CLASS III 



CRINOIDEA 



179 



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

 of as uniserial (Fig. 281, A), or as biserial (Fig. 281, B). The plates of the 

 uniserial arms may be either rectangular or cuiieiform, the major ends being 

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

 of the plates meet midway, so as to form a zigzag suture. The arms invariably 



B 



Fig. 279. 



Agaricocrinus americanus 

 Roein. Ventral disk. r, Uni- 

 serial ambulacrals ; i, Inter- 

 ambulacrals ; o, Anally situ- 

 ated oral ; p, Anterior and 

 lateral orals ; x, Posterior in- 

 terambulacrals (after Wach- 

 smuth and Springer). 



Cactocrinus probosddialis (Hall). A, Plates of tegmen partially 

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

 leading from the arms to the mouth. B, Piated upper surface 

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

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

 an, Anus. 



begin uniserially, the biserial structure being gradually introduced in an upward 

 direction. They either remain simple, or branch in various ways ; the plates 

 upon which a bifurcation takes place are called axillaries. 



In the Camerata, the more highly organised Inadunata, and in all Eecent 



Fig. 281. 

 A, Carpocrinus comtus Ang., showing uni- 

 serial arms. B, Codlicrinus costatus Hising., 

 with biserial arms (after Angeliii). 



Fio. 282. 

 Piated ambulacral furrows of the 

 arms. a, b, Cyathoerinns ramo.fiui 

 Ang., showing covering pioces ; 

 c, Gissocrinus arthriticns Hising., 

 with covering pieces. All figures 

 enlarged. 



Crinoids, the arms are furnished with pinmdes, which are given off alternately 

 from opposite sides, usually one to each arm-plate, more rarely on alternate, 

 or on every third arm-plate ; sometimes they are partially or entirely absent 

 from the lower portion of the arms. The pinnulcs are jointed appendages, 

 which at least basally repeat the general structure of the arms, and in living 



