echinoderm

/i kuy"neuh derrm', ek"euh neuh-/, n.

any marine animal of the invertebrate phylum Echinodermata, having a radiating arrangement of parts and a body wall stiffened by calcareous pieces that may protrude as spines and including the starfishes, sea urchins, sea cucumbers, etc.

[1825-35; taken as sing. of NL Echinodermata, neut. pl. of ECHINODERMATUS < Gk echîn(os) sea urchin + -o- -O- + -dermatos -DERMATOUS]

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Any of various marine invertebrates (phylum Echinodermata) characterized by a hard spiny covering, a calcite skeleton, and five-rayed radial body symmetry.

About 6,000 existing species are grouped in six classes: feather stars and sea lilies (Crinoidea), starfishes (Asteroidea), brittle stars and basket stars (Ophiuroidea), sea urchins (Echinoidea), sea daisies (Concentricycloidea), and sea cucumbers (Holothurioidea). Echinoderms are found in all the oceans, from the intertidal zone to the deepest oceanic trenches. Most species have numerous tube feet that are modified for locomotion, respiration, tunneling, sensory perception, feeding, and grasping. Movement of water through a water vascular system composed of five major canals and smaller branches controls extension and retraction of the tube feet. Most echinoderms feed on microscopic detritus or suspended matter, but some eat plants.

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▪ animal phylum

Introduction

      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 (sea lily) and feather stars (feather star)), Echinoidea (sea urchins), Holothuroidea (sea cucumbers), Asteroidea (starfishes (sea star), 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.

General features

Size range and diversity of structure

      Although most echinoderms are of small size, ranging up to 10 centimetres (four inches) in length or diameter, some reach relatively large sizes; e.g., some sea cucumbers are as long as two metres (about 6.6 feet), and a few starfishes have a diameter of up to one metre. Among the largest echinoderms were some extinct (fossil) crinoids (sea lilies), whose stems exceeded 20 metres in length.

      Echinoderms exhibit a great diversity of body forms, especially among the extinct groups. Although all living echinoderms have a pentamerous (five-part) radial symmetry, an internal skeleton, and a water-vascular system derived from the coelom (central cavity), their general appearance ranges from that of the stemmed, flowerlike sea lilies, to the wormlike, burrowing sea cucumbers, to the heavily armoured intertidal starfish or sea urchin. The general shape of the echinoderm may be that of a star with arms extended from a central disk or with branched and feathery arms extended from a body often attached to a stalk, or it may be round to cylindrical. Plates of the internal skeleton may articulate with each other (as in sea stars) or be sutured together to form a rigid test (sea urchins). Projections from the skeleton, sometimes resembling spikes, which are typical of echinoderms, give the phylum its name (from Greek echinos, “spiny,” and derma, “skin”). The surface of holothurians, however, is merely warty.

      Echinoderms also exhibit especially brilliant colours such as reds, oranges, greens, and purples. Many tropical species are dark brown to black, but lighter colours, particularly yellows, are common among species not normally exposed to strong sunlight.

Distribution and abundance

      Diverse echinoderm faunas consisting of many individuals and many species are found in all marine waters of the world except the Arctic, where few species occur. Echinoids, including globular spiny urchins and flattened sand dollars (sand dollar), and asteroids are commonly found along the seashore. Although many species are restricted to specific temperate regions, Arctic, Antarctic, and tropical forms often are widely distributed; many species associated with coral reefs, for example, range across the entire Indian and Pacific oceans. Many of the echinoderms of Antarctica are distributed around the continent; those with a floating (planktonic) larval stage may be widely distributed, carried great distances by ocean currents. Some species, particularly those in Antarctic and deep-sea regions, have achieved a wide distribution without benefit of a floating larval stage. They may have done so by migration of adults across the seafloor or, in the case of shallow-water species, by passive transport across oceans in rafts of seaweed. Echinoderms tend to have a fairly limited depth range; species occurring in near-shore environments do not normally reach depths greater than 100 metres. Some deep-sea species may be found over a considerable range of depths, often from 1,000 metres to more than 5,000 metres. One sea cucumber species has a known range of 37–5,205 metres. Only sea cucumbers reach ocean depths of 10,000 metres and more.

Importance

Role in nature

      Echinoderms are efficient scavengers of decaying matter on the seafloor, and they prey upon a variety of small organisms, thereby helping to regulate their numbers. When present in large numbers, sea urchins can devastate sea-grass beds in the tropics, adversely affecting the organisms dwelling within. Sea urchins that burrow into rocks and along a shore can accelerate the erosion of shorelines. Other tropical species of sea urchins, however, control the growth of seaweeds in coral reefs, thereby permitting the corals to flourish. Removal of the sea urchins results in the overgrowth of seaweeds and the devastation of the coral reef habitat. Echinoderms can alter the structure of seafloor sediments in a variety of ways. Many sea cucumbers feed by swallowing large quantities of sediment, extracting organic matter as the sediment passes through the intestine, and ejecting the remainder. Large populations of sea cucumbers in an area can turn over vast quantities of surface sediments and can greatly alter the physical and chemical composition of the sediments. Burrowing starfish, sand dollars, and heart urchins disturb surface and subsurface sediments, sometimes to depths of 30 centimetres or more. In addition, echinoderms produce vast numbers of larvae that provide food for other planktonic organisms.

Relation to human life

      Some of the larger species of tropical sea cucumbers, known commercially as trepang or bêche-de-mer, are dried and used in soups, particularly in Asia. Raw or cooked mature sex organs, or gonads (gonad), of sea urchins are regarded as a delicacy in some parts of the world, including parts of Europe, the Mediterranean region, Japan, and Chile. Some tropical holothurians produce a toxin, known as holothurin, which is lethal to many kinds of animals; Pacific islanders kill fish by poisoning waters with holothurian body tissues that release the toxin. Holothurin does not appear to harm human beings; in fact, the toxin has been found to reduce the rate of growth of certain types of tumours and thus may have medical significance. The eggs and spermatozoa of echinoderms, particularly those of sea urchins and starfishes, are easily obtained and have been used to conduct research in developmental biology. Indeed, echinoids have been collected in such large numbers that they have become rare or have disappeared altogether from the vicinity of several marine biologic laboratories.

