xii PHYLUM CHORDATA 349 



the system of vessels. The blood which is forced out from 

 the heart by the contractions of the ventricle passes into a 

 series of vessels (Fig. 204, a. br. a.\ which carry it all to 

 the gills. Here it enters a system of capillaries (G.) in the 

 gills, and these being separated from the surrounding water 

 only by a thin membrane, oxygen readily enters the blood, 

 and the carbon dioxide collected in the various tissues and 

 organs of the body is given off. The blood then enters a 



bra 



a.bra 



i/.ao 

 c a 



FIG. 204. Diagram illustrating the course of the circulation in a Fish. Vessels 

 containing aerated blood red, those containing non-aerated blood blue, lymphatics 

 black. B. capillaries of the body generally ; E. of the enteric canal ; G. of the 

 gills ; K. of the kidneys ; L. of the liver ; T. of the tail. a. br. a. afferent 

 branchial arteries ; au. auricle ; c. a. conus arteriosus ; d. ao. dorsal aorta ; 

 e. br. a. efferent branchial arteries ; h.p.v. hepatic portal vein ; h.v. hepatic 

 vein ; Ic. lacteals ; ly. lymphatics ; pr. cv. v. pre-caval veins ; r.p. v. renal portal 

 veins ; s. v. sinus venosus ; v. ventricle ; v. ao. ventral aorta. The arrows show 

 the direction of the current. 



set of larger vessels (e. br. a.), which combine to form a 

 large trunk, the dorsal aorta (d. ao.). Branches from this 

 distribute blood to all parts of the body, where it enters the 

 systems of capillaries, and whence it is carried back again 

 to the heart by the veins. 



In the Lizard the heart and the circulation are somewhat 

 more complicated than in the Dog-fish. There is a sinus 

 venosus as before. The auricle is completely divided into 



