VASCULAR SYSTEM. 



85 



only in the Leptocephalidae (larvae of the eels) ; it circulates in a 

 closed vascular system, in which a muscular pulsating region or 

 heart is present. The heart (Fig. 47) is placed far forward on 

 the tliroat, ventral to the branchial 

 framework, and is enclosed in a 

 pericardium, the cavity of which 

 communicates with the body cavity 

 in some Plagiostomes, Chlmaera, 

 Acipenser, etc. It is a simple 

 venous branchial heart, and is 

 composed of a sinus venosus, a 

 thin- walled large auricle and a very 

 powerful muscular ventricle. The 

 sinus venosus receives the venous 

 blood returning from the body, and 

 the ventricle forces it through the 

 ventral aorta to the respiratory 

 organs. The aorta begins with a 

 bulbous swelling {hulbus arteriosus) 

 which in the Ganoids, Plagiostomes 

 and Dipnoi is replaced by an inde- 

 pendently pulsating part of the 

 heart, with rows of semi-lunar 

 valves {conus arteriosiis). While 

 the fishes with a simple non-mus- 

 cular bulbus arteriosus have but 

 two semi-lunar valves at its origin, 

 the above-mentioned orders usually 

 have two or 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 bran- 

 chial arteries ; they pass into the 

 branchial arches and give off branches to form the capillary net- 

 works of the gills. From the capillary networks small vessels 

 pass out which unite to form a larger vessel in each branchial 

 arch (epibranchial or efferent branchial artery). The arrange- 



FlG. 47. — Diagram o£ the circulation of 

 a Teleosteau. Ab arterial arches ; 

 Ao aorta descendens into which the 

 epibranchial arteries passing out from 

 the gills unite ; Ba ventral aorta with 

 the arterial arches which carry the 

 blood to the gills ; D intestine ; Lk 

 portal circulation ; N kidneys ; V 

 ventricle. The branchial capillary 

 system is omitted. 



