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Fishes, living as they do in an element totally
different from that in which most other animals
live, are difficult to understand. We can
appreciate better the outlook of pet birds,
say, because they have their being in the
familiar air. That is why, partly, the study
of fishes is so perplexing, especially in regard
to keeping them in captivity.
Nevertheless, we are cognisant of the
functions of the fish's principal organs, and
given this knowledge the aquarist must, perforce, rely on his own common sense.
Fishes do not possess lungs, excepting a
few tropical species ; instead they extract
oxygen from the water with their branchiae.
These consist of six rods of bone situated on
each side of the throat, covered with mucous
membrane well supplied with blood vessels.
The water is taken in by the mouth and passes
over the branchiae and makes its exit at the
gill openings. Blood is pumped from the
heart to the branchiae, where it is purified,
and then distributed to all parts of the
creature's body. The process, you will observe,
is the reverse to that in human beings. With
us it is purified blood that is issued by the heart
to the rest of the body ; with the fish the
blood is cleansed after leaving the heart.
The branchial apparatus of the fish corresponds
to our lungs.
The nervous system is not very complicated,
neither are there many nerves to trouble the
fish, in fact quite large areas are devoid of
nerves and there is little doubt that pain,
as we know it, is almost unknown to the
finny tribe. The surface nerves seem to be
concentrated to one line on each side of the
body, forming the lateral line. This is not
very prominent in the ordinary goldfish, but
in other species can be seen as a dark line
running from the gills to the tail, approximately along the middle of the fish. The
function of this line of nerves is that of receiving
vibrations, in other words it acts as a kind of
auxiliary ear.
The Carp family are peculiar in having
no teeth in their mouth ; the masticating
apparatus is situated in the throat and consists of two irregularly shaped bones and, in
reality, the food is eaten after it is swallowed.
Other fishes, especially those of carnivorous
habits, may have teeth on the upper and lower
jaws, the tongue-this, by the way, is immovable-and the roof of the mouth.
From the branchial arches in the mouth
of most fishes hang comb-like fringes and these
serve as sieves. As the water passes through
the mouth to the gill openings many minute
organisms are retained by these ' gill-rakers '
and so the fish receives an addition to its diet.
The alimentary system differs very little from
that of the higher animals.
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