192 THE INHABITANTS OF THE SEA. 
In most of the cartilaginous fishes, such as the sharks, rays, 
and lampreys, the gills are differently formed, the water not 
passing into a cavity closed by a cover, but flowing directly out- 
wards through five (in the shark) or seven (in the lamprey) 
vents or spiracles. In these species also the gills are fixed, their 
margins being attached. Though the whole breathing apparatus 
of a fish is comprised in a small compass, its surface, if fully 
extended, would occupy a very considerable space; that of the 
common skate, for instance, being equal to the surface of the 
human body. This single fact may convince us of the number- 
less ramifications and convolutions of the gills, in which the 
water is elaborated and attenuated in the course of giving out 
its air; and how wonderfully Nature has contrived to effect her 
purpose with the greatest economy of space. 
Respiration is a species of combustion, and this must neces- 
sarily be very slow in an element which contains so small a 
portion of oxygen. No wonder that the circulation of the blood 
Smaller Circulation. 
Heaite**-t- S25 
\ C4 ---Dorsal Artery. 
Auricless--4<0-052/) _-” 
Veins Arteries. 
Greater Circulation. 
Theoretic representation of the Circulation in Fishes. 
in fishes is equally tardy. Their heart, in comparison with owys, 
is but half a one, as it merely serves to force the venous blood 
into the gills—whence the aérated blood does not flow back to 
the heart as with us, to be rapidly and strongly propelled through 
the body, but proceeds immediately to the arteries. Evidently 
only acold blood could be formed under such circumstances. It 
may seem strange that, when fishes are taken out of the water, they 
die from want of air; such, however, is the case. Their delicate 
breathing membranes collapse in the atmosphere, the blood can 
