586 



ECHINODERMATA CRINOIDEA 



in the centre of its five chambers a median pocket grows down 

 into the centro-dorsal, along the side of which is an extension of 

 the genital stolon. In the arms the mesentery separating the 

 extensions of the oral and aboral coelom persists ; the oral ex- 

 tension consists of two parallel canals called " subtentacular " 

 (Fig. 267, s.c), whilst the aboral space is termed the " coeliac " canal 

 (Fig. 267, c.c). In the tip of the pinnule, that is to say at 

 the extremity of a ramification of the arm, the coeliac and sub- 

 tentacular canals communicate. As portions of the lining of 

 both canals are ciliated, a circulation of the coelomic fluid is thus 



Fig. 267. — Diagrammatic 

 transverse section of 

 arm of Antedon. To 

 compare it with tlie 

 section of an Ophiuroid 

 arm it is inverted from 

 its natural position. 

 6r, Brachial ossicle ; 

 c.c, coeliac canal ; 

 c.p, covering plate ; 

 g.c, genital canal ; 

 g.r, genital rachis ; 

 n.r.d, dorsal nerve- 

 cord ; n.r.v, ventral 

 nerve-cord ; pod^ po- 

 dium ; s.c, subtent- 

 acular cacal ; w.v.r, 

 radial water-vessel. 



pod. 



kept up. The genital stolon gives rise at the level of the 

 remnant of the horizontal mesentery in the disc to a circular 

 genital rachis, whence cords pass down the arms in the tissue 

 separating subtentacular and coeliac canals (Fig. 267, g.r). 

 Each cord is contained in a special tube, the " genital canal," 

 which is probably developed in the same way as the aboral sinus 

 of the Eleutherozoa, i.e. as a special sheltering outgrowth of the 

 coelom (Fig. 267, g.c). In the pinnule the rachis swells out into 

 a genital organ, from which a short duct is developed when the 

 organ is mature. The eggs are large (3 mm. in diameter), and 

 adhere for a considerable period of their development to the 

 pinnules. 



