INTEODUCTOEY EEMAEKS. 



According to the views generally adopted at present, all 

 those Vertebrate animals are referred to the Class of Fishes, 

 which living in water, breathe air dissolved in water by means 

 of gills 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 in space and time, is termed Ichthyology.^ 



^ From Ix&vs, fish, and \oyos, doctrine or treatise. 

 B 



