COUBSE OF THE BLOOD IN ANIMALS. 59 



Smaller Circulation. 



Branchio-Cardiac 

 Canals. 



\ Heart. 



Arteries. 



i 

 I 



Greater Circulation. 



Fig. 41. Crustacea. 



THEOBETIC FIGUBE OF THE CIBCULATION.* 



also been called complete, which means that the whole of the 

 blood circulates through the lungs before being restored to 

 the body. 



Before birth there exists an opening between the right and 

 left auricles, by which the blood conveyed to the heart by the 

 vena cava inferior passes directly into the left auricle. This 

 blood comes mostly from the placenta. The pulmonary 

 artery also, before birth, divides into three branches instead 

 of two, as in the adult; the centre branch passes into the 

 aorta. This peculiar mechanism connected with foetal life dis- 

 appears soon after birth, leaving merely traces of its existence. 

 108. Reptiles. In this class the circulation is not com- 

 plete. The heart has only three cavities instead of four, as in 

 mammals and birds namely, two auricles and one ventricle 

 (Fig. 39) ; the venous blood coming from the various parts of 



* In all these figures the shaded parts represent the veins; the parts 

 simply traced represent the arteries ; the dotted circle represents the heart. 

 Finally, the arrows point out the direction of the sanguine current, which 

 is the same in all the figures. 



