CLASS I CRINOIDEA 129 
inward, and the ambulacra follow the inner floor of the tegmen, forming a 
skeleton of ramifying tubes ; these are conducted along open galleries from the 
mouth to the arm-openings (Fig. 232, 4). 
2. The Arms (Brachia).—The arms of the Crinoid body form the immediate 
prolongation of the radials. The plates of the arms are termed brachials, and 

a 
Fic. 231. 
Agaricocrinus Americanus, 
Roem. Ventral disk. 7, Uni- Actinocrinus proboscidialis, Hall. A, Plates of tegmen partially 
serial ambulacrals ; i, Interam- removed in order to show the covered ambulacral passages (a) 
bulacrals ; 0, Anally situated leading from the arms to the mouth. B, Plated upper surface 
oral ; p, Anterior and lateral orals ; of ambulacral galleries. C, Natural cast of ventral disk with 
x,; Posterior interambulacrals impressions of calyx ambulacra (~) leading to the mouth (0); 
(after Wachsmuth and Springer). an, Anus. 
are arranged either in single or double alternating rows ; and hence are spoken 
of as uniserial (Fig. 233, 4), or as biserial (Fig. 233, B). The plates of the 
uniserial 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 

Fia. 234. 

Plated ambulacral furrows of the 
arms. a, b, Cyathocrinus ramosus, 
Ang., showing covering pieces ; 
Fig. 233. 
A, Carpocrinus comtus, Ang. sp., showing c, Gissocrinus arthriticus, Hising., 
uniserial arms. B, Callicrinus costatus, Hising., with covering pieces. All figures 
with biserial arms (after Angelin). enlarged. 
the plates meet midway, so as to form a zigzag suture. The arms invariably 
begin 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 bifureation takes place are called azillaries. 
In the Camerata, the more highly organised Fistulata, and in all recent 
VOL. I K 
