FISHES. 
CHAPTEE 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 Yertebrates. 
