TYPE ECHINODERMA. 



565 



gland from which these arise. These two last cavities are 

 said to be portions of the general cosloin which become sepa- 

 rated off during development, and are not simple extensions 

 of that portion of the coelom into which the stone-canal opens 

 in the embryo, and which persists as the ampulla. 



mu 

 am pc 



FIG. 259. DIAGRAM SHOWING THE RELATIONSHIPS OF THE STONE-CANAL, 



AXIAL SINUS, ETC., IN AmpMura squamata (after MACBRIDE). 

 am = ampulla of stone-canal. N = riug-uerve. 



U ~ genital bursa. o = ovoid gland. 



gr = origin of genital rachis. pc pore-canal. 



M mouth. ps = peripharyngeal space. 



mp = inadreporite. s = sinus. 



mu = muscle. sc = stone-canal. 



These differences from what occurs in the Asteroidea do not imply, 

 however, a want of homology between the axial sinuses of the two groups. 

 The entire sinus is after all a separated portion of the coelom, and it 

 makes little difference whether it he all separated off at au early stage 

 iu the development, as in the Asteroidea, or only a part of it, the rest de- 

 veloping later from, the general coelom as in the Ophiuroidea and Crinoidea, 

 the dorsal organ of the latter being homologous with the axial sinus 

 in inns the ampulla of the stone-canal of the Ophiuroidea and Asteroidea. 



Lying 011 the aboral surface of each radial nerve-cord is 

 a radial schizocoalic sinus (Fig. 260, br], which communicates 

 with an oral sinus surrounding the mouth. The relations of 

 this system are similar to those of the schizocoalic system of 

 the Asteroidea, and numerous communications between it 

 and the ccelomic cavities occur. It contains, however, a 

 system of canals, which correspond to the lacunse occurring 



