CHAPTER XXI 

 K*HiJkx> IV " ^xx--4-^**-^ LfH-j'tMj} pl; 

 t-J-J .CLASS PISCES FISHES. 



Order i. Elasmobranchii or Selachii, cartilaginous fishes, e.g., skates 



and sharks. With these may perhaps be ranked the 



-^ Holocephali (Chimera and Callorhynchtts.} 



(jz. Ganoidei, such as sturgeon (Acipenser] and bony 



(Lepidosteus] ; numerous extinct genera, only seve 

 extant. 



3. Teleostei, bony fishes, such as cod, herring, salmon, 



flounder, eel. 



4. Dipnoi, mud fishes : Ceratodus^ Protopterus^ Lepidosiren. 



The Dipnoi, or double breathers, are so distinct that some would re- 

 move them from among Fishes, and place them as an independent class 

 between Fishes and Amphibians. 



FISHES form the first markedly successful class of Verte- 

 brates. For while the Tunicates are numerous, most of 

 them are degenerate ; the level attained by the lancelet is 

 represented by, at most, two or three closely related genera ; 

 and the Cyclostomata are few in number and partially re- 

 trogressive. 



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 



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