400 
THE OCEAN WOULD. 
Immediately behind the head, two large openings are observed in 
fishes ; these are the gill-openings. Their anterior edge is mobile, and 
they are raised or lowered to serve the purposes of respiration ; under 
this species of covercle are the gills, or branch he. These usually consist 
of many rows of thin membranous plates, lmng on slender arches of 
bone, placed on each side of the head, usually protected by a bony plate 
made up of several pieces, called the gill-covers. The breathing i s 
produced by water taken in at the mouth, which passes over the 
gill-membranes, and is ejected through an orifice at the hind margin 
of the gill-covers. During the contact of the water with the gills, the 
blood which circulates in the train of this organ, and which common!' 
cates to them the red colour by which we recognise them, combines 
chemically with the oxygen of the air which the water holds in solu- 
tion when it flows freely at the ordinary temperature in presence of 
the air. The blood is thus oxygenized, or made arterial. 
The heart in fishes is placed between the inferior parts of th e 
branchial arch, and consists of a ventricle and an auricle (Fig. 34 li- 
lt corresponds with the right half of the heart in the Mammifera 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 without true ey e ' 
lids ; the skin usually passes 
over the ocular globe, and 
becomes from this point so 
transparent that the luminary 
rays traverse it. This light 
covering is all the eyelid be 
longing to fishes. The int e 
rior of the eye is covered by 
the membrane called choroid' 
the thin external leaf of which* 
in consequence of the p re 
sence of innumerable mi° r0 
scopic crystals, presents the appearance of a gold or silver-colour e 
coating, which gives to the iris that extraordinary brilliancy wine 
