The use of gills, however, as has been
explained in the treatise on Respiration, is limited to such animals
as do not breathe; for no animal can possibly possess gills and at the
same time be a respiratory animal. In order, therefore, that these
Cetacea may discharge the water, they are provided with a blow-hole.
This is placed in front of the brain; for otherwise it would have
cut off the brain from the spine. The reason for these animals
having a lung and breathing, is that animals of large size require
an excess of heat, to facilitate their motion. A lung, therefore, is
placed within their body, and is fully supplied with blood-heat. These
creatures are after a fashion land and water animals in one. For so
far as they are inhalers of air they resemble land-animals, while they
resemble water-animals in having no feet and in deriving their food
from the sea. So also seals lie halfway between land and water
animals, and bats half-way between animals that live on the ground and
animals that fly; and so belong to both kinds or to neither. For
seals, if looked on as water-animals, are yet found to have feet; and,
if looked on as land-animals, are yet found to have fins. For their
hind feet are exactly like the fins of fishes; and their teeth also
are sharp and interfitting as in fishes.
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