II 
THE EVOLUTION OF STRUCTURES CHAR- 
ACTERISTIC OF FISHES 
Ir will be the object of the present chapter to review 
the gradations which occur in some of the characteristic 
structures of fishes and to follow in some degree the 
mode of their evolution. We may thus review the con- 
ditions of the (1) gills, (2) skin defences (including teeth), 
(3) fins, and (4) sense organs. 
The structures of the immediate ancestor of the fishes 
cannot be definitely inferred: the form, however, must 
have been elongate and transversely jointed, for this con- 
dition seems to have existed remotely before fishes — in 
the broadest sense — had become evolved. This segmen- 
tation, or metamerism, of the vertebrate body is best shown 
among water-living forms, sometimes indeed in so perfect 
a way as to suggest the jointed condition of an earth-worm. 
The segmented body of the eel-shaped Lamprey, shown 
in section in Fig. 69, illustrates an interesting condition 
of vertebrate metamerism. Its entire body, from the 
head region to the base of the tail, is composed of drum-” 
like segments which closely correspond to one another 
in size and in component structures. Each segment 
thus resembles its neighbours in its equal portions of the 
vertebral column, digestive tract, nerve tube, muscle > 
plates and blood canal, and in the arrangement of these 
parts with reference to bilateral symmetry. Motion in 
this form requires no more of each segment than that its 
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