A class of the asterozoan group of the phylum Echinodermata, often known as sea stars or starfish. Members of the class Asteroidea, that is, asteroids, are notable echinoderms with a star-shaped body plan (Fig. 1). Unlike members of their sister asterozoan class, the Ophiuroidea (brittle and basket stars), the arms of asteroids are not sharply demarcated from the rest of the body, the ambulacral ossicles never fuse to form vertebrae, and the tube feet are locomotory (Fig. 2). These tube feet are usually suctorial and emerge from an open ambulacral groove. The dominant growth gradients cause the skeletal ossicles to lie in longitudinal rows known as series (for example, adambulacral series, ventrolateral series, and inferomarginal series). Asteroids range in size from about 10 mm to 1 m (0.4 to 40 in.) across. Many sea stars are brightly colored and attractive animals, but some are dowdy and cryptic. Their conjugated carotenoid pigments fade on preservation. Like most echinoderms, sea stars have a life span of about 5 years. See also: Carotenoids; Echinodermata; Ophiuroidea

A class of extinct Pelmatozoa in the subphylum Blastozoa which arose in the Ordovician and flourished in the Carboniferous and Permian. Blastoids did not survive the Permo-Triassic mass extinction event. Historically, two orders of the Blastoidea have been recognized: the Fissiculata with exposed hydrospire slits and the Spiraculata with concealed hydrospires. In recognition of their polyphyletic origins, the Spiraculata are now subdivided into the Troosticrinida, Nucleocrinida, Granatocrinida, and Pentremitida. See also: Pelmatozoa

A class of exclusively suspension-feeding echinoderms with long, slender arms arranged radially around the calyx, a rigid cuplike structure composed of calcareous plates. The radial arm arrangement gives crinoids a flowerlike appearance (Fig. 1). Two basic adult body types are recognized: the sea lilies, with a long, anchored stem vertically supporting the calyx and arms above the sea bottom; and the stemless featherstars, or comatulids, with a whorl of flexible appendages on the calyx. Crinoids have a worldwide distribution and can be found in all seas except the Black and Baltic. They occupy depths ranging from just below sea level to a depth of over 9000 m (29,500 ft). Sea lilies are found only at depths greater than 100 m (330 ft), whereas comatulids are most abundant and diverse in shallow, tropical coral reef environments. See also: Blastoidea; Pelmatozoa

A phylum of exclusively marine animals with a peculiar body architecture dominated by a five-part radial symmetry. Echinodermata [from the Latin echinus (spine) + dermis (skin: “spiny skins”)] include the sea stars, sea urchins and related animals. The body wall contains an endoskeleton of numerous plates (ossicles) composed of calcium carbonate in the form of calcite and frequently supporting 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 closely associated connective tissues and muscles, form a tough and sometimes rigid test (hard shell), which encloses the large coelom. A unique water-vascular system is involved in locomotion, respiration, food gathering and sensory perception. This system is evident outside the body as five rows of fluid-filled tube feet. Within the body wall lie the ducts and fluid reservoirs necessary to protract and retract the tube feet by hydrostatic pressure. The nervous system of these headless animals arises from the embryonic ectoderm and consists of a ring around the mouth with connecting nerve cords associated with the rows of tube feet. There may also be diffuse nerve plexuses, with light-sensing organs, lying below the outer epithelium. The coelom houses the alimentary canal and associated organs and, in most groups, the reproductive organs. The body may be essentially star-shaped or globoid. The five rows of tube feet define areas known as ambulacra, ambs, or radii; areas of the body between the rows of tube feet are interambulacra, interambs, or interradii.

