TYPE ECHINODERMA. 



575 



cate canals. These organs, which are usually quite small, 

 are situated in the vicinity of the ambulacra! pores or near 

 the mouth, and are supposed to have a sensory, perhaps 

 olfactory, function. 



Owing to the presence of the firm test the muscular system 



Gp pp an tt 



oc 



FIG. 



Al 



amp 



an 



as 



au 



br 



Co 







Gd 



Gp 



Or 



lir 



1 



Ir 



M 



m 



265. DIAGRAM SHOWING THE 

 = Aristotle's lantern. 

 = ampulla. 

 = aboral nerve-ring. 

 = axial sinus. 

 = auricula. 



= external branchia. 



= coelom. 



= reproductive organ. 

 = genital duct. 



= genital pore. 



= genital rachis. 

 = hydroccel-ring. 

 = ossophagus. 

 = lacunar ring. 

 = madreporite. 



STRUCTURE OF AN ECHINOID. 



nr = epithelial nerve-ring. 

 oc = ocular plate. 

 og ovoid gland. 

 pa = perianal space. 

 pp = periproctal space. 

 pph -- peripharyngeal space. 

 pv == Polian vesicle. 



R = rectum. 



r?i = radial hydroccel-vessel. 

 rn = radial nerve. 



sc = stone-canal. 



si siphon. 



sp = spine. 



tf = tube-foot. 



tt = terminal tentacle. 



is but feebly developed, being represented by the muscles 

 attached to the spines, pedicellariae, etc., and by those which 

 move the parts of the masticatory apparatus when it is 

 present. 



The coelom (Fig. 265, Co) is comparatively spacious, though 



