Phylum Echinodermata

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Antedon sp., Crinoid (sea lily ), modified from Arnold and Lang, 1896 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) , photo by S. Shaner 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 . 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. Echinoid (sea urchin) , photo by S. Shaner Holothuroid (sea cucumber) , photo by S. Shaner 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.   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: Brusca, R., and G. Brusca. 2003. Invertebrates. Second edition. Sinauer Assoc. Inc. Sunderland, MA, USA. Castro, P., and M.E. Huber. 2005. Marine Biology. 5th edition. McGraw-Hill, New York. N.Y. Meglitsch, P.A., and F.R. Schram. 1991. Invertebrate Zoology. Third edition. Oxford University press, New York., Oxford. 623 pp. Pechenik, J.A. 2005. Biology of the Invertebrates. Fifth edition. McGraw Hill, 590 pp. Ruppert, E.E., R.S. Fox, and R.D. Barnes. 2004. Invertebrate Zoology, A Functional Evolutionary Approach. Seventh edition. Brooks/Cole, 961 pp. For more information beyond this site, search the web! Suggestions for improving, changing, or adding to this website and etextbook should be emailed to:

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