a PHYLUM ECHINODERMATA 427 



vessels given off from it, together with an axial sinus and aboral 

 ring- vessels. These "vessels" are channels with a definite epi- 

 thelial lining, and are of the nature of specialised parts of the 

 COBlomej from which they are developed. In Asteroidea and Ophi- 

 aroidea the radial and ring- vessels, which lie between the corres- 

 ponding parts of the ambulacral and epidermal nervous systems, 

 are divided into two parts by a longitudinal septum, vertical in 

 the radial, oblique in the ring- vessel. The axial sinus is nearly 

 vertical in direction and partly encloses the axial organ in the 

 way already described (p. 384). At its oral end it opens into the 

 inner division of the circum-oral vessel : at its aboral end it opens 

 into, or becomes closely applied to, the aboral vessel, which is in 

 the form of a ring giving off radial branches towards the gonads : 

 it also may communicate aborally with several of the pore-canals 

 of the madreporite, and opens into the madreporic canal itself. In 

 the Echinoidea the arrangement of the parts is modified in certain 

 important respects. An oral ring-sinus is absent unless it be 

 represented by the lantern-coelome. The radial vessels of the 

 system do not open orally into the lantern-coelome : aborally they 

 also terminate blindly, not opening into the aboral ring-sinus. 

 The axial sinus is largely filled by the axial organ : it terminates 

 blindly at the oral end ; aborally it communicates with the 

 madreporic canal and is not connected with the aboral sinus. In- 

 the Holothuroidea there are five radial sinuses extending through 

 the ambulacral areas between the superficial radial nerve and the 

 radial ambulacral vessel, ending blindly aborally and opening 

 orally into an "oral ring-sinus. There is no axial sinus. In the 

 Crinoidea the perihaemal system is greatly reduced, though 

 representatives of the radial sinuses are present in the same 

 situation as in the other classes. 



The general disposition of the lacunar or so-called haemal 

 system in the Asteroidea has been described in the account given 

 of the structure of the Starfish (p. 380). Save for certain minor 

 alterations which are involved in the change in the position of the 

 madreporite, the system is arranged in the Ophiuroidea on the 

 same plan as in the Asteroidea. In the Echinoidea there is an 

 oral ring giving off five radial strands which in the greater part of 

 their course occupy the typical position between the superficial radial 

 nerve and the radial ambulacral vessel; aborally they terminate 

 blindly. A gastrointestinal system given off from the oral ring 

 is highly developed, and there is an axial plexus in the axial organ 

 and an aboral ring, with strands passing to the gonads, as in the 

 Asteroidea. In the Holothuroidea there is an oral ring with 

 radial strands, and a well-developed gastro-intestinal system. In 

 the Crinoidea this system of lacunar is highly developed and 

 complicated in arrangement. 



Whatever be its functions, this system is not a system of blood- 



