THE CRINOIDEA 



109 



articular surface (Fig. XVIII. 5) ; the fibrils are at first developed 

 into elastic ligaments, and later into true muscles. (3) " Perforate 

 articulation," in which there is a highly developed facet, with fulcral 

 ridge, ligamentar depressions, and muscles innervated from an axial 

 cord which perforates the ridge (Fig. XVIII. 6). 



In such a simple crinoid as that under discussion the cup-plates 

 would be united by close suture; the tegminals, probably by 

 primitive suture, or loose suture of the rhomb type ; the brachials, 

 by imperforate articulation, perforate being a later development ; 

 the columnals, by loose suture of the striate type. 



The Skeletal Elements of a crinoid may be thus classified : 

 " Primary Elements," the first to be developed in both ontogeny 

 and phylogeny, divided into " abactinal," developed on the 

 right or aboral coelom, and directly innervated from the chambered 



Fi... XV1I1. 



Forms of Joint. 1, sutural mar- 

 gin of cup -plate of Marsufntet< 

 (original). 2, brachials from dorsal 

 side, x 10. 3, ditto from ventral 

 side, x *,,*. 4, distal face of IIBr :j , 

 a hypozygal. x >. (2, 3, and 4 are 

 of Antedon biftrta, after W. B. Car- 

 penter.) 5, articular facet of radial 

 of Pisocrinus ollula, x 3 (after 

 Bather). 0, articular facet of radial 

 of Bathycriniix aldrichianus, x 8 

 (after P. H. Carpenter), oc, axial 

 canal for nerve ; dl, dorsal liga- 

 ment ; /, fulcral ridge ; il, inter- 

 articular ligament ; vif, muscle 

 fossa ; n, notch for axial nerve ; 

 j), pinnule ; p', pinnule-facet ; ., 



syzygy- 



organ, viz. columnals, cirrals, IBB, BB, RE, Br, and pinnulars 

 (vide infra) ; " actinal," developed on the left or oral coelom, and 

 connected with the various oral ring-systems, viz. A (orals) and 

 ambulacrals (Chapter VIII., Fig. X.). "Secondary or Supple- 

 mentary Elements," which may be intercalated between the primary 

 pieces ; these have not yet been discussed ; they include " inter- 

 brachials" (iBr), " interambulacrals " (iAmb), " interaxillaries " 

 (iAx), some "anals," and a few others of no special importance. 



The terms " proximal " and " distal " are reckoned from the 

 plane separating stem from crown, so that the infrabasals and top 

 columnal are the proximal elements of crown and stem respec- 

 tively. The actinal elements, however, start from the oral centre 

 as proximal point. 



For orientation the crinoid is placed in its natural position, 

 mouth upwards, and is viewed from the anal side. The anal 

 interradius is then posterior ; the radius opposite it is anterior ; 



