CHAPTER XXII. 



CLASS PISCES FISHES. 



Order I. — Elasmobranchii or Selachii, cartilaginous fishes, e.g., skates 

 and sharks. With these may perhaps be ranked the 

 Ilolocephali [Chiinirra 3.vlA Callorhynchus.) 



2. — Ganoidei, such as sturgeon (Acipenser) and bony pike 

 {Lepidosteus) ; numerous extinct genera, only seven 

 extant. 



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

 flounder, eel. 



4. — Dipnoi, mud fishes; CeratodiLS, Protopterits, 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 



