238 GENERAL ZOOLOGY 



wonder, then, that perch usually exceed other fishes in num- 

 bers. They are far too versatile to be kept down by 

 scarcity, or other variations in the food supply. 



When food enters a perch's mouth, it is taken charge of 

 by the digestive system, and passes down the esophagus to 

 be digested and absorbed in the stomach and intestine. 

 There are three little pockets, or caeca, which enter the 

 intestine just behind the stomach and help to increase the 

 surface. The liver, pancreas, and intestinal glands pour 

 their secretions into the intestine to be mixed with the food. 

 Undigested parts collect in the rectum and are eliminated 

 through the anus. Absorbed food enters the circulatory 

 system and in the blood is distributed to all parts of the 

 body. The blood is pumped by the heart to the gills, where 

 aeration takes place. It then passes through the four aortic 

 arches (Fig. 105) before passing to the head and to the great 

 dorsal aorta which distributes it to all parts of the body. 

 All vessels leading away from the heart (arteries) end in 

 minute capillaries, and these in turn open into veins which 

 carry the blood back to the heart, to be pumped round the 

 circuit again. In short, the perch has a " closed" blood 

 system which carries food, waste products, and oxygen. 



The important organs of respiration in a perch are the 

 gills and the air-bladder. There are four pairs of gills, 

 each bearing a double row of soft finger-like branchial 

 filaments, through the walls of which aeration takes place. 

 As water passes over the filaments, the blood in their 

 capillaries gives up its waste products and acquires oxygen. 

 The air-bladder serves as a storage reservoir for oxygen 

 and enables a perch to invade stagnated regions without 

 danger of suffocation. It contains a high percentage of 

 oxygen and this reserve is used when the surrounding water 

 does not contain a sufficient supply. 



Waste products accumulating in the blood are carried to 

 the kidneys for elimination from the body. A kidney con- 

 sists of a vast collection of uriniferous tubules, or nephridia, 

 which are individually somewhat like those described in the 



