CHAPTER XXII. 



CLASS PISCES FISHES. * 



Sub-Class I. ELASMOBRANCHII :- 



Order Plagiostomi (skates and sharks). 



Order Holocephali {Chimccra and CaUorhynchns and several 

 extinct orders). 



Sub-Class II. TELEOSTOMI :- 



Order Crossopterygii (Polyptenis]. 



Order Chondrostei, e.g. sturgeon. 



Order Holostei, e.g. bony pike. 



Order Teleostei, the great majority of living fishes. 



Sub-Class III. DIPNOI :- 



CeratoduS) Protoptents, and Lepidosiren, and many extinct forms. 



FISHES form the first markedly successful class of Verte- 

 brates. For though the Tunicates are numerous, most of 

 them are degenerate ; the level attained by the lancelets 

 is represented by, at most, two or three closely related 

 genera; and the Cyclostomata are also few in number. 



In the possession of a vertebrate axis and central nervous 

 system, in the general integration of their structure, and in 

 their great fecundity, Fishes have an easy pre-eminence over 

 their Invertebrate inferiors. As successfully adapted forms 

 with typically wedge-like bodies, supple muscular tails, 

 fin-like limbs, and the like they may well compare with 

 Birds in their mastery of the medium in which they live. 



Their success may be read in the immense number of in- 

 dividuals, species, and genera, not only now, but in the past ; 

 in the geological record which shows how the cartilaginous 

 Elasmobranchs have persisted strongly from Silurian ages, 

 or how the mysterious decadence of the " Ganoids " has 

 been followed by a yet richer predominance of the modern 

 Bony Fishes ; and, furthermore, in the plasticity with which 



