418 ZOOLOGY SECT 



and are of the nature of specialised parts of the coelorne, from which 

 they are developed. In Asteroidea and Ophiuroidea the radial 

 and ring-vessels, which lie between the corresponding 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. 378). 

 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 may also communicate aborally 

 with several of the pore-canals of the madreporite, and opens into 

 the madreporic canal itself. In the Echinoidea the arrangement 

 I 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- 

 cceloine : aborally they also terminate blindly, not opening into the 

 aboral ring-sinus. The axial sinus is largely encroached upon by the 

 axial organ : it terminates blindly at the oral end ; aborally it com- 

 municates 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 periha3inal 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 hsemal 

 system in the Asteroidea has been described in the account given 

 of the structure of the Starfish (p. 373). 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 gastro-intestinal system given off from the 

 oral ring is highly developed, and there are 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 lacunae is highly developed and 

 complicated in arrangement. 



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

 vessels. It is made up of strands of a kind of gelatinous connec- 

 tive tissue, with many leucocytes, permeated in a very irregular 

 way by minute lacunas without definite walls. The great develop- 



