108 ELEMENTARY BIOLOGY. [XI. 



tion of the latter being much the more easily visible. The lips 

 of the auriculo-ventricular apertures are so disposed that the 

 blood is impeded from flowing back into the auricles, when 

 the ventricles contract, and is forced out, either forwards 

 or backwards, through the two aortas. From these it finds 

 its way to the capillaries, and returns from them to the 

 vena cava; whence it is carried, through the organs of Bo- 

 janus, to the branchiae. Here it becomes purified of carbonic 

 anhydride, and receives oxygen from the water in which 

 the branchiae are plunged ; and it is finally brought back in 

 an arterialized condition to the heart. 



The heart is therefore systemic and propels aerated blood. 



The majority of the vessels which convey the blood from 

 the vena cava to the branchiae, traverse the walls of the dark- 

 coloured organ the organ of Bojanus which has already 

 been mentioned ; and it is probable that they here part with 

 their nitrogenous waste matters the organ of Bojanus, in all 

 probability, playing the part of a kidney. The cavity of 

 the organ of Bojanus communicates, on the one hand, with 

 the pericardium, and, on the other, with the exterior, by an 

 aperture which is situated close to the attachment of the 

 inner gill to the walls of the body. Thus the cavity of the 

 pericardium communicates directly with the exterior, though 

 by a roundabout way. But it also communicates directly 

 with the venous system, by sundry small apertures placed in 

 the anterior part of its floor. Hence it must contain a 

 mixture of blood and water. 



The blood of the Anodonta is colourless, and contains 

 colourless corpuscles, which resemble those of Man in struc- 

 ture and present the same Amoebiform movements. 



The nervous system of the Anodonta consists of three 

 pairs of yellow ganglia ; the cephalic, situated at the sides of 

 the mouth ; the pedal, placed in the foot ; and the parieto- 

 splanchnic, on the under face of the posterior adductor muscle. 



