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(əkī'nōdûr`mətə) [Gr.,=spiny skin], phylum of exclusively marine bottom-dwelling invertebrates having external skeletons of calcareous plates just beneath the skin. The plates may be solidly fused together, as in sea urchinssea urchin,
spherical-shaped echinoderm with movable spines covering the body. The body wall is a firm, globose shell, or test, made of fused skeletal plates and marked by regularly arranged tubercles to which the movable spines are attached.
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, loosely articulated to facilitate movement, as in sea starssea star,
also called starfish, echinoderm of the class Asteroidae, common in tide pools. Sea stars vary in size from under 1-2 in. (1.3 cm) to over 3 ft (90 cm) in diameter. They are commonly dull shades of yellow or orange, but there are many brightly colored ones as well.
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 (starfish), or reduced to minute spicules in the skin, as in sea cucumberssea cucumber,
any of the flexible, elongated echinoderms belonging to the class Holothuroidea. Although sea cucumbers have the basic echinoderm radial symmetry (see Echinodermata), they do not have arms like starfish.
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. The skin usually has warty projections or spines, or both. Echinoderms display pentamerous radial symmetry, that is, the body can be divided into five more or less similar portions around a central axis. Unlike other radially symmetrical animals, they develop from a bilaterally symmetrical larva and retain some degree of bilateral symmetry as adults. There is no head; the surface containing the mouth (the underside, in sea stars and most others) is called the oral surface, and the opposite side, which usually bears the anus, the aboral surface. There are five living classes of echinoderms.

Anatomy and Physiology

The radially symmetrical body cavity contains a system of water-filled canals unique to echinoderms. Called the water-vascular, or ambulacral, system, it connects with the tube feet, or podia, which are extensions of the body wall that generally protrude through holes in the skeleton. The areas with such holes are called ambulacra. The tube feet often have suction cups on their tips and are used for locomotion in most echinoderms; they also function in feeding, respiration, and sensory reception.

The water-vascular system consists of a circular passageway, the ring canal, that surrounds the digestive tract and five radial canals that radiate from the ring canal like spokes of a wheel. Each radial canal underlies an ambulacral area. The ring canal is usually connected to a porous plate in the body wall, the madreporite, by a lime-walled tube called the stone canal. The position of the madreporite varies in the different groups. Seawater enters the system through the madreporite, which is regulated by the animal. Short lateral canals equipped with valves lead from the radial canals into the tube feet. Generally a muscular, water-filled bulb, the ampulla, is connected to each tube foot. When the valve closes and the ampulla contracts, water is squeezed into the tube foot, causing the foot to extend. The foot is retracted by the contraction of the attached muscles, thereby forcing the water back into the ampulla. Sea stars, sea cucumbers, and sea urchins move by alternately extending and retracting groups of tube feet, gripping with the suction cups and pulling themselves along. Because the tube feet are very thin-walled, their surface is suitable for the diffusion of oxygen into the body cavity and the diffusion outward of carbon dioxide and wastes.

The tube feet perform at least part of the respiratory function in most echinoderms; however, many groups have developed auxiliary respiratory structures. Echinoderms have no special excretory organs. Circulation occurs in an open system of channels and sinuses and in the body cavity, which is lined with flagellated cells that create an internal current. The cavity contains large phagocytic cells (amoebocytes) that function in the transport of food and the storage of insoluble wastes. There is a simple nervous system sensitive to temperature, light, and vibrations, with the various body projections serving as sensory receptors. Echinoderms have extensive powers of regeneration of lost or injured parts.

Most species reproduce sexually, and species have separate sexes. Fertilization is external; the gametes are simply shed into the water at spawning time. The floating embryo develops into a ciliated, free-swimming, bilaterally symmetrical larva, which undergoes metamorphosis into the radially symmetrical adult.

Class Asteroidea

Sea stars, or starfish, vary in shape from nearly circular, to pentagonal, to the familiar starlike and flowerlike forms with five or more tapering arms. The arms are extensions of the body; each contains an extension of the body cavity, a radial canal, and body organs. Each arm has an ambulacral groove on the undersurface; in the furrow of the groove is the ambulacral area, or ambulacrum, with holes for the tube feet. The margins of the groove have spines that can close over the ambulacrum. The tip of each arm bears a tube foot that functions as a sensory receptor for chemical and vibratory stimuli, and some have a red pigment spot that serves as a simple eye. The outer surface consists of a latticework of lime ossicles, or plates, between which project thin-walled fingerlike extensions called papulae. The papulae and the tube feet are the principal sites of respiratory exchange. In some groups of sea stars there are also body wall projections called pedicellaria, equipped with tiny pinchers that are operated by muscles and are used to clean the body surface and capture very small prey. Sea stars crawl about on rocks or muddy bottoms, feeding on a variety of living and dead animals. Many feed largely on bivalve mollusks and are notorious as destroyers of commercial oyster beds. There are two or more gonads in each arm; at spawning time these may nearly fill the arms. The swimming larva settles and goes through a sessile (attached) stage while changing to the adult form.

