238 ZOOLOGY 
The pinnules resemble the arms, with the exception that 
the generative rhachis has become functional, producing either 
ova or spermatozoa. The rhachis, both in the arms and in the 
10 1 
Fia. 140.—Transverse section of a Cri- 
noid arm (partly diagrammatic). After 
Milnes Marshall. 
Ambulacral groove. 
Ambulacral nerve. 
Ambulacral water-vessel. 
Tube-feet. 
Pinnule. 
Coeliac (dorsal) canal. 
Subtentacular (lateral) canal. 
cot oO OP ON EF 
Ventral canal: contains genital rhachis. 
Muscles connecting the joints of arm. 
Axial cord. 
H 
aS 
Its branches. 
12. Branch to pinnule. 
pinnules, is surrounded by a blood-plexus, and the whole is 
enclosed by the ventral division of the body-cavity, which is 
relatively much larger in the pinnules, corresponding with the 
enlargement of the rhachis. The generative cells escape through 
a series of special pores. At the tips of both arms and pinnules 
all the sections of the body-cavity communicate with each 
other. 
The mouth is central, and the anus is interradial in 
position and on the oral surface of the disk; the alimentary 
canal is coiled, and lined by a ciliated epithelium. The coelom 
in the disk is much broken up by strands of connective tissue 
which support the viscera. The mouth is surrounded by 
vascular, water-vascular, and nervous rings, which each give off 
extensions into the arms (Fig. 141). The water-vascular ring 
gives off numerous ciliated canals which open into a series of 
vessels which communicate with the exterior by a series of 