      Starfishes that prey upon commercially usable mollusks, such as oysters, have caused extensive destruction of oyster beds. Sea urchins along the California coast have interfered with the regrowth of commercial species of seaweed by eating the young plants before they could become firmly established. The crown-of-thorns starfish, which feeds on living polyps of reef corals, has caused extensive short-term damage to coral reefs in some parts of the Pacific and Indian oceans.

Reproduction and life cycle

      In most species the sexes are separate; i.e., there are males and females. Although reproduction is usually sexual, involving fertilization of eggs by spermatozoa, several species of sea cucumbers, starfishes, and brittle stars can also reproduce asexually.

Asexual reproduction

      Asexual reproduction in echinoderms usually involves the division of the body into two or more parts (fission (binary fission)) and the regeneration of missing body parts. Fission is a common method of reproduction in approximately 60 species of asteroids, ophiuroids, and holothurians (a total of less than 1 percent of the living species), and in some of these species sexual reproduction is not known to occur. Successful fission and regeneration require a body wall that can be torn and an ability to seal resultant wounds. In some asteroids fission occurs when two groups of arms pull in opposite directions, thereby tearing the animal into two pieces. Successful regeneration requires that certain body parts be present in the lost pieces; for example, many asteroids and ophiuroids can regenerate a lost portion only if some part of the disk is present. In sea cucumbers, which divide transversely, considerable reorganization of tissues occurs in both regenerating parts.

      The ability to regenerate (regeneration), or regrow, lost or destroyed parts is well developed in echinoderms, especially sea lilies, starfishes, and brittle stars, all of which can regenerate new arms if existing ones are broken off. Echinoderm regeneration frustrated early attempts to keep starfishes from destroying oyster beds; when captured starfishes were chopped into pieces and thrown back into the sea, they actually increased in numbers. So long as a portion of a body, or disk, remained associated with an arm, new starfishes regenerated. Some sea cucumbers can expel their internal organs (autoeviscerate) under certain conditions (i.e., if attacked, if the environment is unfavourable, or on a seasonal basis), and a new set of internal organs regenerates within several weeks. Sea urchins (Echinoidea) readily regenerate lost spines, pincerlike organs called pedicellariae, and small areas of the internal skeleton, or test.

Sexual reproduction

      In sexual reproduction, eggs (up to several million) from females and spermatozoa from males are shed into the water (spawning), where the eggs are fertilized. Most echinoderms spawn on an annual cycle, with the spawning period normally lasting one or two months during spring or summer; several species, however, are capable of spawning throughout the year. Spawn-inducing factors are complex and may include external influences such as temperature, light, or salinity of the water. In the case of one Japanese feather star (Crinoidea), spawning is correlated with phases of the Moon and takes place during early October when the Moon is in the first or last quarter. Many echinoderms aggregate before spawning, thus increasing the probability of fertilization of eggs. Some also display a characteristic behaviour during the spawning process; some asteroids and ophiuroids raise the centre of the body off the seafloor; holothurians may raise the front end of the body and wave it about. These movements are presumably intended to prevent eggs and sperm from becoming entrapped in the sediment.

Development (biological development)

      After an egg is fertilized, the development of the resulting embryo into a juvenile echinoderm may proceed in a variety of ways. Small eggs without much yolk develop into free-swimming larvae that become part of the plankton, actively feeding on small organisms until they transform, or metamorphose, into juvenile echinoderms and begin life on the seafloor. Larger eggs with greater amounts of yolk may develop into a larval form that is planktonic but subsists upon its own yolk material, rather than feeding upon small organisms, before eventually transforming into a juvenile echinoderm. Development involving an egg, planktonic larval stages, and a juvenile form is termed indirect development. Echinoderm development in which large eggs with abundant yolk transform into juvenile echinoderms without passing through a larval stage is termed direct development.

      In direct development the young usually are reared by the female parent. Parental care or brood protection ranges from actual retention of young inside the body of the female until they are born as juveniles to retention of the young on the outer surface of the body. Brood protection is best developed among Antarctic, Arctic, and deep-sea echinoderms, in which young may be held around the mouth or on the underside of the parent's body, as in some starfishes and sea cucumbers, or in special pouches on the upper surface of the body, as in some sea urchins, sea cucumbers, and asteroids.

      During indirect development, the fertilized egg divides many times to produce a hollow ciliated ball of cells (blastula); cleavage is total, indeterminate, and radical. The blastula invaginates at one end to form a primitive gut, and the cells continue to divide to form a double-layered embryo called the gastrula. Echinoderms resemble vertebrates and some invertebrate groups (chaetognaths and hemichordates) in being deuterostomes; the hole through which the gut opens to the outside (blastopore) marks the position of the future anus; the mouth arises anew at the opposite end of the body from the blastopore. A pair of subdivided hollow pouches arise from the gut and develop into the body cavity (coelom) and water-vascular system.

      The gastrula develops into a basic larval type called a dipleurula larva, characterized by bilateral symmetry; hence the name, which means “little two sides.” A single band of hairlike projections, or cilia, is found on each side of the body and in front of the mouth and anus. The characteristic larvae found among the living classes of echinoderms are modifications of the basic dipleurula pattern.

