CHAPTER XXII. 



Class PISCES— FISHES. 



Elasmobranchii or Selachii, cartilaginous fishes, e.g. skates, dog-fishes, 



and sharks. 

 Holocephali {Chimcera, Callorhynchus , and Harriotta). 

 Ganoidei, such as sturgeon (Acipenser) and bony pike (Lepidosteus) ; 



numerous extinct genera, only seven extant. 

 Teleostei, bony fishes, such as cod, herring, salmon, flounder, eel. 

 Dipnoi, mud-fishes : Ceratodus, Protopterus, Lepidosiren. 



Besides these there are several extinct orders. The Dipnoi, or double 

 breathers, are so distinct that some would place them as an independent 

 class between Fishes and Amphibians. 



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 Ganoid order has 

 been followed by a yet richer predominance of the modern 

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

 many types appear to have assumed particular specialisations, 



