FISHES, Ilf 
that the fish is indebted for its motion : the tail 
contributes greatly towards it ; and, by its alternate 
impulse, advances the head and all the rest of the 
body, in an infinitely better manner than that by 
which a progressive motion is given to a boat by an 
oar placed at the stern, and worked about alternately 
to the right and left. 
The uniform action of the lips and gills of a 
fish is doubtless analogous to our breathing : it is 
thus performed The fish first takes in a quantity 
of water by the mouth, which it drives backwards 
with a power sufficient to lift up the flap or gill 
cover, and force it out behind. During its pas- 
sage through the feather-like process of the gills, 
the greater part of the air contained in the water 
is left behind, to perform its part in the animal 
ceconomy. That this air is essentially necessary to 
support the life of the animal is sufficiently evi- 
dent ; for if the fish be placed in distilled water, or 
under the exhausted receiver of an air pump, it soon 
dies : therefore when a pond is frozen over it is 
necessary to break holes in the ice, that the fish 
may come to the surface and breathe. To stop the 
action of the gills is certain death to the animal, 
who presently becomes convulsed, although in its 
native element. 
The generality of fishes are provided with a blad- 
der or bag of air, which enables them to rise or sink 
in proportion to its being dilated or contracted. 
The body of a fish, which is heavier than the 
quantity of water whose place it fills, must always 