      Because the ciliated band of the dipleurula larva of holothurians becomes sinuous and lobed, thus resembling a human ear, the larva is known as an auricularia larva. The dipleurula larva of asteroids develops into a bipinnaria larva with two ciliated bands, which also may become sinuous and form lobes or arms; one band lies in front of the mouth, the other behind it and around the edge of the body. In most asteroids the larval form in the next stage of development is called a brachiolaria, which has three additional arms used for attaching the larva to the seafloor. Echinoids and ophiuroids have complex advanced larvae closely similar in type. The larva, named pluteus, resembles an artist's easel turned upside down. It has fragile arms formed by lobes of ciliated bands and is supported by fragile rods of calcite, the skeletal material. The echinoid larva (echinopluteus) and the ophiuroid larva (ophiopluteus) usually have four pairs of arms but may have fewer or more. An extra unpaired arm on the plutei of sand dollars and cake urchins extends downward, presumably to help keep the larva upright. The crinoids, which apparently lack a dipleurula larval stage, have a barrel-shaped larva called a doliolaria larva. The doliolaria larva also occurs in other groups; in holothurians, for example, it is the developmental stage after the auricularia larva, which may not occur in some species. A doliolaria larva usually contains large quantities of yolk material and moves with the aid of several ciliated bands arranged in hoops around the body.

      Although most larval stages are small, often less than one millimetre (0.04 inch) in length, some holothurians are known to be 15 millimetres long in the larval stage, and the length of bipinnaria larvae of some starfishes may exceed 25 millimetres.

      After a few days to several weeks in a free-swimming form (plankton), echinoderm larvae undergo a complex transformation, or metamorphosis, that results in the juvenile echinoderm. During metamorphosis, the fundamental bilateral symmetry is overshadowed by a radial symmetry dominated by formation of five water-vascular canals (see below Form and function of external features (echinoderm)). Among holothurians, echinoids, and ophiuroids, the larvae may metamorphose as they float, and the young then sink to the seafloor; among crinoids and asteroids, however, the larvae firmly attach to the seafloor prior to metamorphosis. The average life span of echinoderms is about four years, and some species may live as long as eight or 10.

Life activities

Food and feeding habits

      Echinoderms feed in a variety of ways. A distinct feeding rhythm frequently occurs, with many forms feeding only at night, others feeding continuously. Feeding habits range from active, selective predation to omnivorous scavenging or nonselective mud swallowing.

      Crinoids are suspension feeders, capturing planktonic organisms in a network of mucus produced by soft appendages, called tube feet, contained in grooves on the tentacles, or arms. The arms are spread into a characteristic “fan” at right angles to the prevailing current, and small prey animals are passed to the mouth along the grooves by activity of the cilia and the tube feet.

      Many asteroids are active predators on shellfishes and even upon other starfishes; other asteroids are mud swallowers. When feeding, some asteroid species extrude their stomach through the mouth onto the prey, which then is partially digested externally, after which the stomach is retracted and digestion is completed inside the body. Most ophiuroids feed on small organisms floating in the water or lying on the bottom, which are captured by the arms and tube feet and passed toward the mouth. Ophiuroids with arms branched in a complex manner may feed in a way similar to that of the crinoids. Feeding methods of concentricycloids are not yet known.

      The more primitive, so-called regular, sea urchins are omnivorous or vegetarian browsers, either scraping algae and other small organisms from rocks with their hard teeth or eating seaweed. Several deep-sea regular echinoids feed exclusively on plants carried into the sea from the land. The more advanced irregular echinoids, which usually lack teeth, are burrowers and pass small organisms to the mouth with the aid of spines and tube feet. Several species of sand dollars sometimes feed on suspended organisms carried to them by ocean currents as they lie on the seafloor. Some sea cucumbers remain attached to a surface for indefinite periods of time, capturing plankton in a network of branching, sticky tentacles; others select food from the seafloor and push it into their mouths with their tentacles. A large number of holothurians feed by actively swallowing mud and sand, digesting the organic material, and egesting the waste in the form of characteristic castings, in a manner similar to that of earthworms.

      Under artificial conditions, as in aquariums, echinoderms can survive apparent starvation for several weeks at a time. After a holothurian has autoeviscerated, it is unable to feed during the several weeks required for gut regeneration. Echinoderms may derive a significant amount of nourishment, at least for the outer cell layers of the body, from organic material dissolved in seawater.

Locomotion

      Asteroids and echinoids, which use spines and tube feet in locomotion, may move forward with any area of the body and reverse direction without turning around. The feet may be used either as levers, by means of which the echinoderm steps along a surface, or as attachment mechanisms that pull the animal. Sea daisies presumably move in the same way. Ophiuroids tend to move by thrashing the arms in one of several possible methods, including a rowing motion in which strokes are taken by two pairs of extended arms; the fifth arm either is extended forward in the direction in which the animal is traveling or trails behind.

      Holothurians (sea cucumbers) generally lead with the mouth, or oral, end, movement being carried out by both the tube feet and contraction and expansion of the body; sluglike movement is common. Holothurians of the family Synaptidae are able to pull themselves across a surface using their sticky tentacles as anchors.

      Stalked crinoids (sea lilies), so called because they have stems, generally are firmly fixed to a surface by structures at the ends of the stalks called holdfasts. Some fossil and living forms release themselves to move to new attachment areas. The unstalked crinoids (feather stars) generally swim by thrashing their numerous arms up and down in a coordinated way; for example, in a 10-armed species, when arms 1, 3, 5, 7, and 9 are raised upward, arms 2, 4, 6, 8, and 10 are forcibly pushed downward; then the former group of arms thrashes downward as the latter is raised. Feather stars that do not swim pull themselves across a surface using their arms.

      Swimming is known to occur in crinoids, ophiuroids, and holothurians. Some holothurians, formerly regarded as strictly bottom-living forms, are capable of efficient swimming; others, with gelatinous or flattened bodies and reduced calcareous skeletons, spend most of their lives swimming in deep water.

Righting response

      Among echinoderms a normal position may be with the mouth either facing a surface, as in asteroids, ophiuroids, concentricycloids, and echinoids, or facing away from it, as in crinoids and holothurians. When overturned, echinoderms exhibit a righting response. Starfishes show this response most effectively, using the tube feet and the arms to perform a slow, graceful somersault that restores their normal position. Sea urchins roll themselves over by a concerted action of their tube feet and spines. The flat sand dollar can turn itself over only by burrowing into the sand until its position is vertical, then toppling over. In more agile groups such as holothurians, crinoids, and ophiuroids, righting is performed with relative ease.

