ZOOLOGY FOR MEDICAL STUDENTS CHAP. 



displaced round the right side of the alimentary canal until it was dorsal 

 in the closest proximity to the air-bladder (Fig. 154, D). 



The typical lung is innervated, as has been mentioned, by pulmonary 

 branches from the two vagus nerves. Now in Polypterus with the great 

 increase in size of the right lung a share in its innervation has been 

 handed over to the pulmonary branch of the left vagus, the main trunk 

 of which is continued backwards dorsal to the alimentary canal to be 

 distributed over the left-hand portion of the right lung. This curious 

 arrangement of the pulmonary nerves in Polypterus is of great importance 

 tor understanding the arrangement in the Lung-fishes. 



The intestine of the Crossopterygii is short and it retains the spiral 

 valve seen in the Elasmobranch. An interesting detail is the presence 

 of a pyloric valve in the form of a spout-like projection of the stomach 

 into the commencement of the intestine, for if the space round this spout 

 to become divided up into a number of separate pockets opening 

 into the commencement of the intestine at their hinder ends we should 

 have an arrangement like the pyloric caeca of the teleost, and it has been 



,ested that these very characteristic organs have so arisen in evolution. 



The kidney of the Crossopterygian during the larval stage is a pro- 

 ne phros, with as a rule two tubules on each side, but in the adult the 

 place of this is taken by an opisthonephros which retains the relatively 

 primitive elongated slender form. 



There are two ovaries : the eggs are numerous and small in size, 

 about i mm. in diameter, and the oviducts are short Miillerian ducts- 

 pointing to the probability that the ancestors of teleostean fishes also 

 possessed typical Miillerian ducts. 



The testis is of special morphological importance. In the young 

 larva it forms an elongated ridge of the coelomic lining but only a com- 

 paratively short portion at the front end becomes functional. The 

 hinder and much longer sterile portion has in the adult the appearance 

 of a simple duct and it functions as such, and it is only on examining its 

 minute structure and development that it becomes apparent that it is 

 really a degenerate portion of the testis. It opens at its hind end into 

 the duct of the opisthonephros. 



The skeleton of Polypterus is in the adult very completely bony. 



In connexion with the blood-system the only point requiring special 

 nient ion is that the conus arteriosus remains muscular and that it possesses 

 in its interior numerous pocket-valves arranged in longitudinal rows. 



Turning to the nervous system we should naturally look at those 



the brain that in the Teleost show specially striking peculiarities, 



namely the cerebellum and the hemispheres. Both of those show in- 



