314 GENERAL ZOOLOGY 



flow of the blood. In addition to the blood, a white fluid 

 (lymph) circulates through the body in vessels called lym- 

 phatics. The function of the lymph is supplementary to that 

 of the blood. 



The Respiratory System. The principal organs of respira- 

 tion are the gills, eight in number, four on either side. Each 

 gill consists of a bony arch, on the anterior surface of which 

 are teeth-like gill-rakers; on the posterior surface are the 

 delicate gill filaments. In this position the filaments are 

 constantly bathed by a current of water, which passes from 

 the mouth cavity out beneath the operculum. In the dorsal 

 part of the body cavity is a large air bladder (Fig. 160). In 

 the lining of the wall of the air bladder is a network of 

 blood vessels, grouped into gland-like "red bodies." By 

 the absorption and formation of gas by these blood vessels 

 the weight of the fish can be maintained nearly equal to 

 that of the water it displaces. The air bladder is probably 

 useful also as a reservoir of air, for it has been found that in 

 a perch suffocated in stagnant water the oxygen in the air 

 bladder, which normally amounts to about one fifth of the 

 volume of the inclosed gas, had been entirely absorbed and 

 replaced by carbon dioxide and nitrogen. In some fishes 

 the air bladder communicates with the alimentary canal by 

 means of a tube called the pneumatic duct. In the perch 

 this duct is present in early life, but it soon closes, remain- 

 ing, however, as a fibrous cord. 



The perch, like other fishes, is usually spoken of as cold- 

 blooded, since its body temperature is little above that of 

 the surrounding medium. Compared with the higher ver- 

 tebrates, — the birds, for example, — very little oxygen is 

 required for respiration, and the circulation is compara- 

 tively slow. 



The Excretory System. The principal organs of excretion 

 are the kidneys, placed just above the air bladder and below 



