbers are associated along one line; the starfishes, brittle-stars, and crinoids along another. See the separate articles.