350 



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P/wtograph by J. Arthur Button 

 FIG. 133. Scale of perch, showing ctenoid margin and basal radii. 



is parallel to that of the paired limbs of vertebrates ; fins 

 are more primitive than feet, yet the whale has ac- 

 quired fins, although undoubtedly having an ancestor 

 with legs. It is also easy to observe, when we come to 

 study fish scales, that the so-called ctenoid scales are of 

 very different types, often having little in common. 1 



1 Scales may be prepared for study as follows : Remove them from the 

 middle of the side of the fish, trying to avoid regenerated scales, which have 

 the central sculpture imperfect ; place them, while wet, on a glass slide, first 

 removing the skin which covers the part which was exposed ; put on a square 

 cover glass, or if the scales are large, a second slide ; use a clamp to hold down 

 the cover glass, or two or three^ if a second slide has been used ; put on two 



