194 CoMrARATirE ANATOMY AND PHYSIOLOGY. 



passes through tlie already-mentioned anterior en- 

 largement. 



A very definite system obtains throughout the 

 Vertel>rata ; there is always a centralised ventrally 

 placed heart, which consists of at least two chambers, 

 an auricle and a ventricle ; from the latter, one, or a 

 pair, or several pairs of arterial vessels (aortic 

 arches) are given off; these divide into smaller and 

 smaller arteries, which end in the capillaries found 

 in all organs and parts of the body ; the capillaries 

 pour their blood into the small veins, and the small 

 veins into larger ones, three, or less than three, of 

 which open into the auricular region of the heart. 



Where respiration is effected by gills the blood 

 goes through the aortic arches directly to these organs, 

 distributes itself in the fine gill-capillaries, and then, 

 re-collecting, distributes itself through the body ; the 

 heart, therefore, of a lowly Vertebrate is branchial, 

 and not systemic like that of the gill-bearing cray- 

 fish ; when lung-like structures are superadded to the 

 gills, the heart becomes incompletely divided into two 

 halves, and where lungs altogether take the place of 

 gills there is a tendency, which in the higher forms 

 becomes an accomplished fact, for the heart to become 

 divided into two separate parts ; one of these, that 

 on the right side, collects the blood from the body, 

 and sends it to the respiratory organ, or acts the 

 part of a branchial heart, while the other (left 

 side) receives the blood from the lungs and pumps 

 it into the body, or, in other words, acts as a systemic 

 heart. 



The heart is placed in a membranous pouch or 

 bag, the pericardium, and ordinarily hangs freely 

 in it, though sometimes, as in the eel, the heart is 

 attached to it by fibrous bands ; the successive cham- 

 bers are separated from one another by valves, and the 

 ventricle is likewise separated from the aortic system 



