Echinoidea, a class of Echinodermata (q.v.), including the Sea-urchins. The characters of the class are that the skeleton consists of 20 zones of plates (the number varies in some extinct genera), generally solidly united to form a spherical-, heart-, or disc-shaped body. Each plate bears one or more spines. Ten of the zones are usually smaller than the others, and are perforated for the passage of the "tube feet," and for the ambulacral areas. The mouth opens on the ventral or actinal side of the body, and is typically central in position. The anus opens either at the dorsal or abactinal pole of the body or in some position in one of the interambulacral areas, which is thereby marked out as the posterior. Echinoids are always free, and never have a stalk or any indication of one. The main character of the Sea-urchins is the skeleton, or test. This is composed of five sets of plates, arranged in vertical series. In all living forms there are ten pairs of vertical series, five being ambulacral and five being of larger plates separating these. In many extinct genera the number, however, is irregular. They usually occur in ten alternating series of plates, but each series may consist of more than one pair, as in Melonites; the interradial series may be more than a pair in number, as in Archaeocidaris, or they may each consist of only a single series, as in Bothriocidaris. The plates of the interradial series remain distinct, but those of the ambulacra may be united into compound plates, the nature of which differs in the different groups. In addition to the vertical series of plates there is usually a group of plates arranged as one ring of ten plates, or as two rings of five plates, at the upper pole of the body. In the regular Echinoids the anus opens in the middle of this group, but in the higher forms the anus has worked back till it opens in the posterior interradius, far from this "apical system" (q.v.), or even on the under-surface. In some of the most specialised Echinoids the apical system has been broken up (as in Collyrites), and in others it cannot be traced (Pourtalesia). The most important of the remaining skeletal structures are the jaws, and the "perignathic girdle" of processes around the mouth, which support these. The jaws are not found in the ordler Spatangoidea. The structure of the jaws and details of the anatomy of a typical Echinoid are given under Sea-urchin (q.v.). The plates of the skeleton are usually ornamented by a series of small prominences or tubercles, which bear the spines. These serve for protection and locomotion, the animal often walking on the tips, as on stilts. The largest spines are called "primaries;" but there are series of much smaller ones, borne on small tubercles or granules, known as secondary tubercles or miliaries respectively. In many of the Spatangoids, or Heart-Urchins, there are narrow bands of crowded minute granules, forming the fascioles. These bear small spinelets, and some of the modified spines known as pedicellariae. These structures consist of a short flexible stem, bearing a head formed of two or three jaw-like valves. The use of these pedicellariae is not quite decided, but they appear to act to some extent as organs of defence; while they have also been seen to pass the excreta from the central anus to the margin of the body. The Echinoidea are at present grouped into two main groups - the Palaeechinoidea and the Euechinoidea; the former, however, is a motley collection of extinct forms that must be eventually dispersed. It includes most of the Palaeozoic representatives of the class. The most remarkable form included in it is the abnormal Tiarechinus that lived in the lagoons among the Triassic coral reefs of the Tyrol. The Euechinoidea are divided into five orders - Cidaroida, Diadematoida, Holectypoida, Clypeastroida, and Spatangoida. The oldest known Echinoidea comes from the Ordovician rocks; the fossil forms are most abundant in the seas of the Jurassic, Cretaceous, and Cainozoic. They are of much value to the geologist, as they give so much assistance in problems of geographical distribution, as their range in depth is generally marked by definite characters. All the living Echinoids are marine; they occur in most seas and at all depths, but abound most in the shallower parts of the tropical and subtropical regions.