Class of free-living echinoderms in which the body is enclosed in a globular, cushion-shaped, discoidal, or heart-shaped test built of meridionally arranged columns of interlocking, calcareous plates, which bear movable appendages (spines, pedicellariae, and spheridia). The test is composed of 20 vertical rows of plates arranged in five double rows of perforate (ambulacral) plates and five double rows of imperforate (interambulacral) plates. Tube feet, connected to the internal water-based vascular system, emerge through the pores of the ambulacra. The apical system on the upper surface consists of five ocular plates and up to five genital plates. In all regular echinoids the anus is enclosed within the apical system, but in many irregular echinoids it is in the posterior interambulacrum. The mouth is always on the lower surface and may be central or anterior in position. The class first appeared in the Ordovician; underwent a great adaptive radiation in post-Palaeozoic times, when rigid tests were evolved; experienced a marked reduction in the Permian and Triassic; and thereafter resumed its diversification, which has continued until the present day. Fossils of the inner skeletons of these echinoderms are common in both Mesozoic and Cenozoic sediments. There are about 125 Palaeozoic, 3 670 Mesozoic, 3 250 Cenozoic, and more than 900 extant species.