FISHES 



CHAPTER I 

 GENERAL CHARACTERISTICS CLASS Pisces 



ALTHOUGH in popular language lampreys are included among fishes, while un- 

 til quite recently the lancet was very generally placed by zoologists in the same 

 class, it now seems preferable to make each of these the representative of a distinct 

 class, and the true fishes can consequently be defined with greater precision. In 

 this somewhat restricted sense fishes may be described as cold-blooded vertebrate an- 

 imals, adapted for a purely aquatic life, and breathing almost invariably by means 

 of gills alone. They have a heart consisting generally of only two chambers (three 

 in the lungfishes); the limbs, if present, are modified into fins; there are unpaired 

 median fins, supported by fin rays; and, as in all the higher classes, the mouth is 

 furnished with distinct jaws. The skin may be either naked, or covered with scales 

 or bony plates. As a rule, fishes lay eggs; and the young do not undergo a distinct 

 metamorphosis. 



With the Tailed Amphibians the class is very closely connected by means of 

 the lungfishes, which are furnished not only with internal gills, but likewise with 

 functional lungs, and during the early part of their existence with external gills; 

 while these fishes also differ from the other members of the class in that the nostrils 

 communicate posteriorly with the cavity of the mouth, as in the higher Vertebrates. 



Although the bony fishes of the present day form a specialized side branch, which 

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