He can also live for a
considerable while out of the water, but all this while he keeps up
a dull moaning sound corresponding to the noise made by
air-breathing animals in general; furthermore, when sleeping, the
animal keeps his nose above water, and he does so that he may
breathe the air. Now it would be unreasonable to assign one and the
same class of animals to both categories, terrestrial and aquatic,
seeing that these categories are more or less exclusive of one
another; we must accordingly supplement our definition of the term
'aquatic' or 'marine'. For the fact is, some aquatic animals take in
water and discharge it again, for the same reason that leads
air-breathing animals to inhale air: in other words, with the object
of cooling the blood. Others take in water as incidental to their mode
of feeding; for as they get their food in the water they cannot but
take in water along with their food, and if they take in water they
must be provided with some organ for discharging it. Those blooded
animals, then, that use water for a purpose analogous to respiration
are provided with gills; and such as take in water when catching their
prey, with the blow-hole.
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