Body generally spheroidal, pedunculate or sessile, enclosed by calcareous articulated plates, some of which are usually porous and are connected with respiration, and perhaps with reproduction also. Arms rudimentary, mostly reduced to the pinnulae only. Reproductive organs contained within the interior of the calyx.
The members of this order are all extinct, and are entirely confined to the Palaeozoic period. The body (fig. 107) was, typically, more or less spherical, and was protected by an external skeleton, composed of numerous polygonal calcareous plates accurately fitted together, and enclosing all the viscera of the animal. The body was in most cases permanently attached to the sea-bottom by means of a jointed calcareous "column," or pedicle, but this was much shorter than in the majority of Crinoids, and was rarely altogether absent. Upon the upper surface of the body were two, sometimes three, apertures, the functions of which have been a matter of considerable controversy. One of these is lateral in position, is defended by a series of small valvular plates, and is believed by some to be the mouth, whilst by others it is asserted to have been an ovarian aperture. The most probable view, however, is that this valvular opening is really the anus. The second opening is central in position, and it is believed by Mr Billings to be the "ambulacral orifice," as it is always in the centre of the arms when these are present. The third aperture is only occasionally present, and its true functions are doubtful.
Fig. 107. - Hemicosmites pyriformis, one of the Cystideans. The right-hand figure shows the upper surface of the calyx.
In some Cystoidea there were no arms, properly speaking, but only small pinnulae. In a second section two arms were present, but these were bent backwards, and were immovably soldered down to the body. In one single species (Comaro-cystltes punctatus, Billings), the development has gone further, the arms being free, and provided with lateral pinnulae, as in the true Crinoids.
Many Cystideans are likewise provided with a system of pores or fissures, penetrating the plates of the body, and usually arranged in definite groups. These groups are termed "pectinated rhombs," but their exact function is doubtful. By Mr Billings, however, they are believed, and apparently with good reason, to have admitted water to the body-cavity, and to have thereby subserved a respiratory function; though the recent researches of Ludwig on the genital glands of the Ophiuroids would render it not improbable that they were also connected with the function of reproduction.