CRINOIDEA. 419 



collectively to the " suranal " or dorsocentral plate of the SafeuiadcE, 

 which is represented in the larvae of other Urchins ; but it is now 

 generally admitted that the homologue of this plate in the Crinoids is 

 the plate supporting the discoidal base of the larval stem (fig. 288, d). 

 Succeeding the basals, and alternating with them, are one or more 

 cycles of plates, which are directly superimposed upon one another in 

 longitudinal rows, and which form the foundations of the arms. The 

 lowest of these, up to the first bifurcation, are known as the " radials " 

 (figs. 291, 292, r), and are termed "primary," "secondary," or "ter- 

 tiary " radials, according to their distance from the basals. The last 



Fig. 292.— Diagram of the dissected calyx of Rhodocrinus, a "dicyclic" Crinoid, viewed from 

 below (after Schultae). b, Underbasals ; p, Basals ; r, First radials ; z, Interradials ; a, Anal 

 plates. 



radial plates, or those furthest from the column, are the " axillary " 

 radials, and give origin to the lowest plates of the arms (" brachial " 

 plates) ; or, if the cycle of the primary radials alone is developed, 

 the first brachials rest upon these. The "radial" plates are 

 arranged in a series of five vertical columns, which are, as the name 

 implies, radial in position ; and the primary radials are homolo- 

 gous with the " ocular plates " in the apical disc of the Echinoids. 1 



1 From the strict morphological point of view, these primary radials are the 

 only plates to which the name "radials" is applicable, all those which follow 

 them being really arm-joints ("brachials"). It is, however, convenient for 

 descriptive purposes to give the name of "radials" to all those plates which are 



