Ill] THE PEARL OYSTER 37 



system. On dilatation after each contraction, blood 

 is Slicked into the ventricle from the auricles. The 

 blood can only flow in one direction. It is prevented 

 from re-entering the auricles, when the ventricle 

 contracts, by valves which guard the orifices. The 

 auricles receive their blood from the gills and mantle ; 

 thus the heart only contains blood of one kind — pure. 

 The blood system is much more simple than that of 

 man, where the heart has two ventricles, and pumps 

 out both pure and impure blood. 



The Excretory Orgmis. 



The pearl oyster possesses two so-called "kidneys." 

 They are simply tubes which open at one end into 

 the pericardium and at the other into that part of 

 the external world enclosed by the two shell valves. 



Excrete matter is removed from the blood, which 

 circulates in the walls of these tubes and is passed 

 into the lumen, eventually reaching the exterior. 

 Waste matter may also be excreted into the peri- 

 cardium itself by the walls of the heart. This excrete 

 matter also passes down the lumen of the kidneys to 

 the exterior. 



There now remains to be described the nervous 

 system and sense organs, and finally the reproductive 

 organs to render this general account of the anatomy 

 complete. 



