The dog-fish have all
their gills double, five on a side; and the sword-fish has eight
double gills. So much for the number of gills as found in fishes.
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.
All fishes are saw-toothed excepting the scarus; and the teeth in
all cases are sharp and set in many rows, and in some cases are placed
on the tongue. The tongue is hard and spiny, and so firmly attached
that fishes in many instances seem to be devoid of the organ
altogether. The mouth in some cases is wide-stretched, as it is with
some viviparous quadrupeds....
With regard to organs of sense, all save eyes, fishes possess
none of them, neither the organs nor their passages, neither ears
nor nostrils; but all fishes are furnished with eyes, and the eyes
devoid of lids, though the eyes are not hard; with regard to the
organs connected with the other senses, hearing and smell, they are
devoid alike of the organs themselves and of passages indicative of
them.
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