286 



CHAPTER XV. 



CYSTOIDEA, BLASTOIDEA, AND HOLOTHUROIDEA. 



ORDER V. CYSTOIDEA. 



THE Cystoidea are Echinodermata in which the lody was gen- 

 erally spheroidal, pedunculate or sessile, enclosed ly calcareous 

 articulated plates, some of which are porous and are connected 

 with respiration. Arms rudimentary, mostly reduced to the pin- 

 nulce only. Reproductive organs contained within the interior 

 of the calyx. 



Fig. 171. Hemicosmites pyriformis, one of the Cystideans. The right-hand figure 

 shows the upper surface of the calyx. Lower Silurian. 



In general form the Cystideans are globular, oval, pear- 

 shaped, conical, or sub-cylindrical, and they resemble the 

 Crinoids in consisting of a stem or " column " and a body or 

 " calyx." The column is composed of a succession of cal- 

 careous joints, and in no respect differs from the column of 

 the Crinoids. In Lepadocrinus (fig. 172, D), however, it is 

 doubtful if the column was affixed to any foreign body, for its 



