314 FISHES 



portal," system also exists in connexion with the kidneys in the 

 majority of Fishes. 



There is little doubt that, primarily, the vascular system of 

 Vertebrate animals consisted of a dorsal artery (dorsal aorta), 

 running along the median dorsal line of the alimentary canal, and 

 a ventral or subintestinal vein similarly related to the ventral 

 surface of the digestive tube. The two vessels were connected by 

 a series of pairs of lateral branches, which had their origins from 

 the dorsal vessel, and, by their subdivision, formed a capillary 

 network in the walls of the alimentary canal. From these 

 networks paired veins issued and opened into the subintestinal 

 vein. The simplicity of this primitive arrangement was some- 

 what disturbed in the region of the pharynx by the development 

 of gill-clefts, in the walls of which the blood circulated for 

 respiratory purposes from the ventral to the dorsal vessel ; and 

 also by the development of a hepatic portal circulation in con- 

 nexion with the liver. In the latter instance the subintestinal 

 vein entered the liver and subdivided into capillaries in the 

 substance of that organ, the corresponding efferent vessel, or 

 hepatic vein, becoming continuous with the anterior or pharyngeal 

 section of the subintestinal vein, or, as it is usually termed, the 

 ventral aorta. In this low grade of vascular system, which is 

 perhaps most completely retained in Amphioxus, the circulation 

 of the blood was probably effected by the wave-like contractions 

 of more or fewer of the larger vessels ; but subsequently a definite 

 chambered heart was developed at the origin of the ventral 

 aorta. 



Of Fishes in general it may be said that the primitive dorsal 

 and ventral vessels are present in the embryo, and for a time 

 retain their original relations and physiological importance. To 

 a very unequal extent they may also be retained in the adult, 

 where, however, they co-exist with numerous other vessels, which 

 the increasing differentiation of the body has called into existence. 

 Thus, at a later period of embryonic life, the subintestinal vein 

 becomes somewhat fragmentary. Its caudal section (caudal vein) 

 ceases to be continuous with the precaudal portion, and the blood 

 collected from the muscles and other structures of the tail is con- 

 veyed to the heart by a pair of posterior cardinal veins, which 

 are either directly continuous with the caudal vein, or indirectly 

 through the intervention of a renal portal system in the kidneys. 



