TYPE MOLLUSC A. 



299 



distance from the heart, but sooner or later the blood passes 

 into the system of lacunar spaces constituting the schizocoel, 

 •whence it is again returned to the auricle through a series of 

 veins. The position of the single auricle with reference to the 

 body axis differs in different orders of Gasteropods, in accord- 



FlG. 133. — Strttcturb of Sycotypus canaliculaius. The mantle is divided in 

 the middle line and turned aside, exposing the mantle-cavity. 



an = anus. 



ct = cteuidium. 

 dg = digestive gland. 



i = intestine. 



n = nephiidium. 

 no = nephridial opening. 



= eye. 

 op = operculum. 



OS = osphradium. 

 p = pericardial cavity. 

 pe = penis. 

 pr = proboscis. 

 si = sipho,'^ 1 



i = tentacle. 

 ie = testis. 



V = ventricle. 



vd = vas deferens. 



The arrows show the openings of nephridium to the mantle-chamber and to 



the pericardium. 



ance with the varying position of the branchia. In those 

 forms in which the branchia lies in front of the heart the 

 auricle lies at the-anterior end of the ventricle, while when 

 the branchia is posteriorly situated the auricle lies behind 

 the ventricle. 



The mouth lies in all Gasteropods at the anterior ex- 

 tremity of the body, towards the ventral surface of the head. 



