INTEODUCTOEY EEMAEKS. 



AccoEDiNG to the views generally adopted at present, all 

 those Vertebrate animals are referred to the Class of Fishes, 

 which living ia water, breathe air dissolved m water by means 

 of gnis or branchiae ; whose heart consists of a single ventricle 

 and single atrium ; whose limbs, if present, are modified into 

 fins, supplemented by unpaired, median fins ; and whose skin 

 is either naked, or covered with scales or osseous plates or 

 bucklers. "With few exceptions fishes are oviparous. How- 

 ever, there are not a few members of this Class which show a 

 modification of one or more of these characteristics, as we 

 shall see hereafter, and which, nevertheless, cannot be separated 

 from it. The distinction between the Class of Fishes and that 

 of Batrachians is very slight indeed. 



The branch of Zoology which treats of the internal and 

 external structure of fishes, their mode of life, and their 

 distribution ia space and time, is termed Ichthyology.^ 



^ From IxSvs, fish, and \oyos, doctrine or treatise. 



B 



