Body enclosed in an armour of closely fitting calcareous plates, attached to some foreign body by a slender stem. From the summit of the calyx radiate five transversely striated and longitudinally grooved areas, which carry a row of jointed pinnulae on each side.
The members of this order, like those of the preceding, are all extinct, and are entirely confined to the Palaeozoic period. The body (fig. 108, a) was fixed to the bottom of the sea by means of a short, jointed pedicle; it was globular or oval in shape, and composed of solid polygonal calcareous plates, firmly united together, and arranged in five inter-ambulacral and as many ambulacral areas. (These ambulacral areas are termed by M'Coy "pseud-ambulacra," upon the belief that they were not pierced for tube-feet, but that they carried a double row of little jointed tentacles or arms.) The pseud-ambulacra are petaloid in shape, having a deep furrow down the centre, and striated transversely. They converge to the summit of the calyx (fig. 108, b), and each appears to have carried a row of small jointed "pinnulae" upon each side. The five pseud-ambulacra, radiating from the summit of the calyx, give the upper surface of the body somewhat the appearance of a flower-bud; hence the name applied to the order (Gr. blastos, a bud; eidos, form). Upon the whole, it would seem most probable that the pseud-ambulacra of the Pentre-mites represent the arms of the Crinoids, anchylosed with the calyx, and that the longitudinal furrows of the pseud-ambulacra represent the "brachial grooves" of the Crinoids; but they are peculiar in the fact they are perforated by the apertures of a number of respiratory tubes.
Fig. 108. - Morphology of Blastoidea. a Pentremites pyriformis, viewed sideways, showing a portion of the column; b Summit of the calyx of Pentremites cervinns, showing the pseud-ambulacral areas and the apical apertures ; c Side view of Grana-tocrinus melonoides; d Summit of Granatocrinus neglectus. (Figs, a and b are of the natural size; c and d are slightly enlarged.) After Hall, and Meek and Worthen.
At the summit of the calyx are six apertures, of which one is the mouth, four are ovarian, and the sixth is probably partly ovarian and partly anal.
'The Blastoidea are known more familiarly under the name of Pentremites, and they occur most commonly in the Carboniferous rocks.