530 THE OCEAN WORLD. 
assumes the animal can increase or diminish the specific gravity of 
its body ; that is, it can remain in equilibrium or ascend or descend 
in the bosom of the waters ; it is, moreover, remarked that it is very 
small in those species which swim at the bottom of the water, and, 
as Mr. Gosse says, there is some reason for considering it to be the 
first rudimentary form of the air-breathing lung. 
Immediately behind the head two large openings are observed 
in most fishes; these are the gill openings. Their anterior edges 
are mobile, and are raised or lowered to serve the purposes of 
respiration ; for here, in special cavities, are the gills, or branchiz. 
These usually consist of many rows of thin membranous lamelle, 
hung on slender arches of bone, placed on each side of the head, 
usually protected by bony plates, made up of several pieces, called 
the gil-covers. Respiration is effected by water taken in at the 
mouth, which passes over the gill-membranes, and is ejected through 
the margins of the gill-covers. During the contact of the water 
with the gills, the blood which circulates in these organs, and which 
communicates to them the red colour by which we recognise them, 
combines chemically with the oxygen of the air which the water 
holds in solution when it flows freely at the ordinary temperature 
in presence of the air. The blood is thus oxygenised, or made fit by 
respiration. 
The heart in fishes is placed between the inferior parts of the 
branchial arches, and consists of a ventricle and an auricle 
(Fig. 350). It corresponds with the right half of the heart in the 
Mammals and Birds, for it receives the venous blood from all 
parts of the body and sends it to the gills. From this organ the 
blood is delivered into one great artery, which creeps along the 
vertebral column. 
The eye in fishes is generally very large—we may even say 
enormous, relative to the size of the head—and is generally without 
true eyelids ; the skin usually passes over the ocular globe, and be- 
comes in front of it so transparent that the luminous rays can traverse 
it. This light covering is all the eyelid generally met with in fishes. 
The interior of the globe of the eye is covered by the membrane 
called choroid, the thin external fold of which, in consequence of the 
presence of innumerable microscopic crystals, often presents the 
appearance of a gold or silver-coloured coating, which gives to the 
iris that extraordinary brilliancy which belongs to the fish’s eye. 
The crystalline lens is voluminous, spherical, and diaphanous. When 
the fish is cooked, the crystalline lens constitutes that opaque and 
hard white substance which is so often seen. Cuvier suspected, 
