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 



Fia. 140. — Transverse section of a Cri- 

 noid arm (partly diagrammatic). After 

 Milnes Marshall. 



1. Ambnlaoral groove. 



2. Ambulacral nerve. 



3. Amtulaoral water- vessel. 



4. Tube-feet. 



5. Pinnule. 



6. Coeliao (dorsal) canal. 



7. Subtentacular (lateral) canal. 



8. Ventral canal: oontainsgenital rhachis. 



9. Muscles connecting the joints of arm. 



10. Axial cord. 



11. 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 oif 

 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 



