You will
see that it is made, like your backbone, of a number of small bones which
move upon each other so easily that they enable the fish to turn itself
rapidly, as you see it does. The wonderful way in which these tiny bones
are fitted together by what is called the "ball and socket arrangement" may
best be seen in a large fish, such as the salmon; but a sardine's frame is
made in the same beautiful way.
The scales, overlapping each other as they do, serve to protect the fish
in its journey through watery ways, and their smooth, polished surface
rendered slippery by a sort of natural oil, helps it to move quickly. We
have imitated the scales of a fish in the way in which we arrange slates
and tiles to keeps our houses dry. You know how the slates on the roof of
your house overlap each other, so closely that no rain can get between
them.
When I tell you that there are said to be nine thousand different kinds
of fish in all parts of the world, you will understand that even in a
large aquarium you can see but few varieties. In England alone hundreds of
fresh-water fishes are known, while those whose home is in the sea are much
more numerous still.
It has been found that if fresh-water fish is taken out of its natural
element and put at once into the sea, it will die. But there are some fish,
like the salmon, which live in the sea, but go up the rivers to lay their
eggs, and then back again to their proper home; taking "change of air," as
it were, but taking it gradually, and not plunging into a foreign country
all at once.
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