FISHES. 



505 



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 edge is mobile, 

 and they are raised or lowered to serve the purposes of respiration ; 

 under this species of covercle are the gills, or branchiae. These usually 

 consist of many rows of thin membranous plates, hung 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 is 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 these organs, and which communi- 

 cates to them the red colour by which we recognize 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 fit for respiration. 



The heart in fishes is placed between the inferior parts of the 

 branchial arch, and consists of a ventricle and an auricle (Fig. 342). 

 It 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 de- 

 livered into one great artery, 

 which creeps along the verte- 

 bral column. 



The eye in fishes is gene- 

 rally very large we may even 

 say enormous relative to the 

 size of the head and with- 

 out true eyelids ; 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 belonging to fishes. The inte- 

 rior of the eye is covered by the membrane called clioroid, the thin 

 external leaf of which, in consequence of the presence of innumerable 



o 



C !..... 



Fig. 343. A Fish's Eye. 



t, crystallized pupil; ee', cornea; mm', choroid; ft, pos- 

 terior chambers ; c, optic nerve. 



