580 ECHINODERMATA PELMATOZOA chap- 



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

 divisions as follows : — 



Class I. Crinoidea. — 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 

 witli 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) = Edkioastekoidea (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. Carpoidea (Jaekel). — Pelmatozoa witli a weD- 

 developed stalk. The radial canals and their branches are 

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

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

 horn -like outgrowths of some of its plates. 



Class lY. 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 unl)ranched arms (" fingers "). Special respiratory organs 

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

 " hydrospires." 



CLASS I. CEINOIDEA 



This is the only class wliich has living representatives. There 

 are twelve recent genera, of which eiglit 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 



