Echinoderms have a distinctive radial pattern, which may take the form of five arms or radii, radiating from a central disc or a more globular shape, in which five rays are detectable. However, some species have more than five arms and in others a bilateral shape has been superimposed on the basic radial plan.
Echinoderms possess an internal skeleton. This skeleton is composed of calcareous plates, the ossicles. The skeletal plates can articulate with one another by a flexible integument allowing some movement, or are fused together to form a rigid shell or test. The skeleton might bear a number of different structures: rigid spines, gills or papulae, pedicellariae, sphaeridia and tube-feet.
The spines are usually articulated to special knobs (tubercles) and provided with muscles, being movable in all directions. They are found in sea-urchins, sea-stars, and brittle-stars.
The pedicellariae are peculiar organs, often of very complicate structure, sometimes provided with poison glands. Their function is partly to clean the skin from dirt and all kinds of foreign bodies, partly to catch and hold preys, and partly to serve as defence organs. They are found only in sea-urchins and sea-stars.
The sphaeridia, which are supposed to be organs of taste and smell, or orientation, are found only in sea-urchins.
The tube-feet are cylindrical, highly extensible tubes, generally ending in a sucking disc. Primarily they occur in two series in each radius, but they also occur in four series, arches or without definite order. In sea-stars and sea-lilies they are placed in special furrows (ambulacral furrows or grooves). The tube-feet mainly serve as locomotion organs. They are connected with the water vascular system (wvs in following pictures). This system consists of a ring canal, encircling the oesophagus, and five radial canals issuing from it, one for each radius: in many-rayed forms there is a corresponding number of radial canals. From the radial canals pairs of small side branches issue, each ending in a tube foot. At the base of each tube foot there is generally a small vesicle, the ampulla. When this ampulla is compressed, the water (which fills the whole water vascular system) is pressed out into the tube foot which is thus extended. When the ampulla is relaxed and the muscles of the tube foot contract, the water flows back from the tube foot which is thus shortened. A peculiar valvular structure in the ampullae regulates the filling and emptying of the tube-feet. On the ring canal, one or more outgrowths are found, the Polian vesicles. The stone canal connects the ring canal to the madreporite. The water vascular system is one of three parts of the tubular coelomic system. The other parts are the haemal system (hs) and the perihaemal system (phs), which usually surrounds the haemal system.
Haemal and perihaemal system
The haemal system is a system of communicating spaces in a spongy tissue and has probably the additional function of producing coelomocytes (amoeboid floating cells). The perihaemal system is probably solely vascular.
These systems, together with the nervous system (ns), show the same arrangement as is found in the water vascular system: a ring around the oesophagus from which a main branch issues to each radius.
Excretory organs are lacking; the excretory function is undertaken by amoeboid wandering cells, which are either stored in special places in the body or wander out through the body wall and are thus destroyed. These cells are formed within a glandular organ that accompanies the stone canal: the axial organ or dorsal organ (ao).
Organs of digestion
The organs of digestion consist in sea-stars and brittle-stars of a large sac-shaped stomach; in the other classes, they consist of a long sinuate intestinal canal attached to the body wall by means of a mesentery. All brittle-stars and some sea-stars lack an anal opening, and accordingly, indigestible matter is discharged through the mouth. In sea-stars a pair of large folded caeca proceeds from the stomach into each arm and forms the liver.