64 



sides still project as somewhat hemispherical masses. 

 Hov/ever, internally the liimen remains divided (Tip;, 31) 

 through hall' oi' the extent of the ventricles. At the an- 

 terior end, th.e ventricle has the shape of an elongated 

 cone. The tv/o auricles accompany the ciUs in the poste- 

 rior development of the latter and come to lie side by 

 side like two large vessels, in the posterior half of the 

 pericardial cavity. Each projects into the ventricle on 

 its own side and valves separate the cavities of the au- 

 riclei^ from that of the ventricle (fig. 54). 



The pericardial cavity of the "Ship-worm's "(figs. 

 10, 31, 32) lies on the apparent dorsal side of the vis- 

 ceral mass. It is very large, extending from the poste- 

 rior adductor to the visceral ganglion. In Xylotrya fim- 

 briata , it narrows in front to form a canal which projects 

 beyond the voider part to the posterior addii.ctor muscle. 

 About two- thirds of the distance from the visceral gan- 

 glion to the posterior adductor (fig. 10) the anterior end 

 of the ventricle dips down through the pericardium, into 

 the visceral mass. Th.is point is the end of the ventricle 

 and the beginning of the aortae , 'the end of the ventricle 

 being marked off by two semilunar valves which project 



