1907.] 



NATURAL SCIENCES OF IMllLADEI.PHIA. 



397 



more tumid lips of the two. The left duct leads directly into the main 

 cavity of the excretory organ, while the one of the other side opens into 

 a kind of vestibule, which in turn communicates with the main body 

 of the organ. 



The left kidncn in IJaliotis {fi^. 1, PI. XX.\. LK) has become greatly 

 reduced and highly modified; it is now known generally as the papillary 

 sac, a term applied to homologous organs in other members of the group. 

 It lies against the left side of the rectum (R), and extends backward 

 and to a certain extent over the anterior and left surface of the peri- 

 cardial wall (P). With the exception of its right or inner side, where 

 it is contiguous with the rectum, its walls are produced into well devel- 

 oped jjapillae. 



A— 



Fig. A. — Diagram showing relations oi nephridia, poricardium. gonoduct, etc. 

 For lettering see explanation ol' plate. 



Just what functions are sustained l)y this organ is not clear, though 

 several theories have been advanced. I'elseneer ('96) believes it is 

 phagocytic in character. Its walls and papillae are richly supplied 

 with the purest blood ; the organ seems to be the principal excuse for 

 the existence of the pectiliar inter-auricular sinus (fig. A, 7.4) described 

 by other writers for this genus. This blood sinus connects the right and 

 left auricles, coursing along the outer edge of the wall separating the 

 pericardium and left kidney. In sections a coimection may be traced 

 between the wall of the papillary sac and the basibranchial sinus. Its 



