A starfish is not a fish. A sand dollar is not a form of money. And a sea cucumber is not a vegetable. All of these misleadingly named animals are echinoderms, or "spiny-skinned" marine animals that live on the ocean floor.
The echinoderms are not closely related to any of the other invertebrates. And in one very obvious way, the creatures in this phylum differ from other animals, vertebrates or invertebrates. Most animals, including human beings, have bilateral symmetry—their bodies can be divided into more-or-less-identical right and left halves. Echinoderms, by contrast, are endowed with radial symmetry. Their bodies are built on a circular, or radial, plan. In the center of the body is the mouth. From that hub, the arms or other structures extend outward at regular intervals, much like the spokes of a wheel.
Biologists find the larval forms of echinoderms particularly fascinating because of their close affinities with the larvae of the protochordates—tunicates and other primitive animals whose ancestors gave rise to backboned animals. Anatomy of Echinoderms
The skin of a typical echinoderm covers an internal skeleton of calcareous ossicles, or small bones, from which the creature derives a more or less rigid structure. Projecting outward from the ossicles are numerous calcareous spines.
Inside the skeleton is the large body cavity, or coelom, in which lie the internal organs. The coelom contains a lymphlike fluid that bathes the organs. Amoeba-like cells creep about in the fluid, removing wastes and carrying nutrients to all parts of the animal's body.
Echinoderms have a complete digestive system leading from the mouth, on the underside of the body, to the anus. Digestive glands pour their secretions into the stomach. A unique arrangement called the water vascular system allows water to enter a sieve plate on the body's surface and then circulate by way of another canal to a ring canal, which branches into radial canals. Each radial canal gives off many pairs of tube feet. When these tube feet are distended with water, they are used for locomotion and serve as respiratory surfaces. Encircling the echinoderm's mouth is a nerve ring. Five branches radiate from this ring, extending tiny nerves to the internal organs, the skin, and the tube feet. There are no well-developed sense organs, although the tube feet may play a role in sensory perception.
Echinoderms have sex glands that shed their products into the water, where fertilization occurs. Fertilized eggs give rise eventually to larvae, which swim freely by means of ciliated bands. The larvae go through many stages of development—some in which they have bilateral symmetry—before they begin to resemble miniature versions of their parents.