Scales and Fins. 1 3 



nary teeth. 1 Many scales are of beautiful forms, and 

 they vary very much in outline and surface, some- 



FlG. 4. 



Cycloid scale of roach magnified ; seen in section A, and on surface B. 



times being flat and smooth-edged (fig. 4), or else 

 spinose, ridged, or comblike (fig. 5). These structures 

 can be examined and their varieties observed with the 

 aid of a pocket-lens. The scales of the pike, sole, 

 and perch are especially characteristic forms. 



10. Fins. Along the middle line of the body of a 

 fish there are usually developed extensions of the dermal 



1 It would perhaps be more correct to say that teeth are 

 really in nature a special set of dermal papilla? of the same 

 nature as those which cover the surface of the skin in some 

 fishes, ?nd which, covering the jaw arches, are set apart for 

 grasping and dividing food ; but the relationship is put con- 

 versely, as the tooth form is the more familiar. 



