n 4 THE CRINOIDEA 



of the genital rachis. The pinnule differs in origin, and prob- 

 ably in structure and function, from the brachiole, which is an 

 independent exothecal process (see p. 41). Whether such struc- 

 tures as brachioles occur in Crinoidea is disputed. Wachsmuth & 

 Springer (1897) appear to regard pinnules as independent develop- 

 ments, rejecting the above theory of their origin. Jaekel (1894) 

 accepts the theory for most Inadunata and Neozoic crinoids, and 

 speaks of the organs as ramuli, restricting the term " pinnulae " 

 to the similar organs of the Palaeozoic Camerata (Cladocrinoidea, 

 Jaekel) and to the cystid brachioles, which he regards as homo- 

 logous. His view is admissible, but lacks proof. 



In simple dichotomous arms, each brachial that supports two 

 branches does so by two upper sloping sides, or shoulders, of 

 equal size, each notched by the ventral groove, and pierced by the 

 axial canal, which branches with the arm. Such a brachial is 

 called axillare (ax), and all that part of the arm borne by any 

 single axillary is a " dichotom." The branches of the axial cord 

 are united at their bifurcation by a criss-cross of nerve-strands 

 (chiasma) serving to correlate their activities. As the size of one 

 half of a dichotom is reduced, the supporting shoulder of the 

 axillare is narrowed. Continuance of the process tends to bring 

 the wider shoulder more parallel to the under joint-surface of the 

 axillary. Thus a pinnulate arm of primitive structure consists 

 of a series of axillaries in which the alternate right and left 

 shoulders are wide and almost parallel, while the others are 

 greatly reduced and bear pinnules. 



A pinnulate arm may consist of two rami, or each ramus may 

 bifurcate just as in a simple arm, though never to the same 

 extent. The axillaries on which the rami fork remain unmodified, 

 and with equal shoulders. It is convenient to distinguish these 

 as " main-axils " (Ax). 



Owing to the great variation in the branching of the brachia, 

 it is extraordinarily difficult to devise a consistent terminology, 

 or to denote any particular ossicle in a concise and intelligible 

 manner (for fuller discussion, see Wachsmuth & Springer, 1897 ; 

 and Bather, Ann. Mag. Nat. Hist. Jan. 1892; and Geol. Mag. 

 July 1898). In a non-pinnulate dichotomous arm all brachials up 

 to and including the first axillary may be styled primibrachialia or 

 "primibrachs" (IBr), the axillary being distinguished as "primaxil " 

 (lax); the following Br in each branch are " secundibrachs " (IIBr), 

 with a " secundaxil " (Ilax) ; then succeed " tertibrachs " (IIIBr), 

 " quartibrachs " (IVBr), and so on. In a pinnulate dichotomous 

 arm the IBr do not as a rule bear pinnules, and are therefore 

 homologous -with the IBr of a simple arm ; but of the next series 

 only the proximal brachial of each ramus is strictly homologous 

 with the IIBr of a simple arm, the pinnule borne by it, together 



