308 GENERAL ZOOLOGY 



circulation. Valves in the heart and in the course of the venous 

 circulation prevent the backward flow of the blood. In addi- 

 tion to the blood, a white fluid (lymph) circulates through 

 the body in vessels called lymphatics. The function of the 

 lymph is supplementary to that of the blood. 



The principal organs of respiration 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 cur- 

 rent 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. 153, 19). 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, remaining, however, as a fibrous cord (Fig. 153, 18). 



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 vertebrates, 

 the birds, for example, very little oxygen is required for 

 respiration, and the circulation is comparatively slow. 



The principal organs of excretion are the M^j/s(Fig.l53,24), 

 placed just above the air-bladder and below the back-bone. 

 From the kidneys two tubes, the ureters (Fig. 153, 25), lead, 



