Echinodermata

Echinodermata (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 urchins, loosely articulated to facilitate movement, as in sea stars (starfish), or reduced to minute spicules in the skin, as in sea cucumbers. 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 brittlestars, 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 urchins, heart urchins, and sand dollars—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 lilies and feather stars 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.