392 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



The Water Yascular System. 



This system consists of a circum-oral vessel and five 

 radial vessels which spring therefrom and bifurcate at the 

 base of the corresponding pairs of arms to traverse them 

 and their pinnules to their extremities. In addition to 

 these structures there are numerous water tubes which 

 depend from and open into the circum-oral vessel, and 

 still more numerous ciliated funnels which penetrate the 

 tegmen calycis and open into the body cavity. 



The circum-oral vessel surrounds the mouth at the 

 base of the oral tentacles (PL IV-., fig. 47; PI. V. fig. 52, 

 c.o.w.v.). In common with all other parts of the system, 

 it is lined with endothelium, the cells of which are here 

 rounded, non-ciliated, and have a central nucleus which 

 stains faintly (PI. IV., fig. 47, coe. en.). Longitudinal 

 muscle fibres form a thin but well-defined band upon the 

 side of the vessel nearest to the oesophagus (fig. 47, l.m.f.). 



Tentacular branches are given off directly to the five 

 interradial groups of oral tentacles, and at these points 

 the lumen of the vessel is traversed by little groups of 

 isolated muscle fibres (i.m.f.). Similar groups occur at 

 the points of origin of the radial vessels. The course of 

 the latter coincides exactly with that of the ambulacral 

 grooves of the disc, of the arms, and of the pinnules. A 

 lacuna-like space, the sub-neural sinus (PL IV., fig. 41, 

 s.n.s.), separates the vessel (PL III., fig. 29; PL IV., 

 fig. 41 ; PL V., fig. 52, r.w.v.) from the sub-epithelial 

 nerve band. 



In traversing the arms the radial vessels follow a 

 slightly zigzag course, and give off, at each of the angles, 

 a lateral branch to a corresponding pinnule. Lateral 

 branches are given off also, which, after a short course at 

 right angles to the axis of the arm or pinnule (PL III., 