An order of sea urchins (phylum Echinodermata, class Echinoidea). Members of the Echinoida (Fig. 1) are characterized by the following features: a camarodont lantern that has keeled teeth (Fig. 2), which are T-shaped in cross section; epiphyses enlarged and connected to each other in a continuous arc; a smooth test with relatively small tubercles, which are imperforate and noncrenulate; and ambulacral plates of the echinoid type, with the lowest element being the largest (Fig. 3). Five families are included in the Echinoida, distinguished mainly on the structure of the pedicellariae (pincerlike structures scattered over their surface). Members of the Parechinidae, with three genera, have globiferous pedicellariae with several lateral teeth and polyporous ambulacra (the radial series of plates along which the tube feet are arranged). Echinidae, with six genera, have a pair of lateral teeth on the globiferous pedicellariae. Echinometridae usually have a single asymmetrical lateral tooth; in six of the 13 genera, the test is elongate oval. Strongylocentrotidae, with five genera, have globiferous pedicellariae with or without teeth, but the ambulacral plates are polyporous in all genera, with the pore zones in arcs. In the nine genera of Toxopneustidae, the globiferous pedicellariae lack teeth and the ambulacral plates are usually trigeminate, but with up to five pore pairs in a few species. Approximately 95 of the 800 species of the class Echinoidea belong to the order Echinoida. See also: Echinodermata; Echinoidea

A class of Echinodermata known as the sea urchins, also including sand dollars, sea biscuits and heart urchins. In echinoids, the body is enclosed in a hard shell, or test, formed from regularly arranged plates that bear movable spines (Figs. 1 and 2). There are no arms, but radii are represented by five double rows of tube feet arranged as meridians between the upper and lower poles of the body.

A medium-sized class of primitive, brachiole-bearing, blastozoan echinoderms of the class Crinozoa that ranged from the Early Cambrian to the Middle Silurian, although few eocrinoids survived past the Middle Ordovician. About 32 eocrinoid genera have been described from North America, Europe, North Africa and Australia; other occurrences of distinctive plates that may belong to eocrinoids have also been noted. Eocrinoids are the most diverse class of echinoderms known from the Cambrian with about 15 genera and different members appear to have been ancestral to nearly all of the more advanced brachiole-bearing echinoderm classes that appeared in the Early or Middle Ordovician, such as rhombiferans, parablastoids and coronoids. Eocrinoids have a globular, conical, or flattened theca or body, with many irregularly arranged to partly organized, imbricate or adjacent plates (see illustration). Most Cambrian genera have sutural pores on the plate margins, apparently for respiration. Many early eocrinoids have a multiplated, cylindrical to slightly inflated holdfast for attaching the theca to objects lying on the sea floor. Holdfasts apparently evolved into a true columnal-bearing stem in late Middle Cambrian eocrinoids and most later genera have that advanced type of attachment structure. Eocrinoids typically have two to five short ambulacral grooves radiating from the mouth on the summit to many long, erect, biserial brachioles that were used for feeding. Most eocrinoids were attached, low- to medium-level suspension feeders that used the brachioles to collect small food particles drifting by the theca. Most researchers have argued that eocrinoids are a valid class containing genera that did not develop the foldlike respiratory structures found in more advanced blastozoan classes. However, others have recently proposed that eocrinoids are a paraphyletic stem group that should be discarded, with the included genera reassigned to other classes. See also: Echinodermata

A subclass of Echinoidea (sea urchins, sand dollars and heart urchins) in which the test plates are consistently arranged in double rows or columns. In Euechinoidea, five double rows bear the tube feet (the ambulacral rows) and these alternate with five that do not (the interambulacral rows). Thirteen orders are included within the Euechinoidea. See also: Echinodermata; Echinoidea; Neognathostomata

An order of sea stars (Asteroidea) that includes several ecologically important members that are found primarily in cold to temperate water settings. The Forcipulatida (whose members are referred to herein as forcipulates) occur across a broad depth range, from the near-shore intertidal zone in polar and subpolar regions to the deep-sea abyss [up to 6000 m (19,685 ft)]. See also: Asteroidea; Deep-sea fauna; Echinodermata

A class of Echinozoa characterized by a cylindrical body and smooth leathery skin, which are known as sea cucumbers. In Holothuroidea, there are no arms, but a ring of five or more tentacles may surround the mouth, which is usually at one end of the body. There are no pedicellariae (small grasping organs). Tube feet may be present or lacking. There are no ambulacral grooves, although they are represented by internal epineural canals overlying the radial nerves. See also: Echinozoa