ECHINODERM

any of a variety of invertebrate marine animals belonging to the phylum Echinodermata, characterized by a hard, spiny covering or skin. More than 21 classes have been identified, but only about 6,000 species of six classes are known to still exist. The six are: Crinoidea (feather stars and sea lilies), Asteroidea (starfishes), Ophiuroidea (brittle stars and basket stars), Echinoidea (sea urchins, cake urchins, and heart urchins), Concentricycloidea (sea daisies), and Holothurioidea (sea cucumbers). Echinoderms are widely distributed in all the oceans, occurring in marine environments ranging from the intertidal zone to the deepest oceanic trenches. The most apparent characteristics of echinoderms are the calcite skeleton and the five-rayed, or pentamerous, radial symmetry of the adult body form. The skeletal structure may be a test, a hollow structure formed of solid plates, as found in sea urchins, or it may be composed of many separate smooth or spiny ossicles held together by muscles or ligaments, which is the case in the starfishes. Most echinoderms have numerous small appendages, called tube feet, that are contained in grooves on the animals' tentacles. Movement of the tube feet is controlled by a hydraulic, or water-vascular, system. Water is taken in and passes into five major canals that channel it into branches that carry it to the tube feet. Local muscle action in contracting or releasing the canals functions to extend or retract the tube feet. The tube feet themselves may be variously modified to serve in locomotion, respiration, tunneling, sensory perception, feeding, and grasping. Reproduction among echinoderms is either sexual or asexual. In sexual reproduction, eggs are fertilized in the water by sperm and either pass through a larval stage before transforming into juvenile echinoderms or develop directly into juveniles. One kind of indirect development involves fission of the body and regeneration of the missing parts. Starfishes, for example, can regrow an entire organism from one arm if a small portion of the central disk remains attached. Most echinoderms feed on microscopic detritus or suspended matter (either alive or dead), but many urchins and starfishes graze on plant life. Some starfishes are carnivores and eat mainly mollusks. A few of the echinoderm species are economically important. Some sea urchin roes and the warty sea cucumbers of the genus Holothuria are used for food in certain countries (see bche-de-mer). Various species of starfishes, such as Asterias vulgaris and A. forbesi, that prey on clams and oysters are pests in commercial shellfish beds. any of a variety of invertebrate marine animals belonging to the phylum Echinodermata, characterized by a hard, spiny covering or skin. Beginning with the Lower Cambrian Period almost 570,000,000 years ago, echinoderms have a rich fossil history and are well represented by many bizarre groups, most of which are now extinct. Living representatives include the classes Crinoidea (sea lilies and feather stars), Echinoidea (sea urchins), Holothuroidea (sea cucumbers), Asteroidea (starfishes, or sea stars), Ophiuroidea (basket stars and serpent stars, or brittle stars), and the recently discovered Concentricycloidea (sea daisies). Echinoderms have been recognized since ancient times; echinoids, for example, were used extensively by Greeks and Romans for medicinal purposes and as food. During the Middle Ages, fossil echinoids and parts of fossil crinoids were objects of superstition. In the early part of the 19th century, Echinodermata was recognized as a distinct group of animals and was occasionally associated with the cnidarians and selected other phyla in a division of the animal kingdom known as the Radiata; the concept of a superphylum called Radiata is no longer valid. Echinoderms are separated into 21 classes, based mainly on differences in skeletal structures. The number of extant species exceeds 6,000, and approximately 13,000 fossil species have been described. Additional reading Richard A. Boolootin (ed.), Physiology of Echinodermata (1966), a comprehensive survey of biology and physiology; Ailsa M. Clark, Starfishes and Related Echinoderms, 3rd ed. (1977), an introductory work, with emphasis on living forms, classification, and biology; Libbie Henrietta Hyman, The Invertebrates, vol. 4, Echinodermata. The Coelomate Bilateria (1955), a classic survey of anatomy and biology; Michel Jangoux and John M. Lawrence (eds.), Echinoderm Nutrition (1982), a thorough survey of feeding biology, and Echinoderm Studies (1983), a collection of review articles on all aspects of echinoderm biology; John M. Lawrence, A Functional Biology of Echinoderms (1987), a discussion of echinoderm food acquisition, reproduction, and other aspects of their lives; Raymond C. Moore and Curt Teichert (eds.), Treatise on Invertebrate Paleontology, pt. S, T, and U, Echinodermata (196678), a detailed treatment; David Nichols, Echinoderms, 4th ed. (1969), a general work, including a treatment of fossil forms; and Andrew Smith, Echinoid Palaeobiology (1984), a study of anatomy and paleontology of sea urchins. Vicki Pearse et al., Living Invertebrates (1987), includes a well-illustrated discussion of the living echinoderms. For an exposition at a less advanced level, see Ralph Buchsbaum et al., Animals Without Backbones, 3rd ed. (1987). David Leo Pawson John E. Miller Classification Distinguishing taxonomic features The classification of the echinoderms underwent a great upheaval during the 1970s and 1980s, and much disagreement remains. The five subphyla presented here are based upon combinations of characters: Homalozoa are asymmetrical; Blastozoa are stalked, with simple feeding apparatus; Crinozoa are stalked, with complex feeding apparatus; Asterozoa are star-shaped; Echinozoa are globoid to discoid. Below the subphylum level, the criteria for classification vary, but the skeleton is the most important; most groups can be characterized on the basis of skeletal characters alone. Annotated classification The echinoderms once were divided into two great groups, the Pelmatozoa and the Eleutherozoa, the names referring to living habits; pelmatozoans were attached to the seafloor for at least part of their life cycle while eleutherozoans were unattached animals capable of moving freely over the seafloor. It has been argued that such a separation is confusing, because each group contains a mixture of subgroups bearing no relationship to the evolutionary history of the phylum. The terms pelmatozoan and eleutherozoan are often used to describe the life habits of echinoderms. Some sea cucumbers, for example, have adopted a pelmatozoan habit, attaching themselves to rocks and feeding on plankton; others are eleutherozoan, moving about the seafloor while feeding, or even actively swimming. The classification presented here is based upon current research by paleontologists and zoologists. Totally extinct classes, marked with a dagger (), are known only as fossils.