ECHINODERMATA— WATER VASCULAR SYSTEM 



423 



5. Crinoidea. — Adult Crinoids have at least five, and usually 

 many more or even very numerous stone canals, all of which open 

 into the body cavity. Communication between the exterior and the 

 body cavity is brought about by 

 at least five ciliated pores (Kelch- 

 poren) in the tegmen calycis ; 

 their number is generally far 

 greater, and may mount up to a 

 thousand. Each single pore 

 corresponds with a madreporite 

 with one pore canal. We must 

 not therefore compare the calyx 

 pores of a Crinoid collectively 

 with the numerous pores of a 

 madreporic plate. Oritrinallv, 



, ^ , K 11 Fig. 362.— A stone canal and pore of the calyx 



the number of pores on the calyx of RWzocrinus lofotensis, diagrammatic (after 



no doubt agreed with the number Ludwig). Interradial section in the neighbourhood 



of stone canals. In cases in of the mouth l, Teamen calycis; 2 calyx pore ; 



3, aperture of the stone canal into the body cavity ; 



which both structures are very 4, intestinal epithelium; 5, intestinal cavity; 



numerous, however, no such <^. coelom ; 7, stone canal ; S, ring canal ; 9, circular 



1 ,■ 1 J. 1 T 1 1 nerve; 10, cesophageal epithelium; 11, oesophagus; 



relation can be established._ _ 12, connective tissue. 



In many inadunate Crinoids 

 {cf. p. 303) a madreporite occurs in the posterior interradius of the 

 tegmen. 



Ehizocrinus lofotensis and Actinocrinus verneuilianus have only five 

 interradial stone canals and five interradial pores in the calyx. The 

 openings of the stone canals into the body cavity lie directly below 

 the pores belonging to them. 



For the number and arrangement of the calyx pores, cf. the section on the Tegmen 

 Calycis of the Crinoids, p. 377. 



B. The Water Vascular Ring- and its Appendages. 



1. Holothurioidea. — The water vascular ring always encircles the 

 oesophagus behind {i.e. apically to) the calcareous ring. In all Holo- 

 thurioidea without exception it carries Polian vesicles. As a rule, 

 only one Polian vesicle is present. 



These pear-shaped or tubular creca of the water vascular ring, which project 

 freely backward into the body cavity, vary greatly in size. In extreme cases they 

 may be half as long as the body. 



In tlie Mulpadiidce, and among the Elasipoda in the Psychropotida; and Dcimatidcc, 

 more than one vesicle has never yet been observed, and, in the Elpadiidai, there 

 is, normally, only one. In other divisions, a varying number of species, greatest in 

 the Srjnaptida', have more than one Polian vesicle. In all such species, however, 

 there was originally only one vesicle. Where accessory vesicles occur they vary 

 greatly in number, and appear to have very slight, if any, systematic significance. 



