ECHINODERMATA 



447 



at the time of its establishment, has in the meantime elon- 

 gated as far as the outer body- wall and fused with it (Fig. 

 219), forming in this way the stone canal (Barrois). As in 

 the other Eohinoderms, so also in the Crinoids, at least while 

 they are young, a communication exists between the water- 

 vascular system and the outside world ; this fact was estab- 

 lished by Perrier and confirmed by Barrois. 



As is well known, a large number of stone canals hanging clown into 

 the body cavity occur in the adult Crinoids. Ludwig (No. 32) had 

 already shown that in the penta- 

 crinoid stage of the Antedon larva 

 at first only one stone canal is pre- 

 sent ; but he believed that this 

 also, arising from the water-vas- 

 cular ring, ended free in the body 

 cavity, wheiice it took up into it 

 the water which entered through 

 a pore in the body-wall. This 

 view corresponds nearly to that 

 which was defended upon embryo- 

 logical grounds by Bury. Ac- 

 cording to him, the free process 

 of the fundament of the hydro- 

 coele, which was considered by 

 Bareois as a stone canal, is rather 

 a third ccslomic sac. This en- 

 larges, comes into connection with 

 the body-wall by means of a pro- 

 cess (parietal canal), and thus 

 opens to the outside world by 

 means of the water-vascular pore. 

 It is only secondarily then that 

 the hydrocoele, by means of a 

 stone canal, is united to this part 

 of the body cavity. The descrip- 

 tion of these conditions coincides 

 with that given by Ludwig for 

 Asterina, where the stone canal 



also opens into the enterocoele, and is connected with the dorsal pore 

 only by means of this (comp. supra, p. 436). According to Perrier, 

 the pore described by Ludwig, which lies in one of the oral plates, cor- 

 responds to the external oi^ening of the stone canal. The stone canal 

 is said to be easily separated from the pore in dissecting, and then hangs 

 from the water-vascular ring free in the body cavity. 



Pig. 219.— Longitudinal section of an 

 Antedon larva (after figures by Peeriee). 

 D, intestine ; is, subambulacral, Lv, 

 visceral, part of the body cavity ; N, 

 stalk of the larva ; St, stone canal ; T, 

 tentacle ; V, vestibule ; Wr, water-vas- 

 cular ring, from which spring the ten- 

 tacular vessels and the tstone canal. 