Burrowing

      Many echinoderms burrow in rock or soft sediments. Crinoids do not burrow because their feeding apparatus must be kept clear of sediment. Some urchins use the combined abrasive actions of their spines and teeth to burrow several inches into rock, usually in areas of severe wave and tidal action. The so-called irregular echinoids excavate soft sediments to various depths; most sand dollars burrow just below the surface, and some heart urchins may be found at depths of 38 centimetres or more. Holothurians use tentacles and contraction of the body wall in burrowing that generally is related to feeding. Several asteroid species bury themselves in sandy or muddy areas. The characteristic position of several ophiuroid groups involves burying the body into a surface and leaving only the tips of the arms projecting for food gathering.

Ecology

Habitats

      Echinoderms are exclusively marine animals, with only a few species tolerating even brackish water. Among the exceptions are a few tropical holothurians that can withstand partial drying if stranded on a beach by a receding tide. Most echinoderms cannot tolerate marked changes in salinity, temperature, and light intensity and tend to move away from areas where these factors are not optimal. The behaviour of a large proportion of shallow-water species is regulated by light; i.e., individuals remain concealed during the day and emerge from concealment at night for active feeding. Echinoderms are found in the warmest and coldest of the world's seas; those species that can tolerate a broad temperature range usually also have a broad geographic range. The horizontal or vertical distribution of many species is also governed by water temperature. The influence of pressure upon echinoderms has not yet been thoroughly investigated.

      Echinoderms occupy a variety of habitats. Along a rocky shore, starfishes and sea urchins may cling to rocks beneath which sea cucumbers and brittle stars are concealed. Some sea urchins have special adaptations for coping with surf pounding against rocks (e.g., particularly strong skeletons and well-developed tube feet for attachment). In sandy areas starfishes, brittle stars, irregular sea urchins, and sea cucumbers may bury themselves or move on the surface. Large populations of all living groups of echinoderms can be found in mud and ooze offshore. In some marine areas, echinoderms are the dominant organism; in the deepest ocean trenches, for example, holothurians may constitute more than 90 percent by weight of the living organisms. Perhaps the most unusual habitat is exploited by sea daisies and a small family of asteroids; these animals occur only on pieces of waterlogged wood on the deep-sea floor.

      Echinoderms frequently use other animals as homes; thousands of brittle stars, for example, may live in some tropical sponges. Sea cucumbers may attach themselves to the spines of sluggish Antarctic echinoids, and one sea cucumber attaches itself to the skin of a deep-sea fish. On the other hand, echinoderms are also hosts to a wide variety of organisms. Various crustaceans and barnacles, for example, cause the formation of galls, or tumourlike growths, in the skeletons of sea urchins, and crinoids are hosts of specialized parasitic worms. Commensal worms, which do no damage, are associated with most groups; an interesting case of commensalism is the association between various tropical sea cucumbers and the slender pearlfish, which often is found in the rectum of the holothurian, head protruding through its anus. Pinnotherid crabs may be found in the rectum of echinoids and holothurians in Peru and Chile, and highly modified parasitic gastropod mollusks are frequently found in the body cavities of holothurians. A conspicuous parasitic sponge grows on two species of Antarctic ophiuroids.

Predation and defense

      Although echinoderm populations do not generally suffer from heavy predation by other animals, ophiuroids form a significant part of the diet of various fishes and some asteroids. Echinoids are frequently eaten by sharks, bony fishes, spider crabs, and gastropod mollusks; crows, herring gulls, and eider ducks may either peck their tests (internal skeletons) or drop them repeatedly until they break; and mammals, including the Arctic fox, sea otters, and humans, eat them in considerable numbers. Asteroids are eaten by other asteroids, mollusks, and crustaceans. Some holothurians are eaten by fishes and by humans. Crinoids appear to have no consistent predators.

      Echinoderms can protect themselves from predation in a variety of ways, most of which are passive. The presence of a firm skeleton often deters predators; echinoids, for example, have a formidable array of spines and, in some cases, highly poisonous stinging pincerlike organs (pedicellariae), some of which may cause intense pain and fever in humans. Some asteroids use chemical secretions to stimulate violent escape responses in other animals, particularly predatory mollusks. Some holothurians eject from the anus a sticky mass of white threads, known as cuvierian tubules, which may entangle or distract predators; others produce holothurin, a toxin lethal to many would-be predators.

Aggregation

      Echinoderms tend to aggregate in large numbers and evidently also did so in the past; fossil beds consisting almost exclusively of large numbers of one or a few species are known from as early as the Lower Cambrian. In present-day seas, ophiuroids may cover large areas of the seafloor; vast aggregations of echinoids are also common. Holothurians, crinoids, and some asteroids also often show a tendency to aggregate.

      The phenomenon of aggregation apparently is a response to one or more environmental factors, chief of which is availability of food; e.g., large numbers of ophiuroids and crinoids occupy areas in which strong currents carry large amounts of plankton. An ophiuroid raises some arms into the water to capture food, using other arms to hold on to other nearby ophiuroids; in this way, a large aggregation can maintain its position in an environment in which a single ophiuroid or a small clump of them would be swept away. As stated previously, aggregation also enhances possibilities for successful propagation of a species and possibly may afford some protection from predators. Aggregation may be a passive phenomenon resulting from interactions between individuals and the environment as well as a demonstration of true social behaviour, a result of interactions among individuals.

Form and function of external features

General features

      Echinoderms have a skeleton composed of numerous plates of mineral calcium carbonate (calcite). Part of the body cavity, or coelom, is a water-vascular system, consisting of fluid-filled vessels that are pushed out from the body surface as tube feet, papillae, and other structures that are used in locomotion, feeding, respiration, and sensory perception. The conspicuous five-rayed, or pentamerous, radial symmetry of living echinoderms tends to obliterate their fundamental bilateral symmetry.

