The phylum Echinodermata (“spiny-skins”) consists of about 6,000 species of exclusively marine organisms, which include such well-known animals as starfish and sea urchins. The description “spiny-skins” is derived from the limy plates that form the endoskeleton of these animals, which in some species is covered with spines. These spines are particularly prominent in sea urchins.
The most obvious feature of these animals, however, is their five-rayed radial symmetry. Zoologists believe that this characteristic, far from primitive, developed at a later stage in echinoderm evolution because fossils of extinct species show that some were bilaterally symmetrical. The fact that echinoderm larvae are bilaterally symmetrical is further evidence of this evolutionary change.
The relationship between the echinoderms and other invertebrate phyla is somewhat obscure. It was initially thought that echinoderms and hemichordates (primitive chordates) were closely related because of the similarity between the larvae of the two groups. It is now believed, however, that the resemblance is due not to genetically similar characteristics but to the adaptation of different organs to perform the same function, a process that is known as convergent evolution.
The 1,600 species of starfish, more properly called sea stars (subclass Asteroidea), are the most familiar echinoderms. They consist of a central disk, from which five arms arise, though some species have more than five arms. These animals move using tube feet called podia, which are found in a groove under each arm. Each podium ends in a suction disk, which allows the sea star to stick to rocky surfaces. When moving over sand, however, the sea star uses its tube feet as stiff legs.
Sea stars also use their tube feet to capture and hold prey, which usually consists of clams and oysters. The tube feet exert enough pressure to open them slightly so that the sea star can push its stomach, which it can turn inside out, into the gap—the stomach can squeeze through a space as small as .004 inch (0.1 millimeters). The stomach then secretes enzymes that slowly digest and absorb the victim. The sea star intestine has five branches, one in each arm. indigestible fragments are usually ejected via the mouth.
Within each arm there is also a pair of sex organs. Reproduction in most echinoderms is a simple process. The sexes are separate, and sperm and eggs are released into the water, where fertilization occurs. The larvae that hatch are called bipinnariae and metamorphose gradually into the adult form. A few species copulate, and the eggs develop in a brood chamber without going through a larval stage. Sea stars are also able to reproduce by fragmentation— or regeneration, as this process is sometimes called—but this happens only occasionally.
In large numbers, sea stars can have a devastating ecological effect. The population of the crown-of-thorns sea star, for example, has increased in the Pacific Ocean in recent years, and, because it feeds on the live corals, has destroyed whole areas of coral reef.
The 2,000 species of brittle stars (subclass Ophiuroidea), which occur at ail depths of the ocean, are easily distinguished from sea stars by their round central disk and their very long arms. They also differ from sea stars in several other ways. Brittle stars move not by means of tube feet but by muscular movements of the arms. Most brittle stars feed on small living or dead organisms, but do not have an intestine or anus, and indigestible fragments are ejected through the mouth. Sea stars and sea urchins breathe by means of skin gills, which are located all over the body surface, whereas brittle stars use respiratory pouches, called bursae, which occur near the arm bases.
Sea urchins and sand dollars
Sea urchins belong to the class Echinoidea, which has about 1,000 members. Other members of this class include heart urchins and sand dollars. Most sea urchins have a spherical body, with the skeletal plates fused together to form a hard shell called the test. In most species, the body is further protected by sharp spines, which may be poisonous. Some of the spines are movable, attached to the test in sockets, and are used for walking. The spines of some species of urchin are also used for boring holes in coral or rock, into which the animals wedge themselves to prevent removal. In addition to spines, urchins and sea stars have tiny pincerlike structures on stalks, called pedicellariae, on the skin surface. They use the pedicellariae for defense, for catching small prey, and for cleaning their body surface.
The tube feet of echinoids protrude through holes in the skeletal plates in 10 rows. In heart urchins and sand dollars, the podia are modified for respiration, and they move by means of the spines only. These urchins also burrow in sand. As a result, the heart urchin has reduced spines to assist burrowing, giving it a furry appearance, and the sand dollar’s body is covered with tiny movable spines used for crawling and digging.
Sea urchins are omnivorous, often scavenging on organic debris. The mouth is on the underside, and the anus is on the upper side.
As their name suggests, sea cucumbers (class Holothuroidea) are cucumber-shaped echinoderms. They have soft bodies covered with a glandular skin, and their skeletal plates have been reduced to microscopic bony structures.
Sea cucumbers burrow in sand. To breathe, they have tubes known as “respiratory trees,” which carry seawater into the body from the anus, so that gaseous exchange can occur internally. They aIso use their podia and skin surface for breathing.
Sea lilies and feather stars
Sea lilies and feather stars (class Crinoidea) are among the most primitive of echinoderms. Paleontologists have identified crinoid fossils dating back 600 million years. The 550 species of crinoids basically resemble an inverted sea star.
In the case of the sea lily, this starlike structure is attached to the seabed by a stalk. Most fossil echinoderms are of this form. The sea lily skeleton, which stands on the stalk, is cupshaped with arms. Tube feet are present in a groove on the upper surface of the arms, but they are used solely for respiration. The arms collect food, usually plankton, and cilia direct it to the mouth on the upper surface. Sea lilies are deepwater creatures.
Feather stars resemble sea lilies, differing, however, in that they are free-living as adults, either creeping along the seabed or swimming.