Class Ophiuroidea

The brittlestarsbrittlestar,
common name for echinoderms belonging to the class Ophiuroidea. The name is derived from their habit of breaking off arms as a means of defense. New arms are easily regenerated.
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, or serpent stars, are so called for their long, slender, fragile arms, which are set off sharply from the circular, pentagonal, or slightly star-shaped body disk. The arms of brittlestars are flexible and appear jointed because of the conspicuous plates of the outer surface. They bear a row of spines along each edge. In one group, the basket stars, they are repeatedly branched, forming a large mass of tentaclelike limbs. Each arm contains a radial canal (or one of its branches), but it does not contain body organs. Brittlestars feed on detritus and small organisms. The mouth leads to a large saclike stomach that fills most of the body cavity. There is no intestine or anus, and solid waste is extruded through the mouth. The stomach is folded into ten pouches, between which lie ten respiratory sacs that open by slits onto the oral surface. The cells lining the sacs have flagellae, which create a current of water moving in and out. Respiratory exchange occurs chiefly through the thin lining of the sacs.

Class Echinoidea

Echinoids—sea urchinssea urchin,
spherical-shaped echinoderm with movable spines covering the body. The body wall is a firm, globose shell, or test, made of fused skeletal plates and marked by regularly arranged tubercles to which the movable spines are attached.
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, heart urchins, and sand dollarssand dollar,
common name for a marine animal in the same phylum as the starfish (see sea star). The sand dollar has a rigid, flattened, disk-shaped test, or shell, made of firmly united plates lying just beneath the thin skin.
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—are echinoderms without arms and with a spiny shell, or test, formed of tightly fused skeletal plates. The sea urchins (regular echinoids) are hemispherical in shape, round on top and flat on the lower surface. They have very long, prominent spines and are often brightly colored. The test of a sea urchin is divided into ten parts from pole to pole, like the sections of an orange. Five of these are ambulacra, with openings for tube feet; these alternate with wider sections, called interambulacra, that lack tube feet. However, spines and pedicellaria are found over the entire surface of the test. Urchins move by pushing against the substratum with the spines and extending the tube feet in the direction of movement. If turned over they can right themselves by means of the tube feet on the aboral surface. The mouth, located in the center of the undersurface, is surrounded by a thickened region bearing five pairs of short, heavy tube feet and sometimes five pairs of bushy gills. Within the mouth is an elaborate five-sided jaw structure called Aristotle's lantern that can be partially extruded from the mouth. It is able to grind up calcareous exoskeletons of plants and animals. The anus is at the center of the aboral surface and is surrounded by a thin-walled area without skeletal plates.

Sand dollars and heart urchins (irregular echinoids) have a dense covering of short spines, and locomotion is exclusively by movement of the spines. There are two groups of podia-bearing ambulacra, one arranged in a petallike pattern on the upper surface and the other forming a similar pattern on the lower surface. The upper tube feet function as respiratory organs (there are no gills around the mouth), and the lower ones are specialized for gathering food particles. Sand dollars are extremely flattened and oval in outline; the anus is on the oral surface. Heart urchins are somewhat flattened and are heart-shaped; a deep ambulacral groove running from top to bottom creates a secondary bilateral symmetry. The anus is on the aboral surface, opposite the groove.

Class Holothuroidea

The sea cucumbers are long-bodied echinoderms with the mouth at or near one end and the anus at or near the other. Because of their elongation along the oral-aboral plane, they lie on their sides rather than on the oral surface. In nearly all sea cucumbers the skeleton is reduced to microscopic ossicles imbedded in the leathery skin. Sea cucumbers have no arms, but tube feet around the mouth have been modified to form a circle of 10 to 30 tentacles of varying lengths and shapes that function in gathering food particles from the ocean bottom. The gut of the sea cucumber terminates in a chamber called the cloaca that opens into the anus. Two unique structures called respiratory trees, found in most sea cucumbers, also terminate in the cloaca. These are systems of highly branched tubes, one on either side of the body. The animal pumps water into the respiratory trees by contracting the cloaca, and oxygen diffuses through from the walls of the trees into the fluid of the body cavity. The madreporite in most sea cucumbers opens into the body cavity rather than to the outside and receives its fluid from the cavity. In a few sea cucumber species there is a large mass of tubules at the base of the respiratory tree that can be shot out of the anus if the animal is irritated. The extruded tubules, which may engulf and incapacitate an intruder, break off; they are then regenerated by the sea cucumber. In other species the respiratory trees, gonads, and part of the digestive tract are shot out through the anus; this evisceration is followed by regeneration of the lost organs.

