PISCES 239 



earthworm (page 206), but are not metamerically arranged. 

 The ureter is a duct which carries urine from the kidney 

 outside the body. 



Self-protection. The chief enemies of perch are large 

 predaceous fishes (the pickerel, bass, dogfish) and man. 

 Piscivorous enemies usually lurk in the shore vegetation, 

 and capture their prey by fierce and sudden rushes. From 

 these perch are protected to some extent by various habits 

 and adaptations, but from man, with his nets, hooks, 

 spears, and traps, there is no escape. 



A perch is well protected from slight injuries by its cover- 

 ing of scales. If caught by a pickerel, or some other enemy 

 (Fig. 94, A), it spreads its fins so as to inflict painful pricks 

 when the bony rays are touched. There are also sharp saws 

 along the edges of the opercula which may cause severe 

 cuts. These are made more effective by spreading the 

 opercula to their greatest extent when danger threatens. 

 The slimy skin and the backwardly directed scales make a 

 perch difficult to hold, for with every wriggle the body 

 tends to slip forward a little. 



Fishes have a row of peculiar sense organs extending 

 along either side of the body. These are known as the lat- 

 eral lines, and are believed to serve for the perception of very 

 slow vibratory movements in the water, such as would 

 result from the splashing of falling bodies. The lateral 

 line organs would therefore be of value in giving notice of 

 disturbances in the water. 



The body of a perch is " countershaded " by being paler 

 below, so that when light strikes it from above, the dark 

 back with its brighter illumination is equal to the lighter 

 belly in the shadow. The dark vertical bands blend readily 

 with the lights and shadows in the shore vegetation, or 

 merge with the ripples on the surface of the water when 

 seen from below. A perch, like other fishes, may change 

 its color somewhat to suit the background on which it 

 customarily rests. Such changes are brought about by the 

 expansion and contraction of certain pigmented cells in the 



