TYPB EGHINODERMA. 



565 



glaud from which these arise. These two last cavities are 

 said to be portions of the general coelom 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. 



am' po 



l"l&. 359.— DiAGEAM SHOWING THE RELATIONSHIPS OP THE StONB-CANAL, 



AxiAii Sinus, btc. , in Amphiura squamata (after MacBbide). 



am = ampulla of atone-caual. iV = ring-nerve. 



6r ~ genital bursa. o — ovoid gland. 



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



M = mouth. ps = peripharyngeal space. 

 mp = madreporite. s = sinus. 



mu = mnsck'. sa = stone-canal. 



These diflferences 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 be all separated off at an early stage 

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

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

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

 tninus the ampulla of the stone-canal of the Ophiuroidea and Asteroidea. 



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

 a radial schizocoelic sinus (Fig. 260,-Z>r), which communicates 

 with an oral sinus surrounding the mouth. The relations of 

 this system are similar to those of the schizocoelic system of 

 the Asteroidea, and numerous communications between it 

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

 system of canals, which correspond to the lacunae occurring 