Class Crinoidea

The sea liliessea lily,
stalked echinoderm of the class Crinoidea. Sea lilies are ancient, having reached their peak in the Middle Mississippian period; about 5,000 fossil species are known. About 80 modern species remain.
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 and feather starsfeather star,
common name of a class of echinoderms that, as juveniles, are attached to the sea bottom by a stalk with rootlike branches; the mouth side faces upward. In the adult stage they break away from the stalk and move about freely.
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 are members of an ancient group of stalked, sessile, detritus-feeding echinoderms. Most of the sea lilies remain stalked throughout life; their movements include bending the stalk and the arms and crawling. Feather stars break off the stalk and become free-living as adults. Crinoids, whether free or stalked, always have the oral side upward, and the ring of arms about the mouth gives them a flowerlike appearance. They have at least 10 arms, but some sea lilies have up to 40 and some feather stars up to 200 arms. The stalk and the arms have a jointed appearance, and each arm has a row of projections, the pinnules, on either side, giving a feathery appearance. A ciliated ambulacral groove runs along each arm and branches into the pinnules; the groove contains feathery, tube feet arranged in triads. These react to the presence of minute food particles in the water by bending inward, sweeping the particles into the groove, where they are trapped in mucus and swept by the cilia toward the mouth. Gametes develop in some of the pinnules, which rupture at spawning time. The free-swimming larva eventually settles and develops a stalk and a crown.

Class Concentricycloidea

The sea daisies, which were discovered in 1986, have disk-shaped flat bodies and are less than 0.39 in. (1 cm) in diameter. The two known species were located on wood found in deep waters off the coasts of New Zealand and the Bahamas. They have a water-vascular system, with tube feet on the body surface around the edge of the disk. They have no obvious arms or mouth, and appear to absorb nutrients through the membrane surrounded their bodies.


A unique group of exclusively marine animals with a peculiar body architecture. They are headless with a fivefold radial symmetry. The body wall contains the endoskeleton, made of numerous independent calcareous plates which frequently support 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 associated muscles and connective tissue, form a tough and sometimes rigid test which encloses the large coelom. A unique water-vascular system is involved in locomotion, respiration, food gathering, and sensory perception. This shows outside the body as rows of fluid-filled tube feet in conspicuous double lines or ambulacra. Within the body wall lie the ducts and fluid reservoirs necessary to protract and retract the tube feet by hydrostatic pressure. The nervous system arises from the embryonic ectoderm and consists of a ring around the mouth with connecting nerve cords associated with each ambulacrum. There may also be diffuse nerve plexuses lying below the outer epithelium. The coelom houses the alimentary canal and associated organs and in most groups the reproductive organs.

The larvae are usually planktonic with a bilateral symmetry, but the adults are usually sedentary and benthic. They inhabit all seas and oceans, ranging from the shores to the ocean depths.

The phylum comprises about 6000 existing species and many fossils, providing a good fossil record. Echinoderms first appeared in the Early Cambrian and have been evolving over 600 million years. During this vast time several divergent patterns have arisen. The surviving groups show few resemblances to the original stock. The existing representatives fall into three subphyla: Crinozoa (class Crinoidea: sea lilies and feather stars); Asterozoa (class Asteroidea: starfishes, and class Ophiuroidea: brittle stars), and Echinozoa (class Echinoidea: sea urchins, sand dollars and heart urchins); and class Holothuroidea: sea cucumbers). The fourth subphylum, Homalozoa, has no living representatives.

Echinoderms evolved very rapidly near the beginning of the Paleozoic Era. During the Paleozoic, numerous well-marked evolutionary trends are discernible in nearly all echinoderm groups, including free-moving forms (especially echinoids) as well as crinozoans. Many small classes of echinoderms became extinct during the Paleozoic, and the surviving groups, especially the crinoids, lost many members at the great Late Permian mass extinction. All groups of modern echinoderms have their origin in early Paleozoic stocks, and the lines of their phylogeny are mostly indicated by the fossil record. Echinoids predominate in Mesozoic and Cenozoic echinoderms.



a phylum of invertebrate animals. They are marine free-moving or sessile animals, with secondary radial symmetry, a calcareous skeleton, and an ambulacral (water-vascular) system. The phylum Echinodermata, together with the phyla Hemichordata and Chordata, belongs to the subdivision Deuterostomia.

