The echinoderms (echino means "spiny;" derm means "skin") are large, conspicuous, entirely marine invertebrates. Today, this group inhabits virtually every conceivable oceanic environment, from sandy beaches and coral reefs to the greatest depths of the sea. They are also common as fossils dating back 500 million years. These less-familiar fossil types are represented by a bizarre variety of animals, some of which reveal their relationship to the living echinoderms only at close inspection.
The species living today are generally regarded as belonging to five subgroups: sea lilies and feather stars (Crinoidea, 650 species); starfish (Asteroidea, 1,500 species), brittlestars and basket stars (Ophiuroidea, 1,800 species), sea cucumbers (Holothuroidea, 1,200 species); and sea urchins and sand dollars (Echinoidea, 1,200 species).
Sea lilies have a central body, or calyx, surrounded by feathery, usually heavily branched arms. This whole arrangement sits at the end of a stem-like stalk attached to the sea bottom. The feather stars lack this stalk. Starfish (also called sea stars) have a central disk that is not marked off from the unbranched arms, of which there are usually five. Occasionally, one will encounter starfish species with more than five arms. Brittlestars also typically have five relatively long, flexible arms, but these are well differentiated from the central disk.
Sea cucumbers are soft-bodied and wormlike, with a cluster of tentacles around the mouth at one end. Sea urchins usually have a rigid body of joined plates upon which is mounted a dense forest of spines. The sea urchin body can be almost spherical, with long spines, or flattened to varying degrees with very short spines in types such as the sand dollars.
Anatomy and Physiology
In general, echinoderms are characterized by several unique features not found in any other animal phylum . They have a limestone (calcium carbonate) skeletal meshwork called "stereom" in their tissues, especially the body wall. The porous structure of stereom makes the skeleton light yet resistant to breakage. Echinoderms possess a special kind of ligament that can be stiffened or loosened at will so that these animals can maintain a posture without expending energy by muscular contraction. Echinoderms have an internal set of plumbing tubes, the "water vascular system" that manipulate flexible external tube feet. Tube feet are the "hands" and "feet" of echinoderms, and are involved in sensory, locomotory, feeding, and respiratory activities.
Males and females are separate, and fertilized eggs develop into a typically free-swimming larva that changes (or "metamorphoses") from a bilaterally symmetric form to an adult possessing a body structure with the five radiating rays that makes adult echinoderms so distinctive. Even the worm-like sea cucumbers and sea lilies show this five-part structure because the feeding tentacles and arms are usually present in multiples of five.
Echinoderms are relatives, although distant ones, of the vertebrates. Like vertebrates, and unlike other animal phyla, echinoderms are "denterostomes," meaning the mouth pore forms after the anal pore during early development. This makes them ideal subjects for studies that shed light on human development and evolution. In addition, the ecological importance of echinoderms, combined with their sensitivity to environmental degradation, gives them a key role to play in environmental research.
see also Animalia; Coral Reef; Development
Hyman, Libbie Henrietta. The Invertebrates, Vol. 4: Echinodermata. New York, McGraw-Hill, 1955.
Lawrence John M. A Functional Biology of Echinoderms. Baltimore, MD: Johns Hopkins, 1987.
Mooi, R., and B. David. "Skeletal Homologies of Echinoderms." Paleontological Society Papers 3: 305–335.
Nichols, David. Echinoderms. London: Hutchinson, 1962.
An echinoderm is a spiny-skinned invertebrate (an animal without a backbone) that lives in the ocean. Most echinoderms, like starfish and sand dollars, have a distinctive five-part body plan, an endoskeleton (an internal skeleton), and many tiny, sucker-tipped tube feet with which they take in seawater in order to move.
As one of the more unusual animals, the echinoderm (meaning "spinyskinned") numbers about 6,000 related species and includes such exotic animals as the five-armed starfish, the pincushion-like sea urchin, and the bottle-shaped sea squirt. With their armor-like external spines that protect them, it might seem that an echinoderm has an exoskeleton (like a lobster or an insect). In fact, the spines are only extensions of an interior or endoskeleton that makes echinoderms unique among invertebrates. This internal skeleton is made of plates under the skin that have spiny projections. Because of this internal skeleton, echinoderms are considered closer to vertebrates that any other invertebrate phylum. However, unlike vertebrates, echinoderms have no head or centralized nervous system. Nor do they have an excretory or respiratory system. Finally, echinoderms have a circulatory system that is unique in the animal kingdom since they use seawater as their circulatory fluid. Echinoderms use their specialized tube feet to suck in water and create a suction effect that allows them to move about and to feed on other animals.
The starfish is an excellent example of features that make an echinoderm such a different animal. Also called sea stars, this invertebrate has a radial symmetry, meaning that its body parts are arranged around a central area or hub. Animals with radial symmetry can be divided into mirror-image halves along many lengthwise lines. A human body that has bilateral symmetry only can be divided perfectly down the middle and, therefore, have only two mirror-image halves. However, a starfish with five arms can be divided evenly several different ways. A starfish has a skeleton made up of hard plates beneath its skin. It also has many canals inside its body that help it operate a water pumping system, allowing it to move about and find the food it needs. These canals are connected to tube feet that are located on its underside and are hollow, suction-cup-like structures that suck in water. This forces the tube feet outward. A starfish can walk on the tips of its arms, using the sucker action of the tube feet to cling to a surface. When the suction is released, it moves forward. The starfish also uses its tube feet to wrap its arms around a clam shell and pry its hinged shells apart. A starfish's mouth is located in the center of its underside. It eats the soft-bodied mollusk by pushing part of its own stomach through its mouth and engulfing its prey, thereby digesting the clam inside the shell.
Finally, the starfish and all other echinoderms reproduce sexually by the union of male sperm and female egg. They also have remarkable powers of regeneration. If an echinoderm's arm breaks off, it will grow back, and if a part of its central core breaks off, it will grow into a complete organism. Many clam and oyster fishermen viewed the starfish as a rival, since clams and oysters are the starfish's main diet, and would tear these creatures apart whenever they found them. Unfortunately the fishermen were only multiplying their problem.