Greek echinos, sea urchin; derma, skin
Echinoderms are characterized by the presence of tube feet, bulbed structures belonging to a unique water vascular system that develops from part of the larval coelom. The tube feet serve for locomotion, food handling, and respiration. Water is forced through the vascular system by muscle action.
There are about 6000 living species of echinoderms; they are all marine organisms. Most are intertidal or subtidal, and a few dwell in deep-ocean trenches. Thousands of fossil species are known. Familiar members of the group include starfish, sea urchins, sand dollars, sea lilies, and sea cucumbers. Human beings eat some echinoderms; for example, sea urchins (gonads only) and sea cucumbers (also known as bêche-de-mer and trepang).
Adult echinoderms lack heads, brains, and segmentation; most of them are radially symmetrical. The body generally has five symmetrically radiating parts, or arms, reflecting the internal organization of the animal. The body surface is covered by a delicate epidermis, which is stretched over a firm endoskeleton made of movable or fixed calcareous plates. In most cases, the plates are arranged in a genetically determined pattern and bear spines. The plates over certain areas of the arms are perforated; from these areas, called ambulacra, the tube feet project.
Echinoderms feed by suspension feeding, predation, grazing, scavenging, swallowing substrate, or ingesting substrate selectively; simple organic nutrients are absorbed through the epidermis from the sea. The digestive system of most echinoderms is simple and complete: the tract begins with a mouth and ends with an anus, which opens to the exterior. In some species, the anus is lacking.
The circulatory system of echinoderms also radiates in five directions; it circulates a colorless blood that differs little from coelomic fluid. In fact, it is so rudimentary in many species that the coelom itself performs the circulatory and respiratory functions. The coelom is lined by a ciliated membranous tissue called the peritoneum, and the coelomic fluid contains free amoebocytes that engulf foreign particles. In some groups, respiration is by minute gills or papillae that protrude from the coelom into the sea. Some species respire by using their tube feet to exchange fluid with the exterior. The sea cucumbers respire by means of special cloacal structures called respiratory trees. The nervous system consists of nerves in a ring around the mouth and also extending radially out into the body.
Most echinoderms are able to regenerate lost parts easily. Reproduction is asexual by fission in some sea stars, sea cucumbers, and brittle stars. The sexes are usually separate, although parents of the opposite sex often look very much alike. Fertilization is external; most echinoderms are oviparous. The fertilized eggs develop into bilaterally symmetrical,-ciliated larvae, which may pass through several distinct stages before they metamorphose into adults. Development is direct in some species.
There are many classes of extinct echinoderms, but only four living classes, which are organized into two subphyla. The first is the subphylum Pelmatozoa, which contains only the class Crinoidea, the sea lilies. The crinoid mouth and anus are both on the upper surface of the body, which is cup shaped over a cup-shaped skeleton. Most are attached to the substrate by a stalk on the aboral surface, that is, the surface away from the mouth. The earliest known crinoids were preserved about 530 million years ago in the Burgess Shale. All the other living echinoderm classes belong to the subphylum Eleutherozoa: the sausage-like Holothuroidea (sea cucumbers), the Echinoidea (sea urchins and sand dollars), the Stelleroidea (starfish or sea stars, brittle stars, and basket stars). These echinoderms lack a stalk, and the mouth and anus are on their lower surface, facing the rock or sediment.
Like the chordates, echinoderms are deuterostomes: the blastopore or opening of the blastula develops into the anus rather than the mouth. The cleavage of the egg, the pattern of blastulation and gastrulation in egg development, the formation of the three germ layers (ectoderm, mesoderm and endoderm), and the existence of a true coelom suggest that echinoderms and chordates have common ancestors.