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Phylum Echinodermata Crinoid (sea lily ), from Pratt, 1923.

The Echinoderms are among the most obvious and colorful organisms of many temperate seashores. Echinoderms are a very old group, dating from the early Cambrian , and are well represented in the fossil record. There are about 7,000 extant species of echinoderms , which can be divided into six classes: Crinoidea Asteroidea Ophiuroidea Echinoidea Holothuroidea Concentricycloidea The Crinoidea , popularly known as Sea lilies and Feather stars, have the body oriented so that the mouth faces up. They may or may not have a stalk. There are about 625 living species of crinoids . The Asteroidea (sea stars or starfish ) are among the more familiar forms to most people. The oral surface is typically ventral, they never have a stalk, and usually have multiple "arms" surrounding a central disk. there are about 1,500 living species of Asteroids .

Asteroid (sea star)

Ophiuroid (brittle star), from Halstead (1965)

The class Ophiuroidea includes about 2,000 living species of animals that are usually called brittle stars or serpent stars. They have five arms radiating from a central disk, and the arms are typically flexible and used for locomotion. They have a ventral mouth, but no anus .

Echinoid (sea urchin)

The class Echinoidea includes the sea urchins and sand dollars . The body is often globular, without distinct arms, and they usually have a rigid "test" of fused skeletal plates. Some of the Echinoids have secondarily become bilaterally symmetrical. The class holothuroidea contains about 1,150 living species of cylindrical organisms (cucumber shaped), which are drawn out along their oral-aboral axis . They are relatively soft-bodied, since their skeletal elements are usually reduced to small and isolated ossicles . they typically have a circle of tentacles around the mouth that are used for feeding.

Holothuroid (sea cucumber)

Concentricycloidea, after Baker et. al., 1986.

The class Concentricycloidea (the sea daisies )is a recently discovered group that inhabits deep water environments where there is decaying wood. Sea daisies have a discoid body form, and are without arms (although they have five ambulacral areas), and with their skeletal plates arranged concentrically. They have an incomplete digestive system (no anus ) and sometimes lack a gut. They are generally less than 1 cm. diameter, and only a few species are known.

The general Characteristics of the Echinodermata are: Larvae, when present, are bilaterally symmetrical, but adults have a pentamerous radial symmetry. Deuterostomous embryogeny with radial cleavage . Complete gut, however, the anus is secondarily lost in some forms. Echinoderms have no excretory organs (as a result, there are no freshwater forms). A calcareous endoskeleton is derived from mesodermal tissue, and consists of numerous ossicles (or plates), derived from the mesoderm . A water vascular system derived from the coelom , which consists of a system of fluid filled canals. A diffuse nervous system consists of a nerve ring, nerve net, and radial nerves. The circulatory system, if present, consists of a haemal system that is derived from coelomic sinuses. REFERENCES Barnes, 1980 Brusca & Brusca, 1990 Barnes, Calow, and Olive, 1993 Halstead, 1965 Meglitsch & Schram, 1991 Pratt, 1923.

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