Symmetry and body form

      Many of the earliest echinoderms either lacked symmetry or were bilaterally symmetrical. Bilateral symmetry occurs in all living groups and is especially marked in the larval stages. A tendency toward radial symmetry (the arrangement of body parts as rays) developed early in echinoderm evolution and eventually became superimposed upon the fundamental bilateral symmetry, often obliterating it. Radial pentamerous symmetry is conspicuous among all groups of living echinoderms. Although the reasons for the success of radial symmetry are not yet completely understood, it has been suggested that a pentamerous arrangement of skeletal parts strengthens an animal's skeleton more than would, for example, a three-rayed symmetry.

      Pentamerous structure is evident in the arrangement of the tube feet, which usually radiate from the mouth in five bands. Many of the major organ systems, including the water-vascular system, muscles, hemal system (a series of fluid-filled spaces of indeterminate function), and parts of the nervous system are also pentamerous. The skeleton follows a pentamerous pattern, except in holothurians, where it is usually reduced to microscopic ossicles (bones).

      Distinct growth patterns among the echinoderms provide some basis for separating the phylum into subphyla. Among homalozoans, the pattern is asymmetrical. In crinozoans and blastozoans, bands of tube feet radiate from the mouth, cross the theca (i.e., sheath or calyx), and extend onto the brachioles or arms; in asterozoans the bands of tube feet radiate outward from the mouth onto the arms to produce a star shape; and in the armless echinozoans, the tube feet form five meridians on the spherical or cylindrical body.

      The crinoid (sea lily, feather star) mouth is centrally located on the cup-shaped theca, from which arise a variable number of arms resembling fern fronds. Although five is the primitive number of arms, they branch once or several times in most living forms to produce 10 to 200 arms. Crinoids either are supported on a stem (or stalk) attached to the underside of the theca, or they lack stalks, as is the case with most living forms, and attach themselves by means of slender appendages adapted for grasping (prehensile cirri).

      Asteroids have a large central disk from which radiate five or more hollow arms containing parts of the major internal organ systems. The underside (oral surface) of the disk contains a centrally located mouth; the underside of each arm contains five or more bands of tube feet in special grooves called ambulacral furrows. The upper (aboral) surface of the disk has a centrally located anus (often absent) and the sieve plate (madreporite) of the water-vascular system (see below Form and function of internal features (echinoderm)). Seven-armed starfish species are not unusual, a deep-sea family has six to 20 arms, and one Antarctic genus may have up to 50 arms. Concentricycloids have a discoid body; the dorsal surface is plated and the ventral surface is naked.

      Ophiuroids have a small disk from which five arms radiate. The larger internal organs usually are confined to the disk. The centrally located mouth is on the underside of the disk as are the tube feet, which are not arranged in special grooves. Although most ophiuroids have five arms, a few have six or more, and in one group, the basket stars, the arms are branched to form a complex network.

      In echinoids the skeleton forms a rounded, or globular, test of solid plates; tube feet, which emerge through holes in the plates, form five conspicuous bands, or ambulacra. Spaces between bands of feet are called interambulacra. Regular echinoids are roughly spherical in shape, with a centrally located mouth at the junction of the five bands containing tube feet (ambulacra); the anus is located on the side of the body opposite the mouth (aboral). Irregular urchins are elongated or flattened in shape, with the anus on the oral or aboral surface of the body. In regular and some irregular echinoids, the mouth is equipped with five teeth operated by a complex system of plates and muscles called Aristotle's lantern.

      Holothurians are elongated, with mouth and anus at opposite ends of the body. The spaces between the tube feet, which are arranged in five rows, or radii, are known as interradii. The tube feet may be more numerous on the underside of the body than elsewhere, scattered over radii and interradii, or absent. Most holothurians are soft-bodied animals because the skeleton is reduced and the skeletal units, called ossicles or spicules, are microscopic in size. Holothurians usually show bilateral symmetry outside, radial symmetry inside.

skeleton

      The skeleton is dermal but nonetheless conspicuous in echinoderms, with the exception of most holothurians, and forms an effective armour. Each skeletal unit (ossicle) usually consists of two parts, a living tissue (stroma) and a complex lattice (stereom) of mineral calcium carbonate, or calcite, which is derived from the stroma. In living echinoderms, certain properties of calcite are not evident in the stereom because of its latticed structure and the presence of soft stroma. In fossils, however, the stroma may be replaced by secondary calcite (i.e., calcite laid down in continuity with the original skeletal calcite), and recognition of fragments of echinoderm skeletons in fossil strata is easier because no other animal group has the same type of skeleton. Each ossicle is formed from granules in the dermal layer that, after secretion from special lime-secreting cells, enlarge, branch, and fuse to build up a three-dimensional network of calcite. Parts of the skeleton enlarge as an animal grows, and resorption and regeneration of the skeleton may occur.

      Echinoderms exhibit a variety of skeletal structures. In the echinoids, a hollow test (skeleton) consisting of 10 columns of plates bears large and small spines as well as pincerlike organs (pedicellariae) used in defense and in the removal of unwanted particles from the body. Pedicellariae, also found in the asteroids, are absent from crinoids, ophiuroids, and holothurians. The complex feeding apparatus (Aristotle's lantern) of echinoids consists of 40 ossicles held together by muscles and collagenous sutures.

      Crinoids have a hollow sheath (theca or calyx) composed of two or three whorls, each consisting of five skeletal plates; the stalk and the slender appendages (cirri) of unstalked forms consist of a series of drum-shaped ossicles. The asteroid skeleton is composed of numerous smooth or spine-bearing ossicles of various shapes held together by muscles and ligaments, permitting flexibility. The arms of asteroids are hollow, those of ophiuroids solid, with the central axis of each arm consisting of elongated ossicles called vertebrae. The microscopically sized ossicles of holothurians are highly variable in form, ranging from flat lattice plates with holes to exquisitely symmetrical wheels, and are usually numerous; one tropical species, for example, has more than 26,000,000 ossicles in its body wall. A ring of plates, called the calcareous ring, surrounds the tube leading from the mouth to the stomach (i.e., the esophagus) of holothurians. Although located in a similar position to that of the echinoid Aristotle's lantern, the calcareous ring functions as a point of insertion for muscles, not as a feeding apparatus.

