104 



THE CRINOIDEA 



such ring is at the level of the radials. If the crinoid have a 

 monocyclic base, the cords that pass to the basals join one another 

 in a ring immediately surrounding the chambered organ, the 



^=4-. 



Fio. XI. 



Course of axial nerve-cords in Isocrimis. Diagrammatised from sections figured by P. H. 

 Carpenter. B, basal ; 1{, radial; ax, axial organ ; ch, five chambers of chambered organ ; 1, 

 nerve-cord from It to B ; 2, cord passing down B ; n3, cord from E to radially placed lobes of 

 chambered organ. 



lobes of which in this case correspond with the basals, i.e. are 

 interradial. If the base be dicyclic, the ring forms a commissure 

 at the level of the centres of the basals ; and from these points 



h-Sl 



Fie. XII. 



Course of axial nerve-cords in Dicyclic (D), Pseudomonocyclic (P), and Monocyclic (M) 

 Crinoids. c.o, lobes of chambered organ, the connecting nervous sheath omitted for greater 

 clearness ; r.c, ring commissure in radials ; other letters as usual. 



the cords again fork towards the adjacent irifrabasals, where 

 they join in another ring round the chambered organ, the lobes 

 of which in this case correspond with the infrabasals, i.e. are 



