THE CARP. 121 



can act efficaciously only under water, although that 

 liquid contains but a small proportion of air in solution. 

 Fish die in the air because the branchial plates, no longer 

 floating and separated by the water, stick together and 

 present but a small surface to the air. 



In the circulatory apparatus the heart has only two 

 cavities, one auricle and one ventricle. It is a venous 

 heart, and represents the right heart of the mammals. 

 The venous blood arrives from the body to the interior 

 of the auricle, passes hence to the ventricle, and is then 

 propelled to the respiratory apparatus. This is the 



FIG. 67. 



SKELETON OF A FISH. 



function of the right heart in mammals. From the 

 gills, however, instead of returning to a left heart, the 

 blood is at once distributed over the system ; there is but 

 one circuit, and the circulation is said to be single, in- 

 stead of double as in the other vertebrates. It is a com- 

 plete circulation, for there is no mixture of venous and 

 arterial blood. We may add that the ventricle is followed 

 by a sort of swelling or aortic bulb. 



The skeleton of the carp, like that of other fishes, is 

 constructed according to the general plan of the vertebral 

 F 11 



