138 



BIOLOGY: GENERAL AND MEDICAL 



ment is unconnected with important changes in the 

 heart or great vessels. 



In Protopterus and Lepidosiren, however, the fishes 

 are not only provided with gills, but also with two lungs 

 of considerably larger size and of greater importance. 



c. v. I. 



p.c. 



FIG. 49. Diagram of the heart and branchial arches in Ceratodus (one of the 

 Dipnoi). Position and lettering as in the preceding cut. ab. air bladder (lung) ; 

 p. a, pulmonary artery; p.c, postcaval vein (right); p. v, pulmonary vein. (Gallo- 

 way.) 



In considering the changes necessitated through this 

 improvement we must, however, bear in mind that the 

 fishes by preference and under all favorable conditions, 

 continue to live the life of fishes, remaining under water, 

 and aerating their blood by means of gills and that it is 

 only under exceptional circumstances that the use of 



