310 ELEMENTARY BIOLOGY. [CHAP. 



communicate by valvular apertures with the ventricle. The 

 ventricle gives off two aortic trunks, one of which, the ante- 

 rior, runs forwards in the middle line, above the rectum, 

 while the other runs backwards, below the rectum. From 

 these two aortse branches are given off which divide into 

 smaller ramifications for the different regions of the body, 

 and for the viscera, and finally terminate in channels which 

 answer to the capillaries of the higher animals. 



The pericardial cavity, in which the heart is lodged, is 

 situated in the posterior half of the dorsal region of the 

 body. Through its thin dorsal wall, and, still better, when 

 it is carefully laid open, the heart can be seen beating. The 

 auricles contract, and, after them, the ventricle; the wave- 

 like contraction 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 ventricle contracts, and is forced out, 

 either forwards or backwards, through the two aortse. 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 

 Bojanus, to the branchiae. Here it becomes purified of car- 

 bonic 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. As the heart, in most animals, first appears either 

 in close proximity to, or in the substance of, the wall of the 

 embryonic alimentary canal, there is no real anomaly in its 

 position in the Anodonta. 



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 majority of the vessels which convey the blood from 



