CHAPTER XXII 



Class PISCES— FISHES 



Sub-Class I. Elasmobranchii : — 



Order Plagiostomi (skates and sharks). 



Order Holocephali {Chimcera and Callorhynchus). 



Several extinct orders, e.g. Acanthodei. 

 Sub-Class II. Teleostomi : — 



Order Crossopterygii [Polypterus). 



Order Chondrostei, e.g. sturgeon. 



Order Holostei, e.g. bony pike. 



Order Teleostei, the great majority of living fishes. 

 Sub-Class III. Dipnoi : — 



Ceratodus, Protopterus, 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 Cyclostomes 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. With their typically 

 wedge-like bodies, supple muscular tails, fin-like limbs, and 

 the like — they are well adapted to the medium in which 

 they live. 



Their success may be read in the immense number of 

 individuals, 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 predomin- 

 ance of the modern Bony Fishes ; and, furthermore, in 



the plasticity with which many types appear to have 



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