240 GENERAL ZOOLOGY 



skin. If kept on a black background for a month, a perch 

 will become very dark, and if allowed to remain on a white 

 bottom, will assume a very pale color. 



Perch have considerable power of regenerating parts of 

 the body after loss through accident or injury. A fin may 

 be entirely lost, and will grow again. Many fish die, how- 

 ever, after comparatively slight injuries because fish moulds 

 (Saprolegnia) , bacteria, and protozoans gain entrance to the 

 body through the wounds. Perch are very commonly 

 infested by parasites. Nearly every individual examined 

 will be found to have the white " bladders " of larval tape- 

 worms in the liver or among the visceral organs. These 

 cysts if taken into a bass or pickerel will grow into mature 

 tapeworms. The perch's muscles, bile ducts, caeca, and 

 intestine are also often infested with trematodes. 



Perch adjust themselves readily to conditions in the 

 temperate regions of the earth. In winter most of them 

 retreat into the depths of lakes and rivers, where the tem- 

 perature is low but conditions are uniform and small 

 aquatic animals furnish a plentiful food supply. In migrat- 

 ing from one level to another modifications of the internal 

 pressure in the body are necessary, and such adjustments 

 are brought about largely by slow variations of the gas in 

 the air bladder. Perch remain active throughout the year 

 and may be caught on a hook and line at any season. 



Race Preservation. Perch, and all other teleosts, are of 

 separate sexes. Both the males and females have large 

 gonads (testes or ovaries) and are nearly mature before 

 cold weather begins, During the winter both sexes become 

 fully ripe and are ready to begin spawning in April and 

 May. After the ice goes out the males move into shallow 

 water and are soon followed by the females. When a fe- 

 male comes inshore, she is attended by one or more males. 

 As she deposits her "egg string" (Fig. 94, C), which is a 

 crinkled tube of jelly containing 2000 or 3000 eggs, the male 

 squirts sperm over it and fertilization is thus accomplished. 



