CLASS I 



CRIXOIDEA 



129 



inward, and the ambulacra follow the inner floor of the tegim-n, forming a 

 skeleton of ramifying tubes ; these are conducted along open galleries from the 

 mouth to tin* arm opening ( 1'V -">-, A). 



2. 77*0 Arms (Brachin). The arms of the Crinoid body form the immediate 

 prolongation of the radials. The plates of the arms are termed brachiah, and 



Fio. 231. 



Aijiinnx-r'nnis Aniericanus, 

 Roen'i. Ventral disk, r, Uni- 

 serial anibnlarrals ; /', Interaiii- 

 bulacrals ; n, Anally situated 

 oral ; /i. Anterior and lateral orals ; 

 Posterior interaniljulacrals 

 (after \Vai-hsnnith and Springer). 



Fio. 232. 



Actinocrinus proboscidicdis, Hall. A, Plates of tegtnen partially 

 removed in order to show the covered ambulacral passages (a) 

 leading from the arms to the mouth. Ji, Plated upper surface 

 of ambulacral galleries. C, Natural cast of ventral disk with 

 impressions of calyx ambulacra (n) leading to the mouth (o); 

 an, Anus. 



arc arranged either in single or double alternating rows ; and hence are spoken 

 of BB uniserial (Fig. 233, A), or as biserial (Fig. 233, B). The plates of the 

 i mist-rial arms may be either rectangular or cuneiform, the major ends being 

 directed alternately to the right and left. In biserial arms the smaller ends of 



unfit.*, An;;, sp., showing 



uniserial arms. /;, i;,l/;,-ri,iii.* <;>xt<itus, llising., 

 with hiserial aims (after Angi-lin). 



Fio. 234. 



Plated ambulacral furrows of the 

 arms, a, b, Cyathocrhm* /-HHIUS*'*, 

 Ang., showing covering pieces ; 

 c, GissQcrinus arthritiots, Uising., 

 with covering pieces. All liguivs 

 enlarged. 



th<- plates meet midway, so as to form a zigzag suture. The arms invariably 

 bi'gin uniserially, the biserial structure being gradually introduced in an upward 

 direction. The arms either remain simple, or branch in various ways ; the 

 plates upon which a bifurcation takes place are called axilla ri<*. 



In the Camerata, the more highly organised Fistulata, and in all recent 



VOL. I K 