Form and function of internal features

Water-vascular system

      The water-vascular system, which functions in the movement of tube feet, is a characteristic feature of echinoderms, and evidence of its existence has been found in even the oldest fossil forms. It comprises an internal hydraulic system of canals and reservoirs containing a watery fluid, the system consisting of a sieve plate, or madreporite, and a ring vessel, or water-vascular ring, that are connected by a frequently calcified vessel called the stone canal. Five radial water canals extend outward from the ring vessel and give rise to branches that end in the tube feet, which are in contact with the sea. The ring vessel in ophiuroids, asteroids, concentricycloids, and holothurians has bulbous cavities called Polian vesicles, which apparently maintain pressure in the system and hold reserve supplies of fluid; ophiuroids have four or more vesicles, asteroids five, holothurians from one to 50. Crinoids lack Polian vesicles, and echinoids have five structures known as either Polian vesicles or spongy bodies.

      The madreporite, which is usually located externally, takes in water from outside the body; if internally located, as is the case in many holothurians, fluid is taken from the body cavity. The water or fluid passes from the madreporite to the ring vessel and along the radial canals to the tube feet. The tube feet are extended by contractions of localized muscle areas in the radial canals (ophiuroids) or by contractions of offshoots of the radial canals called ampullae (asteroids, concentricycloids, echinoids, and holothurians); the contractions force fluid into the tube feet, which then extend.

      The structure of the system varies from group to group; asteroids frequently have more than one madreporite, and in holothurians, the madreporite is usually internal, hanging in the coelom. Radial canals may lie inward or outward from the skeleton. The tube feet may have well-developed suckers with great holding power, may taper to a point, or may be adapted for respiration, feeding, burrow building, mucus production, or sensory perception. Attachment of tube feet to hard substrates is achieved through a combination of suction and mucus production. The mucus contains adhesive and de-adhesive mucopolysaccharides. Respiratory tube feet have high oxygen uptake; they are usually located on parts of the body where water flow is unimpeded. Tube feet have been implicated in photoreception and chemoreception; the eyespots in the terminal tentacles of asteroids are the most conspicuous photoreceptors.

      The tube feet of crinoids are arranged in clumps of three on the arms and on the pinnules. They secrete and spread a net of sticky mucus that traps small organisms. In ophiuroids the tube feet are used to gain a hold on a surface and to pass food to the mouth. The numerous tube feet of asteroids are used in locomotion; asteroids with suckered feet may use them to exert a continuous pull on the valves of shellfish (e.g., oysters, mussels) until muscles holding the valves tire and open slightly, allowing the asteroid to insert its stomach. In sea daisies the ring of tube feet is probably used for attachment to substrates. Holothurians use tube feet for the same purpose. Tentacles around the mouth of holothurians are modified tube feet used to capture food; tentacles used to capture plankton are branched and sticky, while those used to scoop mud and shovel it into the mouth have a simpler structure.

      The tube feet of echinoids serve a variety of functions. The mouth of regular echinoids is surrounded by sensory tube feet, and tube feet farther from the mouth are used in locomotion. On the upper side of the body near the anus, the tube feet have respiratory and sensory functions. The tube feet of irregular echinoids, which burrow, are modified in various ways for feeding, burrow construction, and sensory and respiratory functions.

Body wall and body cavity

      The outer body wall ( epidermis) contains hairlike projections (cilia (cilium)) in most echinoderms except ophiuroids; the body wall of crinoids has relatively few. The cilia produce a waving motion that carries food particles toward the mouth or removes unwanted particles from the body. The epidermis also contains glandular and sensory cells. The epidermis of skeletal elements such as spines and pedicellariae, which project from the body surface, often is worn away. The next layer, the dermis, includes the calcareous skeleton and connective tissues. Internal to the dermis are circular and longitudinal muscle layers. The extensive body cavity (coelom) is modified to form several specialized regions. Two subdivisions of the coelom are the perivisceral coelom and the water-vascular system. The perivisceral coelom is a large, fluid-filled cavity in which the major organs, particularly the digestive tube and sex organs, are suspended. Other regions of the coelom include the axial sinus (absent from adult holothurians and all echinoids), the madreporic vesicle, and the hyponeural sinus (often called the perihemal system).

Alimentary (alimentary canal) and blood systems

      The digestive canal consists of a tube, which is almost straight (asteroids and ophiuroids), coiled in a clockwise direction (crinoids and holothurians), or coiled first clockwise, then counterclockwise (echinoids). The tube may be divided into esophagus, stomach, intestine, and rectum. Specialized branches of the digestive tube enlarge the digestive surface and may serve other functions; e.g., digestive glands of asteroids, diverticula of echinoids and crinoids, siphons in echinoids, and respiratory trees in holothurians. The anus, absent in ophiuroids and a few asteroids, is present in most groups. The mouth is near the centre of the oral surface, at the point of convergence of the areas containing the tube feet.

      The blood system is a complex system of spaces that are neither part of the coelom nor true vessels. A hemal ring and five radial hemal canals surround the esophagus and radial canals of the water-vascular system. A sixth hemal space arises from the hemal ring and enters the axial organ. In addition, a complex network of hemal spaces is associated with the alimentary canal and gonads.

Axial organ

      The axial organ, a complex and elongated mass of tissue found in all echinoderms except holothurians, represents the common junction of the perivisceral coelom, the water-vascular system, and the hemal system. Although its functions are not yet well understood, the axial organ plays a part in defense against invading organisms, can contract, is responsible for a circulation of fluids, and may have excretory and secretory activity.

Nervous system and sense organs

      The echinoderm nervous system is complex. In all groups, a nerve plexus lies within and below the skin. In addition, the esophagus is surrounded by one to several nerve rings, from which run radial nerves often in parallel with branches of the water-vascular system. Ring and radial nerves coordinate righting activity.

      Although echinoderms have few well-defined sense organs, they are sensitive to touch and to changes in light intensity, temperature, orientation, and the surrounding water. The tube feet, spines, pedicellariae, and skin respond to touch, and light-sensitive organs have been found in echinoids, holothurians, and asteroids.

