organs which are placed on each side of the neck, and which are called
gills. It is curious to watch the process of breathing as it is
performed by the finny tribes. It seems to be so continuous, that it
might almost pass for an illustration of the vexed problem which
conceals the secret of perpetual motion. In performing it, they fill
their mouths with water, which they drive backwards with a force so
great as to open the large flap, to allow it to escape behind. In this
operation all, or a great portion, of the air contained in the water, is
left among the feather-like processes of the gills, and is carried into
the body, there to perform its part in the animal economy. In proof of
this, it has been ascertained that, if the water in which fishes are
put, is, by any means, denuded of its air, they immediately seek the
surface, and begin to gasp for it. Hence, distilled water is to them
what a vacuum made by an air-pump, is to most other animals. For this
reason, when a fishpond, or other aqueous receptacle in which fishes are
kept, is entirely frozen over, it is necessary to make holes in the ice,
not so especially for the purpose of feeding them, as for that of giving
them air to breathe.