62 ZOOLOGY. 



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

 lation in reptiles resembles somewhat that of the lu-tus of the 

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

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

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

 reunite to form a single trunk (42). In some reptiles, the 

 crocodile for example, the circulation is Bomewhat 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. 40). 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. 40, p. 58). 



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. H //). 

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

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

 blood from the respiratory apparatus (d), which this liquid 

 reaches by venous canals more or less complete (>?). This is 

 the case in snails, oysters, and all the class gasteropoda and 

 acephala; but sometimes there exist no auricles, and a kind 

 of venous hearts is found altogether distinct from the aortic 

 ventricle, and situated at the base of the respiratory organs ; 

 this takes place in the sepia, and other cephalopoda. How- 

 ever it maybe, the arterial blood in all these animals tra- 

 the heart, then proceeds to all the parts of the body, 

 and is afterward* directed towards the respiratory organ's. 

 But in this latter part of its course, the blood is not alwavs 

 contained in vessels properly so called. Sometimes the veins 

 are altogether wanting, their place being supplied hy lacuna', 

 or void spaces filling up the intervals of the organs : at other 



