64 ZOOLOGY. 



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. 48) ; the venous blood coming from the various parts of 

 the body is poured by the right auricle into the single ven- 

 tricle, which receives also the arterial blood from the left 

 auricle a portion of this mixed blood is returned to the 

 lungs, and the rest proceeds to nourish the body. The circu- 

 lation in reptiles resembles somewhat that of the foetus of the 

 higher classes of animals. From the heart (ventricle) there 

 proceed two arteries or aortce, which, after having each fur- 

 nished a cross or arch, one to the right the other to the left, 

 reunite to form a single trunk (51). In some reptiles, the 

 crocodile for example, the circulation is somewhat different. 

 We shall describe it when speaking of these animals. 



109. Fishes. In Fishes the circulation may be said to 

 be still more simplified. The heart has only one ventricle 

 and one auricle. This auricle receives only venous blood 

 returned to it from all parts of the body. From this cavity it 

 passes into the ventricle, from which springs a single artery, 

 having at its origin a strong arterial bulb (Fig. 49). Through 

 this single artery it is conveyed first to the gills, and the 

 vessels returning from these unite to form a single dorsal 

 artery (the aorta), by whose branches the blood is conveyed to 

 all parts of the body, returning by the veins to the auricle 

 from which it started. Nevertheless, the circulation is here 

 complete, since all the blood is aerated before its employ- 

 ment in nourishing the body (Fig. 49, p. 63). 



110. Mollusca. In most of the mollusca the circulation 

 resembles that of fishes, but the heart is aortic and not pul- 

 monary that is to say, it is placed in the course of the blood 

 proceeding from the respiratory apparatus to the body, and 

 the venous system is more or less incomplete. The heart in 

 these animals is composed usually of a ventricle (Fig. 53 h), 

 whence spring the arteries (i), and of one or two auricles in 

 communication with the vessels (o), which carry the arterial 



