120 A COURSE ON ZOOLOGY. 



driven out. When this happens the \vater is no longer 

 able to sustain the life of fish, and the experiment is easy 

 to perform. It must not be forgotten, however, that 

 water dissolves a certain quantity of air with great ease, 

 and that when the dissolved air has been expelled by 

 boiling it is soon absorbed again when the water is ex- 

 posed to the air. 



The respiratory organs of fishes are located at the 

 back and sides of the mouth, in a cavity that communi- 

 cates with the exterior by two lateral openings, called 

 the gill-openings, These organs, called the gills or 

 branchiae, are generally composed of plates arranged like 

 the teeth of a comb upon four pair of bony arches, called 

 the branchial arches. They are directly under the bony 

 plates that form the cheeks of the fish, and that attain 

 a great development and are very distinctly striated in 

 the carp. These are the opercula or gill-covers, and 

 when they are raised the gill-openings gape behind them. 



Certain rays, three only in number in the carp, sup- 

 port a membrane which closes the respiratory cavity 

 below and at the sides. 



The branchial plates receive innumerable little blood- 

 vessels ; the water which is drawn in at the mouth 

 comes in contact with these respiratory organs, yielding 

 to the blood through the thin membrane separating the 

 two liquids a part of the oxygen it holds in solution, 

 and taking in exchange carbonic acid gas that must 

 be removed. After having served its purpose for res- 

 piration the water is then expelled through the gill- 

 openings. 



Fishes thus respire in reality like other vertebrates, 

 that is, they absorb oxygen and eliminate carbonic acid, 

 but their respiratory organs are so constituted that they 



