8o EVOLUTION AND DISEASE. 



Embryology is eloquent in furnishing evidence sup- 

 porting the view that the ancestors of existing vertebrata 

 were aquatic in their habits, that respiration was carried 

 on in them by means of gills, and that many structural 

 .peculiarities in mammals result from the transformation 

 of an aquatic into a terrestrial animal. 



The type of respiratory organs in these ancestral 

 forms is best preserved in elasmobranch fish, such as 

 the dog-fish, or in a marsipobranch, like the lamprey. 

 In such forms the water, charged with air, enters the 

 mouth and is forced through openings in the walls of 

 the pharynx. The pharyngeal orifices, or branchial slits, 

 are furnished with vascular processes known as gills. In 

 the gills, or branchiae, the blood and water are merely 

 separated from each other by an extremely delicate 

 layer of tissue. Hence venous blood circulating in the 

 gills readily gives up the excess of carbon dioxide, and 

 as readily obtains oxygen from the surrounding water. 

 The gills of fish and batrachians are supported upon a 

 cartilaginous or bony framework known as the branchial 

 bars, and in such fish as sharks a small cutaneous fold 

 projects from each bar and covers the gill-slit as with a 

 lid ; these cutaneous lids are named, in consequence, 

 opercula. The gill-slits, with the opercula, are sketched 

 in fig. 36, as they are seen in a dog-fish. The first slit 

 bears no gills in the adult fish, and is known as the 

 spiracle, or blow-hole. In the embryo it is furnished 

 with beautiful external delicate vascular tufts. The 

 neck of a mammalian embryo is furnished with four 

 similar slit-like orifices, communicating with the pharynx, 

 as in the dog-fish, but are fewer in number. The gill- 



