80 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 branchia, 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 witha 
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- 
