II 



THE EVOLUTION OF STRUCTURES CHAR- 

 ACTERISTIC OF FISHES 



It 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 (i) 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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