CYSTOI'DEA (Neo-Lat. norm pl., from Gk. sOcrra, kystis, bladder + claos, eidos, form). A class of extinct echinoderms of the subphylum Pelmatozoa (q.v.), allied to the crinoids and blastoids, but differing from these chiefly in the irregular arrangement of the plates of the calyx and the imperfect development of their arms. in general appearance the, Cystoidea resemble the crinoids, with the remains of which they are often found associated in the •Ordovician and Silurian strata. The cystoid body was inclosed in a case or 'calyx' of variable form, spherical, cylindrical, hemispherical, or discoid, which is made up of polygonal calcareous plates without regular arrangement. The calyx-plates seem to have been, in many early genera, loosely united to each other, so that they became easily dis sociated after death of the animal. This ex plains the rarity of perfect individuals in the Ordovician rocks, where the fragments are often exceedingly abundant. Another characteristic is the presence of pores that perforate the plates and that are arranged in rhombic series. These pores are supposed to have been connected with the respiratory apparatus. The number of plates in the calyx is very variable, from 10 or 12 to over 100, and as a rule those forms with the largest number of plates show the greatest ir regularity in their arrangement. In some of the forms, with less number of plates, these are arranged in regular transverse rows, and the calyx then approaches more nearly the aspect of the simpler forms of erinoids.
The arms are absent in many genera of cys toids. and when present are seldom found at tached to the calyx. They are never pinnulated like those of the erinoids, though they are com posed in a similar manner, and are often sup plied with grooves. In those cystoids without free arms there is generally found on the ven tral surface of the calyx a system of ambulacral furrows that radiate irregularly from the mouth.
opening. These furrows, representing the arms, are bordered by rows of small plates that often bear pinnules ot delicate construction, as seen in Callocystites, Glyptospluerites. and Agela erinus. The more primitive cystoids have neither arms nor ambulacral furrows. The openings of the calyx are four in number. The mouth is
central or subeentral on the upper or ventral surface; the anal opening is eccentric. and is generally closed by a pyramid of small, triangu lar plates; a third opening, often present near the anal opening and generally closed by triangu lar plates, is considered to be the genital orifice; and a fourth small. slit-like aperture. present in only a few genera, is of problematic nature. The calyx of the cystoid is usually elevated on a stem which often resembles that of the crinoid in being composed of a single series of plates pierced by a central canal. In some genera (as Dendroeystites) the stem is made up of plates arranged in transverse rows, and the central cavity is then much enlarged and continuous with the general cavity of the calyx. In Echino spluerites the stem is reduced to a tubercle on the dorsal surface, and the animal seems to have been a free living form. The discoid genera Agelacrinus and its allies are sessile, and are attached either by a pedestal or by cementation of the d'orsal surface of the calyx to foreign ob jects, generally the shells of mollusks.
Classification of the Cystoidea is a matter of difficulty, not alone because of the general im perfection of the material, but also on account of the great diversity of structure seen within the class, which contains a number of synthetic or ancestral types that seem to have given rise to all the other more specialized groups of the Eehinodermata. Through assumption of a more regular arrangement of the plates and the de velopment of the arms, with consequent rear rangement of the ventral surface. as in Crypto crinus, Poroerinus, and Caryocrinus, they gave rise to the Crinoidea. Reduction of the plates and enlargement of the amhulaeral grooves, with the assumption of the pentameral gemmiform shape, as in Asteroblastes, leads to the Blastoi dea. Agelaerinus is suggestive of the star-fish (Asteroidea) and brittle stars (Ophinroidea), and finally the Echinoidea and Holothuroidea may be imagined to have been derived from the more spherical forms of armless cystoids. the echinoids presenting a series of progressive evo lution, the holothurians a regressive series.