FISHES. 



CHAPTER I. 



General Characteristics, — Class Pisces. 



ALTHOUGH in popular language lampreys are included among fishes, while until 

 quite recently the lancelet 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 

 animals, 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 lung-fishes) ; 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 lung-fishes, 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. 



