314 FISHES CHAP. 
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. 
