Again,

fishes differ from other animals in more ways than as
regards the gills. For they are not covered with hairs as are
viviparous land animals, nor, as is the case with certain oviparous
quadrupeds, with tessellated scutes, nor, like birds, with feathers;
but for the most part they are covered with scales. Some few are
rough-skinned, while the smooth-skinned are very few indeed. Of the
Selachia some are rough-skinned and some smooth-skinned; and among the
smooth-skinned fishes are included the conger, the eel, and the tunny.