146 PISCES. 



vascular system, in which, except in Amphioxus, a muscular pulsating 

 region or heart is present. The heart (fig. 595) is placed far forward 

 on the throat, ventral to the branchial framework, and is enclosed 

 in a pericardium, the cavity of which communicates with the body 

 cavity in some Plagiostomes, Chimcera, Acipenser, etc. It is a simple 

 venous branchial heart, and is composed of a thin-walled large 

 auricle and a very powerful muscular ventricle. The auricle receives 

 the venous blood returning from the body, and the ventricle forces it 

 through an ascending aorta to the respiratory organs. The aorta 

 begins with a bulbous swelling (bulbus arteriosus), which in the 

 Ganoids, Plagiostomes, and Dipnoi is replaced by an independently 

 pulsating part of the heart with rows of semi-lunar valves (conus 

 arteriosus). While the fishes with a simple non -muscular bulbus 

 arteriosus have but two semi-lunar valves at its origin, the above 

 mentioned orders usually have two to four, or rarely five rows of 

 three, four or more valves each in the conus arteriosus. The aorta 

 at once divides into a number of paired vascular arches corresponding 

 to the embryonic aortic arches. These are the branchial arteries ; 

 they pass into the branchial arches and give off branches to form the 

 capillary networks of the gills. From the capillary networks small 

 vessels pass out which unite to form a larger branchial vein in each 

 branchial arch (epibranchial artery). The arrangement of these 

 veins corresponds to that of the branchial arteries; they unite to 

 form the large aorta descendens or dorsal aorta. Before they unite 

 the cephalic arteries pass off from the epibranchial arteries of the 

 anterior arch. The arrangement of the principal venous trunks in 

 fishes is most nearly related to the embryonic^condition. Correspond- 

 ing to the four cardinal veins of the embryo, two anterior and two 

 posterior vertebral * veins (jugular and cardinal veins) bring back 

 the blood from the anterior and posterior part of the body re- 

 spectively. These veins unite on each side to form two transverse 

 veins the ductus Cuvieri which enter the sinus venosus of the 

 heart. The course of the returning venous blood is complicated by 

 the insertion of a double portal circulation. The caudal vein passes 

 directly into the posterior cardinal veins only in Cyclostomes and 

 Selachians : in all other fishes there is a renal -portal circulation, in 

 that the caudal vein breaks up into capillaries in the kidneys, from 

 which the blood passes into the posterior cardinal veins. For the 

 hepatic portal circulation on the other hand the venous blood of the 



* Often called the anterior and posterior cardinal veins. 



