Fishes,

then, as has been observed, have no breasts and no
passage for the genitals visible externally. But they have an
exceptional organ in the gills, whereby, after taking the water in the
mouth, they discharge it again; and in the fins, of which the
greater part have four, and the lanky ones two, as, for instance,
the eel, and these two situated near to the gills. In like manner
the grey mullet-as, for instance, the mullet found in the lake at
Siphae-have only two fins; and the same is the case with the fish
called Ribbon-fish. Some of the lanky fishes have no fins at all, such
as the muraena, nor gills articulated like those of other fish.