Blastoidea, an extinct class of Echinodermata belonging to the group in which the body (calyx) is usually supported on a stem; in many of the Blastoids, however, this structure is absent. The calyx is small and ovoid or globular, and formed of a series of plates of which the most important are arranged in three zones: the lowest consists of three "basal" plates, above which is a circle of five radials, and partly between but mainly above these is a circle of five "interradial" plates. The radial plates are forked, and in the angle of each is the ambulacral field; at the sides of these are rows of pores which open below to a series of chambers known as the "hydrospires," which may be respiratory, reproductive, or both. The mouth occurs in the centre of the upper part of the calyx, and is surrounded by a circle of apertures, known as the spiracles, which lead to the hydrospires. The anus also opens in this circle. The group lived in the Silurian, Devonian, and Carboniferous periods, and in the last it obtained its maximum development and became extinct. The typical genus Pentremites is not found in England, the forms referred to it belonging really to the genus Granatocrinus.