26 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, ¢.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. 
