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There is one particular organ of the fish,
however, that calls for more than passing
notice. This is the ' swim bladder,' a long,
double-lobed transparent affair, connected to
the stomach at one end and the head at the
other. It is filled with gases, principally
oxygen and nitrogen, and its chief function
is to maintain the fish's balance. This is
effected by involuntary muscular pressure on
the part of the fish-when the fore-end is
contracted the tail part is lighter and so the
head dips down ; when the posterior end is
contracted then the head rises. From chills,
congestion and other causes the swim bladder
may be deranged and the fish will be unable
to swim other than on its side, or it may lose
its buoyancy, when it cannot raise itself from
the bottom without an effort. On the other
hand, if it becomes over-inflated the fish must
float on the surface.
So much for the interior of the fish ; the
exterior is too well known to require description.
In the following list of fishes, where the external
parts are referred to, they will be given the
names most commonly used among aquarists ;
in the accompanying diagram (Fig. 11) the
same names are used.
All the fishes described in the following
pages, unless marked with an asterisk, are
sufficiently hardy to withstand ordinary winter
conditions out of doors. The less hardy fishes
should be removed to indoor aquariums at
the commencement of the cold season.
Over ninety per cent, of the pond fishes are
of the Carp persuasion and so all their fins
are soft; none are spiny. The British carps
fall into three main groups which may be
recognised, roughly, as follows : Dorsal fin
long, anal fin short-Carp and Crucian Carp.
Both short-Gudgeon, Tench, Dace, Roach,
Minnow and Rudd. Dorsal fin short, anal
fin long-Common Bream and White Bream.
The Common Carp.
In the Middle Ages
carp were bred largely for the table, most
monasteries and large houses had their own
fish stews, and on the Continent to-day this
fish is esteemed as an article of diet. Owing
to their usual dun colour they are of little
interest to the pond keeper except the large
specimens. The smaller kind, particularly if
the water is at all greenish, may remain
invisible for days. They are said to attain
a great age, two hundred years old or more ;
the largest specimen found in this country
was three feet in length and weighed twenty-
five pounds.
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