VIII 



ECHINODEEMATA 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 

 madrepoi'ic plate. Originally, 



no doubt agreed with the number 

 of stone canals. In cases in 



FIG. 362. A stone canal and pore of the calyx 



the number Ot pores On the Calyx of Rhizocrinus lofotensis, diagrammatic (after 



Ludwig). Interradial section in the neighbourhood 

 of the mouth. ] , Tegmen 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 ; 



6, coelom ; 7, stone canal ; 8, ring canal ; 9, circular 

 nerve ; 10, cesophageal epithelium ; 11, oesophagus ; 

 12, connective tissue. 



numerous, however, no such 

 relation can be established. 



In many inadunate Crinoids 

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

 tegmen. 



Eliizocrinus 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 

 O3sophagus 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 cnsca 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 the Molpadiidce, and among the Elasipoda in the Psychropotidcc and Dcimatida:, 

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

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

 the Synaptidce, 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. 



