48 INTRODUCTION. 



FISHES, IN RELATION TO OTHER ANIMALS. 



By people altogetlier uneducated, every animal 

 is regarded as a fish which is an inhabitant of 

 the water; and although persons somewhat bet- 

 ter informed do not use the term in quite so 

 comprehensive a sense as this, but exclude the 

 animals commonly called shell-fish, belonging to 

 those classes wliich are destitute of an internal 

 skeleton, they still commonly embrace under this 

 title all the inhabitants of the waters w^hich possess 

 such a skeleton, and wdiich move by fins. Even 

 this, however, is a more extensive sense than that 

 in which the word Fish is employed by Naturalists, 

 who confine this appellation to an animal which, 

 besides being possessed of the above-mentioned 

 characters, breathes by means of gills, and not by 

 true lungs, has a single instead of a double heart cir- 

 culating cold instead of warm blood. Now, this is not 

 the case with whales, dolphins, porpoises, and many 

 other tribes of aquatic animals ; all of which breathe 

 by lungs, have a double heart, aie warm-blooded, 

 and are, consequently, with propriety, excluded from 

 the class of fishes. The whale, and other aquatic 

 animals, resemble the mammalia in their structure ; 

 and it is, accordingly, in the same class that they 