Reproductive system

      The masses of sex cells that compose the gonads (gonad) of crinoids fill special cavities in the arms or pinnules. Crinoids are the only echinoderms with gonads outside the main body cavity, probably because its volume is reduced. Asteroids typically have 10 gonads, two in each arm, which are located near the arm base, appearing as a feathery tuft or a mass of tubules resembling a bunch of grapes. The gonads of some species are arranged in rows along each arm. Concentricycloids have five pairs of saclike gonads. Ophiuroids have gonads attached to sacs that hang into the body cavity; the sacs, which open outside the body at the bases of the arms, may have one gonad or as many as 1,000.

      Regular echinoids typically have five gonads attached to the interambulacra. A duct from each gonad opens to the exterior near the anus. Most irregular echinoids have four gonads, some have three or five, a few have two; the ducts are on the upper surface of the body. Holothurians differ from all other living echinoderms in having a single gonad, which consists of branching or unbranched tubules; the tubules open into a single duct, which opens to the exterior near the front end of the body. Since many early fossil echinoderms have a single genital opening, or gonopore, it is assumed that these forms also had only one gonad; the condition in holothurians thus is regarded as primitive.

Paleontology and evolution

Extinct echinoderms

      Because the phylum Echinodermata was already well diversified by the Lower Cambrian Period, a considerable amount of Precambrian evolution must have taken place. A Precambrian fossil from Australia has triradiate symmetry and a superficial resemblance to an edrioasteroid; it has been suggested that the triradiate condition may have been a precursor of pentamerous symmetry, and that this fossil is a “pre-echinoderm.” Scientists speculate that the lack of Precambrian fossil echinoderms indicates that while the earliest echinoderms may have possessed a water-vascular system, they lacked a calcite skeleton and thus did not fossilize. While the fossil record of echinoderms is extensive, there are many gaps, and many questions remain concerning the early evolution of the group. Ancient echinoderms exhibited an extraordinary variety of bizarre body forms; the earliest classes seemed to be “experimenting” with body shapes and feeding mechanisms; most were relatively short-lived. Early echinoderms were adapted to life on the surface of hard or soft seafloors, though the burrowing habit may have been acquired relatively early by sea cucumbers.

Extant echinoderms

      Relationships among the living classes of echinoderms have been the subject of debate for many decades. Some scientists believe that larval stages reflect the interrelationships of the groups; thus, because sea urchins and brittle stars have pluteus larvae, they form a natural group, and starfishes and sea cucumbers form another for the same reason. Some biochemical studies support this scheme. On the other hand, comparative anatomy and some paleontology studies suggest that brittle stars and starfishes may have originated from a crinoidlike ancestor and should be placed together, and their general star shape would support this. Modern sea cucumbers and sea urchins share a globoid body but little else; however, some fossil sea urchins with overlapping skeletal plates share several features with some sea cucumbers.

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.

Phylum Echinodermata (echinoderms)

 Marine invertebrates worldwide in distribution; skeleton composed of calcium carbonate in the form of calcite; most fossils and all living representatives with 5-part body symmetry (pentamerous); part of body cavity (coelom) comprises a water-vascular system. Cambrian to Recent. About 6,000 extant species, about 13,000 extinct species described.

      †Subphylum Homalozoa (carpoids (carpoid))

 Middle Cambrian to Middle Devonian about 365,000,000–570,000,000 years ago; without 5-part symmetry; with fundamentally asymmetrical flattened body.

      †Class Stylophora

 Middle Cambrian to Upper Ordovician about 460,000,000–540,000,000 years ago; with unique single feeding arm sometimes interpreted as a stem.

      †Class Homostelea

 Middle Cambrian about 540,000,000 years ago; no feeding arm, but with stem of essentially 2 series of plates.

      †Class Homoiostelea

 Upper Cambrian to Lower Devonian about 400,000,000–510,000,000 years ago; with a feeding arm and a complex stem composed in part of more than 2 series of plates.

      †Class Ctenocystoidea

 Middle Cambrian about 540,000,000 years ago; no feeding arm and no stem, but with unique feeding apparatus consisting of a grill-like array of movable plates around mouth.

      †Subphylum Blastozoa (blastozoans)

 Cambrian to Permian about 280,000,000–540,000,000 years ago. Stalked echinoderms with soft parts enclosed in a globular theca (chamber) equipped with simple, erect food-gathering appendages (brachioles).

      †Class Eocrinoidea

 Lower Cambrian to Silurian about 430,000,000–570,000,000 years ago; body usually consisting of stem, theca, and feeding brachioles.

      †Class Blastoidea (blastoid)

 Silurian to Permian about 280,000,000–430,000,000 years ago; stem, theca with 18–21 plates arranged in 4 rings; numerous feeding brachioles; distinctive infoldings of theca (hydrospires) well developed.

      †Class Paracrinoidea

 Middle Ordovician about 460,000,000 years ago; with stem, theca, and arms with barblike structures (pinnules); plates of theca with pore system of unique type.

      †Class Parablastoidea

 Lower to Middle Ordovician about 460,000,000–500,000,000 years ago; resemble Blastoidea but differ in structure of ambulacra and in numbers of thecal plates.

      †Class Rhombifera

 Lower Ordovician to Upper Devonian about 350,000,000–500,000,000 years ago; theca globular; respiratory structures rhomboid sets of folds or canals.

      †Class Diploporita

 Lower Ordovician to Lower Devonian about 400,000,000–500,000,000 years ago; theca globular; respiratory structures pairs of pores.

      Subphylum Crinozoa

 Both fossil and living forms (Lower Ordovician about 500,000,000 years ago to Recent); with 5-part symmetry; soft parts enclosed in theca, which gives rise to 5 or more complex feeding arms.

      Class Crinoidea (crinoid) (sea lilies and feather stars)

 Lower Ordovician about 500,000,000 years ago to Recent; with, or secondarily without, a stem; theca reduced to small, cup-carrying hollow, usually branching, feeding arms with numerous small pinnules; includes fossil subclasses Camerata, Inadunata, and Flexibilia; living subclass Articulata, which includes stalked sea lilies and unstalked feather stars; about 700 living species.

