86 CLASS PISCES. 



ment of these vessels corresponds to that of the afferent 

 branchial arteries ; they unite to form the large aorta descendens 

 or dorsal aorta. Before they unite the cephalic arteries pass off 

 from the efferent vessels of the anterior arch. 



The arrangement of the principal venous trunks in fishes is 

 most nearly related to the embryonic condition. Corresponding 

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

 posterior cardinal veins bring back the blood from the anterior 

 and posterior part of the body respectively. 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 does not pass directly into 

 the posterior cardinal veins, but breaks up into capillaries in the 

 kidneys, from which the blood passes into the posterior cardinal 

 veins. There is thus a renal-portal circulation. For the hepatic 

 portal circulation on the other hand the venous blood of the 

 intestine is used ; this blood after passing through the capillaries 

 of the liver is returned to the heart by one or more veins, which 

 open into the sinus venosus between the two ductus Cuvieri. 

 Such capillary systems must be a considerable hindrance to the 

 circulation of the blood and explain the development of the so- 

 called accessory hearts on the caudal vein of the eel and on the 

 portal vein of Myxine. 



The urinogenital organs are described under the different sub- 

 classes. With regard to them the following general remarks may 

 be made. A pronephros is present and functional in the larva of 

 all fishes except Elasmobranchii, in which there is no larval stage. 

 It has been maintained, and a great deal has been written on the 

 subject, that there is a vestige of a pronephros in the embryos of 

 Elasmobranchs ; but if there is it is very feebly developed and 

 never possesses a glomerulus. 



The pronephros is the anterior and first developed portion of a 

 longitudinal gland, which extends, in the embryo at least, the 

 whole length of the body cavity from the pericardium to the 

 hind end. This extended excretory organ consists of nephridia, 

 which in Elasmobranchs are developed, as was first shown by 

 Sedgwick,* from the portions of the body cavity which connect 

 the lower ends of the muscle plate cavities with the general body 

 * Q. J. M. S., 20, 1880, p. 164. 



