530 THE OCEAN WORLD. 



assumes the animal can increase or diminish the specific gravity of 

 its body ; that is, it can remain in equiUbrium 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 branchise. 

 These usually consist of many rows of thin membranous lamellae, 

 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 gill-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, 



