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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 of osseous plates or bucklers. With few exceptions fishes are oviparous.

However, 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.