84 The Vertebrate Organ Systems 



More familiar fish such as the sunfish, perch, and trout have rounded 

 scales which may be of one of two types : cycloid or ctenoid. They 

 overlap one another in a manner similar to that of shingles. Cycloid 

 scales (Fig. 22,B) are rounded in shape and thicker in the center. 

 Ctenoid scales (Fig. 22,A) are very similar to these except for their 

 possession of a row of small teeth along the outer margin. Both of 

 these types of scales are dermal in origin. As the fish grows in size, 

 the scale also increases in size. Since the yearly increments of growth 

 alternate with periods of little or no growth, annual rings can be 

 seen. The age of the fish can be estimated from these rings. 



Fig. 22. — Types of fish scales. A, Ctenoid; B, cycloid; C, longitudinal 

 section of placoid scale; D, ganoid. 



Among fossil amphibians, there were some with scales ; nearly all 

 modern forms, however, are completely scaleless. 



The scales of the reptiles are quite different in their origin from 

 those of the fish. A thickening first appears in the dermis; over this 

 the stratum, germinativum begins to divide rapidly to produce flattened, 

 cornified cells which form into scales or scalelike areas. Unlike the 