The body of echinoderms (measuring from several millimeters to 5 m in existing species and up to 20 m in fossils) has the shape of a star (starfish, or Asteroidea, and ophiuroids, or Ophiuroidea); a flower (crinoids, or Crinoidea); a pouch (many fossil echinoderms); a sphere, disk, or heart (sea urchins, or Echinoidea); or a cucumber or worm (holothurians, or Holothuroidea) (see Figure 1). The body is usually divided into ten alternating parts—five radii, or ambulacra, with tube feet, and five interradii, or interambulacra, without tube feet. However, there may be four, six, 13, 25, and even more than 40 radii. Radial symmetry is also expressed in the structure of the internal organs. The mouth in free-moving echinoderms is usually on the ventral (oral) surface of the body, facing the sea floor, while the anus is on the opposite, dorsal (aboral) surface. In sessile echinoderms both intestinal openings are close together and face upward. The concepts of ventral and dorsal surfaces are arbitrary, since they do not correspond morphologically to those of bilaterally symmetrical animals.

Figure 1. Diagrams of the external structure of various echinoderms: (1) crinoid, (2) starfish, (3) ophiuroid, (4) and (5) sea urchin, (6) holo-thurian; (a) ambulacrum, (b) interambulacrum, (c) mouth, (d) anus

The body wall in echinoderms is rigid and consists of ciliated epidermis and a connective tissue layer in which the skeleton and muscles are found. Soft integuments are typical only of holothurians. In addition to calcareous plates, the skeleton consists of spines and pedicellariae. The spines are usually movable, especially in sea urchins in which they attain a length of 30 cm and serve for defense and locomotion. The pedicellariae of starfish and sea urchins are small, pincer-like appendages equipped with muscles and serve for defense against enemies and removal of dirt. The water-vascular system develops from rudiments of the secondary body cavity—the coelom—and serves for locomotion, respiration, tactility, and excretion. The nervous system consists of a nerve ring around the mouth and radial nerves. The sense organs are poorly developed and are represented by poorly differentiated organs of smell, taste, touch, sight, and equilibrium. The intestine is shaped like a long tube (crinoids, sea urchins, holothurians) or a pouch (starfish, ophiuroids). Starfish have an inactive peristomial skeleton. In sea urchins it forms the complex apparatus termed Aristotle’s lantern, with five sharp “teeth” that serve to scrape off food from the substratum. In crinoids and holothurians there is no peristomial skeleton. The circulatory system consists of an oral and an aboral ring vessel and radial vessels as well as vessels of the so-called axial organ and the intestine. Excretion in echinoderms is accomplished by amoeboid cells which, together with decomposition products, escape through a cleavage in the body wall or are deposited in the body cavity.

The sexes in echinoderms are separate. The sex products are shed into the water. The secondary body cavity is well developed and lined with ciliated epithelium. The axial complex of organs is characteristic only of echinoderms. It consists of the axial organ, the genital rachis, which gives rise to the TEXT $sex glands, the left and right axial sinuses, the genital sinus, the stone canal, and the madreporite (see Figure 2). The right axial sinus, which encloses part of the axial organ, is comparable to the pericardium of chordates. The axial complex regulates osmotic pressure in the water-vascular system, participates in excretion, and causes movement of the blood.

Figure 2. Circulatory system and axial organ of starfish: (a) general diagram, (b) ring canals and axial organ; (1) oral ring canal, (2) aboral ring canal, (3) vessels of the axial organ, (4) radial canals, (5) vessels of the intestine, (6) vessels of the sex glands

Echinoderms are sluggish animals. Stalked crinoids can only move their “arms”; stalkless crinoids are able to swim and crawl by means of “arms.” Ophiuroids move jerkily by extending two pairs of “arms” forward and abruptly bending them backward. Starfish creep slowly by means of ambulacral tube feet. Sea urchins walk on their spines as if on stilts and also pull themselves along on their tube feet. Many echinoderms are predators; some feed on vegetable matter and detritus. Crinoids feed on diatoms and pelagic animals, which enter the mouth with a stream of water along the ambulacral grooves. The food of starfish consists of worms, mollusks, crabs, fish, and so on. Ophiuroids feed on worms and mollusks, which they trap in their “arms.” Sea urchins feed on small animals, algae, and detritus. Holothurians catch planktonic animals with the sticky tentacles around their mouths or eat detritus.

