3IO ELEMENTARY BIOLOGY. [CHAP. 



communicate by valvular apertures with the ventricle. The 

 ventricle gives off two aortic trunks; from these branches 

 are given off which divide into smaller ramifications for the 

 different regions of the body, and for the viscera, and finally 

 terminate in channel's 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 aorta?. 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. There is 

 reason to believe that the mantle, which is highly vascular, 

 may perform an accessory respiratory function, in a manner 

 similar to that of the inner wall of the Crayfish's carapace 

 (cf. p. 182). 



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 amoeboid corpuscles. 



The majority of the vessels which convey the blood from 