      Subphylum Asterozoa

 Fossil and living forms (Lower Ordovician about 500,000,000 years ago to Recent); radially symmetrical with more or less star-shaped body resulting from growth of arms in 1 plane along 5 divergent axes; central mouth; 5 arms; dorsal tube feet and mouth.

      Class Stelleroidea

 Features as subphylum above.

      †Class Somasteroidea

 Lower Ordovician to Upper Devonian about 350,000,000 years ago. Superficially like Asteroidea, without a groove for tube feet.

      Class Asteroidea(starfishes (sea star) or sea stars)

 Fossil and living forms (Middle Ordovician about 460,000,000 years ago to Recent); about 1,800 living species; arms broad, hollow; pinnate structure or arrangement of arms disrupted by dominant longitudinal growth axes; tube feet numerous, carried in grooves on the oral surface of the body; tube feet pointed or equipped with terminal suckers; respiration often by interradial gills on aboral surface of body; includes living orders Platyasterida, Paxillosida, Valvatida, Spinulosida, Forcipulatida, Notomyotida, and Brisingida.

      Class Ophiuroidea(brittle stars (brittle star) or serpent stars)

 Fossil and living forms (Ordovician about 460,000,000 years ago to Recent); disk sharply distinct from long, slender, solid arms; no furrow for tube feet; no suctorial tube feet; no anus; no pedicellariae; respiration by interradial gills on oral surface of body; includes living orders Oegophiurida, Phrynophiurida, and Ophiurida; about 2,000 living species.

      Class Concentricycloidea(sea daisies)

 Body flattened, disk-shaped, without obvious arms; water-vascular system with tube feet on oral surface of body; water-vascular canals form double ring; includes order Peripodida; 2 living species.

      Subphylum Echinozoa

 Fossil and living forms (Lower Cambrian about 570,000,000 years ago to Recent); radially symmetrical with fundamentally globoid body secondarily cylindrical or discoid; outspread arms or brachioles totally absent.

      †Class Cyclocystoidea

 Middle Ordovician to Middle Devonian about 375,000,000–460,000,000 years ago; small, disk-shaped; theca composed of numerous plates; ambulacral system with multiple branching.

      †Class Edrioasteroidea

 Lower Cambrian to Lower Carboniferous about 340,000,000–570,000,000 years ago; discoid to cylindrical; 5 well-developed straight or curved ambulacral food grooves radiate from a central mouth.

      †Class Edrioblastoidea

 Middle Ordovician about 375,000,000 years ago; stalked form with spheroidal theca; 5 well-developed food grooves.

      †Class Helicoplacoidea

 Lower Cambrian about 570,000,000 years ago; pear-shaped or spindle-shaped body with many plates arranged spirally.

      †Class Ophiocistioidea

 Lower Ordovician to Upper Silurian about 395,000,000–500,000,000 years ago; dome-shaped body partly or completely covered by well-developed test; 5 ambulacral tracts carry plated tube feet relatively enormous in size.

      Class Holothuroidea (sea cucumbers (sea cucumber))

 Fossil and living forms (Ordovician about 460,000,000 years ago to Recent); cylindrical body, elongated orally–aborally, with mouth at or near one end, anus at or near the other; mouth surrounded by conspicuous ring of feeding tentacles; no spines or pedicellariae; single interradial gonad; skeleton usually reduced to form microscopic spicules; includes living orders Dendrochirotida, Dactylochirotida, Aspidochirotida, Elasipodida, Molpadiida, and Apodida; 1,100 living species.

      Class Echinoidea (sea urchins, heart urchins, sand dollars)

 Fossil and living forms (Ordovician 460,000,000 years ago to Recent); globular, discoid, or oval in shape, with complete skeleton (test) of interlocking plates bearing movable spines and pedicellariae; mouth directed downward; anus present; 5 or fewer interradial gonads. Includes subclass Perischoechinoidea with living order Cidaroida, and subclass Euechinoidea with living superorders Diadematacea and Echinacea (comprising the “regular” echinoids), and Gnathostomata and Atelostomata (comprising the “irregular” echinoids); 900 living species.

Critical appraisal

      No classification satisfies everyone, and this is especially true for the echinoderms. Some modern scientists argue that fossils contribute little to our understanding of the interrelationships of living groups because fossil forms are different from recent forms and because many of the forms that link the groups in a classification scheme are missing. They believe instead that the higher classification of the echinoderms should be based upon a study of the embryology and anatomy of living groups and that the fossil groups should be inserted wherever they best seem to fit. In other classification schemes, scientists regard the fossils as a logical starting point—the “roots of the tree.” A classification proposed in the early 1960s based upon growth patterns enjoyed wide acceptance until further research showed numerous flaws in the overall scheme. The current classification is decidedly a “hybrid,” incorporating data from several fields of biologic research.

      It has been strongly argued that some members of the subphylum Homalozoa are true chordates rather than echinoderms. This theory has not received wide acceptance. If it eventually proves to be correct, a drastic reevaluation of the Echinodermata would be required. The phylum would then share with chordates a latticelike calcite skeleton and a water-vascular system.

      New discoveries and new theories are continually reshaping the classification. The subphylum Blastozoa, proposed in the early 1970s, has gained wide acceptance. The extinct classes Helicoplacoidea and Ctenocystoidea were suggested in the 1960s, and their discovery caused a reassessment of the classification of the phylum. Extinct classes Lepidocystoidea and Camptostromatoidea have been eliminated and their members distributed among other echinoderm groups. The extant class Concentricycloidea was described in 1986 and is the first new class of living echinoderms to be named since 1821. Some argue that the concentricycloids are extreme forms of starfish that properly belong in the class Asteroidea. Less systematic importance is attached to the characters that are regarded as of class rank.

Additional Reading

Richard A. Boolootián (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” (1966–78), 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

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