All echinoderms reproduce sexually, and some also reproduce asexually by division of the body into two parts with subsequent regeneration of the missing half (starfish, ophiuroids, and holothurians). Fertilization of the egg and embryonic development occur in the water. The egg undergoes radial cleavage, and then the embryo passes through the blastula and the invaginated gastrula stages. Development proceeds with the formation of a pelagic larva and a complex metamorphosis. The initial form of the larva is a bilaterally symmetrical dipleurula with three pairs of coelomic sacs; its modifications are the auricularia in holothurians, bipinnaria in starfish, ophiopluteus in ophiuroids, and echinopluteus in sea urchins. The development of sea urchins, starfish, and ophiuroids is accompanied by complex changes— the destruction of the greater part of the larva and its organs and the formation of a young echinoderm from a small radial rudiment on the left or ventral side of the larva.

Upon being attacked by an enemy or under unfavorable conditions, some echinoderms may discard “arms” (starfish), a part of the body (ophiuroids, crinoids), or even a large part of the body with internal organs (holothurians) and subsequently regenerate them. This is known as autotomy.

Echinoderms are exclusively marine, predominantly benthic, animals. Only a few isolated forms of holothurians are pelagic. Echinoderms are found at all depths: from the surf zone to depths of more than 10,000 m. Often they are the leading forms of many marine communities. Approximately 6,000 species of existing echinoderms are known. The majority live in warm seas; there are only 122 species in the arctic region. In seas of the USSR having low salinity there are few echinoderms: two species live in the Baltic Sea and 13 in the Black Sea; there are no echinoderms in the Sea of Azov or the Caspian and Aral seas, and there are 83 species in the northern seas. The Far Eastern seas are very rich in echinoderm fauna, with more than 200 species in the Sea of Japan, the Sea of Okhotsk, and the Bering Sea. The Sea of Okhotsk and the Bering Sea are rich in deep-water echinoderms.

The most ancient echinoderms were bilaterally symmetrical, and some had gill slits; radial symmetry developed under the influence of sessile life. Initial bilateral symmetry, secondary formation of the mouth, the mode of development of the secondary body cavity (coelom), a stage of development with three pairs of coeloms, and other characteristics indicate the kinship of the Echinodermata with the Hemichordata and Chordata.

The phylum Echinodermata is divided into two subphyla— the sessile Pelmatozoa and the free-moving Eleutherozoa. The extinct classes Carpoidea, Cystoidea, Blastoidea, and Edrioas-teroidea and the extant class Crinoidea belong to the first sub-phylum. The second subphylum consists of two extinct classes—Ophiocistia and Somasteroidea—and four extant ones—As-teroidea, Ophiuroidea, Echinoidea, and Holothuroidea.

Echinoderms probably appeared in the Precambrian; remains of Carpoidea, Edrioasteroidea, Holothuroidea, and some forms closely related to Crinoidea have been found in the Cambrian. Representatives of all classes are known from the Ordovidian. During the Middle and Late Paleozoic many classes of echinoderms were already becoming extinct. Cystoidea, Carpoidea, Ophiocistia, and Somasteroidea became extinct during the Devonian; Edrioasteroidea, at the beginning of the Carboniferous; and Blastoidea, in the Permian. In the remaining classes the process of extinction of certain large groups continued, but the appearance of new ones was occurring simultaneously. There is no unanimous view with respect to the classification of ancient extinct echinoderms. Many investigators separate part of the Cambrian and Ordovician forms that are closely related to Crinoidea into the special classes Eocrinoidea and Para-crinoidea. In the opinion of some authors, the class Carpoidea is heterogeneous and should be divided into three independent ones. It has been suggested that one of them (Stylophora) should be separated into a special phylum—Calcichordata.

Owing to their wide distribution in massive quantities, echinoderms have great significance in the marine economy. In China and Japan certain holothurians (trepang) are used for food in boiled form. In the Mediterranean Sea there are sea urchins whose roe is used for food. In the USSR (in the Sea of Japan) holothurians (trepang, Cucumaria) that are dried or canned have some commercial value; the roe of sea urchins is also canned. A number of building materials (marbles and limestones) are composed chiefly of echinoderms. Many fossil echinoderms are important guiding forms for stratigraphy. Starfish cause considerable damage by destroying oyster beds.


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(invertebrate zoology)
A phylum of exclusively marine coelomate animals distinguished from all others by an internal skeleton composed of calcite plates, and a water-vascular system to serve the needs of locomotion, respiration, nutrition, or perception.
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