447 



CHAPTER XXVI. 



PELMA TOZOA— continued. 



CYSTOIDEA AND BLASTOIDEA. 



Class II. Cystoidea. 



The Cystideans are an extinct group of Echinoderms in which the 

 body (or " calyx ") is spherical or ovate, and is enclosed in a case com- 

 posed of more or fewer calcareous plates, which are usually united by 

 suture but irregularly arranged, and do not exhibit perfect radial sym- 

 metry. The plates of the calyx are in general pierced by more or less 

 numerous pores, which probably commwiicated ivith internal organs, 

 and which may be regarded as almost certainly connected with the 

 function of respiratio?i. A jointed column may be present or absent. 



Fig. 318. — Hemiscosmites pyriformis, one of the Cystideans. The right-hand figure 

 shows the upper surface of the calyx. 



The tipper surface of the calyx usually shows radiating ambulacral 

 grooves, a central oral aperture, and a lateral anal (?) opening, zvith 

 sometimes a small ovarian (?) ope?iing. Arms are imperfectly developed, 

 or may be wanting. 



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



