xx] 



CIRCULATORY SYSTEM 



507 



arrangement will be described when we come to deal with Amphibia. 

 But the resemblance to the vascular system of land animals does 

 not end here. The blood brought to the kidneys by the renal 

 portals filters through them, arid emerges into two subcardinal veins 

 running along the inner sides of the 

 kidneys. In Ceratodus these two sub- 

 cardinals are prolonged forwards in the 

 usual way as two posterior cardinal 

 veins which enter the ductus Cuvieri, 

 but a median vein, the vena cava in- 

 ferior, arises by two forks from the 

 subcardinals and passes forwards to 

 enter the sinus venosus of the heart. 

 In Protopterus and Lepidosiren much 

 the same arrangement is found, but 

 the left posterior cardinal has dis- 

 appeared. Prof. Kerr, to whom more 

 than to anyone else we owe the eluci- 

 dation of the anatomy and develop- 

 ment of these fish, suggests that the 

 vena cava arose through the long lobe 

 of the liver touching and effecting an 

 adhesion with the dorsal wall of the 

 splanchnocoel in the region of the 

 kidneys. In this way some of the 

 blood from the subcardinals can enter 

 the branches of the hepatic vein, and a 

 short road was thus provided by which 

 it could reach the heart, and the en- 

 largement of this channel formed the 

 inferior vena cava (Fig. 248). 



When we turn to the kidneys and 

 the generative system, the researches 

 of Prof. Kerr have brought to light 

 an interesting series of modifications. 

 In all three genera the oviduct opens 

 by a funnel into the coelom, but it is much longer than in most 

 Osteichthyes. In all three genera kidney ducts and generative ducts 

 open with the alimentary canal into a cloaca, as in Chondrichthyes. 



In Ceratodus the testis is connected with the kidney by a number 

 of vasa efferentia. These arise from a longitudinal duct running 



FIG. 248. Diagram to show ar- 

 rangement of the principal 

 veins in a Dipnoan. 



1. Sinus venosus gradually 

 disappearing in the higher 

 forms. 2. Ductus Cuvieri 

 = superior vena cava. 3. In- 

 ternal jugular = anterior car- 

 dinal sinus. 4. External 

 jugular = sub-branchial. 5. 

 Subclavian. 6. Posterior 

 cardinal, front part = venae 

 azygos and hemiazygos. 7. 

 Inferior vena cava. 8. Renal 

 portal= partly hinder portion 

 of posterior cardinal. 9. Cau- 

 dal. The hepatio portal 

 system is omitted. 



