580 ECHINODERMATA PELMATOZOA chap. 



of the classification, we may give the diagnoses of the principal 

 divisions as follows : — 



Class I. Ceinoidea. — Pelmatozoa provided typically with a 

 well-marked stem ; calyx consisting of an aboral " patina " of two 

 or three circles of plates, and a flexible " tegmen " or oral surface 

 with small plates or none ; radial canals supported by long 

 branched arms, which are developed as direct prolongations of 

 the uppermost circle of plates in the patina. 



Class II. Thecoidea (Jaekel) = Edbioastekoidea (Bather). 

 — Pelmatozoa without a stalk, fixed to the substratum by the 

 whole aboral surface. The radial canals run out over the oral 

 surface in grooves, which are closed by specially modified plates ; 

 but there are no arms of any kind. 



Class III. Caepoidea (Jaekel). — Pelmatozoa with a well- 

 developed stalk. The radial canals and their branches are 

 devoid of a skeleton, and either produce no modifications at all 

 on the skeleton of the calyx, or at most are supported by short 

 horn-like outgrowths of some of its plates. 



Class IV. Cystoidea. — Pelmatozoa which typically possess a 

 well-developed stalk, a sac-like calyx contracted at the mouth 

 and covered with plates, some of which are pierced with pores or 

 slits ; the radial canals, though they may for part of their course 

 run over the surface in grooves, have their terminal portions 

 supported by free unbranched arms (" fingers "). 



Class V. Blastoidea. — Pelmatozoa provided with a well- 

 developed stalk and ovoid bud-like calyx. Prom the mouth the 

 radial canals run backwards over the calyx, as in Echinoidea, but 

 they give rise to numerous lateral branches, which are supported 

 by free unbranched arms (" fingers "). Special respiratory organs 

 occur on the interradial areas in the form of parallel folds called 

 " hydrospires." 



CLASS I. CEmOIDEA 



This is the only class which has living representatives. There 

 are twelve recent genera, of which eight retain the stalk through- 

 out life ; the remaining four lose it when adult, retaining only 

 a stump, termed the "centro-dorsal," covered with fixing organs 

 ("cirri"). The stalked forms are confined to considerable depths, 

 and can only be obtained by deep dredging, whereas many of the 



