TEEMINOLOGY. 33 



bearing ones, constitute the nodal joints, and those interposed between them 

 the iniernod'il ]oixii^. The term dorsocentral is used for the enlarged terminal 

 joint of the stem, by which the young Crinoid is attached to other objects; 

 and centrodorsal for the modified, cirrus-bearing top joint of the Comatulse 

 as well as for the plate within the infrabasal ring of the Marsupitidse. The 

 longitudinal canal, passing through the centre of the stem, is the axial canal. 

 In speaking of the form of the stem, allusion is made to the transverse 

 section. 



The croiun consists of catt/x and arms ; the former encloses the visceral 

 cavity; the latter constitute the free appendages, passing off from, and con- 

 necting with the calyx. 



The cali/x is composed of the dorsal cup, and the ventral disk or tegmeii, the 

 arm regions forming the line of demarkation between them. The dorsal 

 cup conforms in a general way to the apical or abactinal system of the 

 Echinoderms, the ventral disk to the oral or actinal system. 



The dorsal cup, in its simpler form, is constructed of hasals, infrabasals, 

 when present, and radials ; to which must be added the anal plates, which, 

 however, are not always represented. In the more complex form it includes 

 also some of the lower brachials, which have been incorporated into the 

 walls, either by lateral union among themselves, or by means of interradial 

 and interaxillary plates ; the last mentioned plates, in that case, also form- 

 ing part of the cup. All Crinoids belonging to the simpler form are dis- 

 tinguished as Crinoidea Inadunata ; those of complex form, when the calyx is 

 rigid, as Crinoidea Camerata, but when flexible, as Crinoidea Articidata. 



The lase, or part next to the column, may be composed of one or two 

 rings of plates, which are distinguished as hasals and infrahasals . The basals 

 adjoin the radials and alternate with them, being interradial in position. 

 The infrabasals, when present, are radially disposed below the basals. Cri- 

 noids in which the base consists of a single ring of plates are called mono- 

 ay oXio, ; those with two rings, dicyclic. 



The radials consist of the first plate of each ray, and all plates beyond 

 this in radial succession are brachials ; fixed brachials so far as they take 

 part in the calyx, /r^^ brachials or arm plates when they do not. In some of 

 the earlier Crinoids one or more of the radials are bisected transversely, in 

 which case the two parts are distinguished as super -radials and infer -radials. 



The arms may be simple or branching. When \h^ divisions are of equal 

 size, and rise to the same general height, they are regarded as parts of the 



