are mostly covered with a kind of horny
scales; but some are almost entirely without them, or have them so
minute as to be almost invisible; as is the case with the eel. The
object of these is to preserve them from injury by the pressure of the
water, or the sudden contact with pebbles, rocks, or sea-weeds. Others,
again, are enveloped in a fatty, oleaginous substance, also intended as
a defence against the friction of the water; and those in which the
scales are small, are supplied with a larger quantity of slimy matter.

203. THE RESPIRATION OF FISHES is effected by means of those comb-like
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.

204. THE POSITIONS OF THE TEETH OF FISHES are well calculated to excite
our amazement; for, in some cases, these are situated in the jaws,
sometimes on the tongue or palate, and sometimes even in the throat.
They are in general sharp-pointed and immovable; but in the carp they
are obtuse, and in the pike so easily moved as to seem to have no deeper
hold than such as the mere skin can afford. In the herring, the tongue
is set with teeth, to enable it the better, it is supposed, to retain
its food.

205. ALTHOUGH NATURALISTS HAVE DIVIDED FISHES into two great tribes, the
_osseous_ and the _cartilaginous_, yet the distinction is not very
preci