2 6 EVOLUTION OF SCALES 



plete but flexible body armouring, made additionally 

 strong by an interlocking articulation of its elements 

 (24 A). 



In this form the enamel-like surface layer ("ganoine") 

 of the ganoid plates is believed to be derived from the 

 dentine substance, and not deposited by the epidermis : 

 they bear numerous shagreen denticles during an early 

 period of life. 



The most complete encasement of a fish's body by 

 dermal plates is shown in Fig. 26, v. p. 172. The met- 

 ameral conditions have here permitted extended fusions, 

 a single dermal plate enclosing the upper, or lower 

 division of the muscle-plate of either side. 



The thin horn-like scales of the majority of recent 

 fishes, e.g. carp or perch (Fig. 31 A) are probably 

 derived from a condition not widely different from that 

 of Fig. 24. They take their origin, however, in a deeper 

 layer of the derma, thence grow outward, arising as 

 if from deep and flattened pockets. Their substance 

 becomes horn-like, rather than limy, and they enlarge in 

 outline, rather than in thickness. Their hinder margins, 

 often crenulate, overlap widely the neighbouring scales ; 

 their arrangement is in direct relation to the underlying 

 metameres, and their surface is densely slime-coated. 

 The dermal armouring they thus constitute is both light, 

 tough, and flexible. 



Degeneration of scales is shown to occur in many 

 types. In some forms their size may become micro- 

 scopic (eel), in others enormously enlarged (mirror carp). 

 In cases they may entirely disappear (leather carp). 

 The fusions of the dermal plates of the trunk-fish or 

 of the sea-horse (p. 177) are probably degenerate. 



