Fishes, again, resemble these two groups in all their
parts, excepting that, while these, being land animals, have a lung,
fishes have no lung, but gills in its place. None of these animals,
excepting the tortoise, as also no fish, has a urinary bladder. For
owing to the bloodlessness of their lung, they drink but sparingly;
and such fluid as they have is diverted to the scaly plates, as in
birds it is diverted to the feathers, and thus they come to have the
same white matter on the surface of their excrement as we see on
that of birds. For in animals that have a bladder, its excretion
when voided throws down a deposit of earthy brine in the containing
vessel. For the sweet and fresh elements, being light, are expended on
the flesh.
Among the Serpents, the same peculiarity attaches to vipers, as
among fishes attaches to Selachia. For both these and vipers are
externally viviparous, but previously produce ova internally.
The stomach in all these animals is single, just as it is single
in all other animals that have teeth in front of both jaws; and
their viscera are excessively small, as always happens when there is
no bladder. In serpents these viscera are, moreover, differently
shaped from those of other animals. For, a serpent's body being long
and narrow, its contents are as it were moulded into a similar form,
and thus come to be themselves elongated.
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