FISHES. 



THIS class, lowest but one of the vertebrates, is of great 



interest and importance. Our British fishes are about two 



hundred in number, of which, roughly speak- 



n ! s * ing, rather over three-fourths are either marine 



species. 



or anadromous forms, the former passing their 

 whole existence in salt water, the latter entering rivers for 

 the purpose of spawning. A few, like the eel and flounder, 

 go down to the sea to spawn, passing most of their ex- 

 istence in fresh water, and these are termed " catadromous." 

 Of our fifty or more fresh-water fishes, a few, non-migratory 

 members of the salmon family for the most part, have 

 from time to time been introduced. As in the case of 

 birds, several divisions of the subject might have been 

 permissible in a small work of this kind, such as the sep- 

 aration of sea- and fresh- water forms, the commercial and 

 non-commercial fish, or like arbitrary groups. It has been 

 thought best, however, to follow the systematic classifica- 

 tion in general use, though an attempt has been made to 

 give some indication, by means of two familiar signs, of the 

 division between the purely fresh-water and purely marine 

 forms, as well as of the third group, embracing members 

 of widely different orders, that are able to exist in both 

 fresh and salt, some entering one or the other at regular 

 intervals and with a fixed purpose, others rather frequent- 

 ing brackish estuaries at all seasons indifferently. 



The definition of a fish, which it is here necessary 



