Only a very few out of each million
become "grown-ups"--the rest are eaten by enemies, or smothered in mud
or sand. In a year or so they are as big as half-a-crown. In five years
they are fine, fat grown-up Oysters--that is to say, if they have not
been dredged up from their bed and sent to market.
Their shells open and shut like a trap. You may have seen a picture of
an inquisitive mouse trapped by an Oyster. Thinking to have a nice taste
of Oyster, the mouse had poked its head into the open shells, but they
were snapped together, and the mouse was firmly held in the trap.
Between the hinge of the two shells there is a pad, which acts like an
elastic spring, and forces the shells open. The Oyster can close them by
means of a strong muscle. They are its only defence, so it closes them
at the least hint of danger.
Even these thick walls are sometimes of no avail, as we saw in our talk
on "Five-fingered Jack." We saw how the starfish forces the shells open
with the help of its strong tube-feet. The whelk and his cousins know
how to bore a hole in the shell, and suck out the helpless Oyster.
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