CRENOIDEA. 



275 



FIG. 190. Analysis of the calyx of a dicyclic in- 

 aduuate form (from Lang). ' ba basal ; ib infra- 

 basal; C2 second arm plate (primibrach 2); ian 

 anal interradial (special anal). 



In the larva of Antedon and in what we may call the simplest 

 forms i.e. the Larviformia and Marsupites amongst extinct 

 forms, Holopus and the Hyocrinidae amongst recent the calyx is 

 composed entirely of the 

 plates of the apical sys- 

 tem ; whereas in the majo- 

 rity of Crinoids the lower 

 ends of the arms are incor- 

 porated in it and together 

 with some interradial 

 plates assist in forming its 

 walls. The apical system 

 when fully developed con- 

 sists of the following 

 plates (Fig. 190) : five 

 infrabasals (ib) in contact 

 at the apical pole and 

 radial in position, five 

 basals (ba) outside the 

 infrabasals, in contact 



with each other, and interradial in position, five * radials (r) 

 beyond the basals, in contact with each other and radial in 

 position. Of these plates the infrabasals are sometimes absent, 

 in which case the apical system is said to be monoeyclie, as 



opposed to dicyclic when they are 

 present. 



This character the presence or ab- 

 sence of infra-basals seems to be vari- 

 able in most of the orders of Crinoidea. 

 In the Flexibilia the base is said always 

 to be dicyclic, but in the other orders 

 both conditions are found. It is known 

 that in many supposed monoeyclie forms 

 the infra-basals are really present in a 

 reduced condition or have fused with the 

 top joint of the stem (pseudo-monocyclic 

 forms, e.g. Antedon). Wachsmuth and 

 Springer assert that the mono- or dicyclic 

 character of the calyx may be determined 

 by inspection of the stem. In dicyclic 



forms the lobes of the chambered organ are radial and the cirri are 

 attached to the stem radially, and, if the stem be pentagonal, the angles 

 are interradial ; with monoeyclie calices the reverse arrangement holds. 



* Sometimes called primary radials. 



FIG. 191. Analysis of the calyx of 

 Marsupites testudinarius (from Zit- 

 tel). erf central; ib iufratasal ; 6 

 basal ; r radial. 



