Of course the teeth wear away.
[Illustration: COWRIES.]
But only part of the toothed ribbon is used at a time, so there are
plenty of teeth behind the worn ones, ready to take their place.
The shell, as we have seen, is made of _limestone_. But the teeth are
made _of flint_. This is a hard substance, so hard that it is used for
striking sparks.
Now we will look at a shell-builder, the Whelk, who uses his flinty
tongue in quite another fashion. The Whelk does not care for a vegetable
dinner. He prefers to eat other molluscs--he is carnivorous, a
flesh-eater; but these other molluscs do not wait to be eaten. As the
enemy draws near they retire into their shells, and shut themselves up
as tight as they can. The Whelk, however, is a clever burglar; he knows
how to make a way into the hardest of shelly houses.
His front part--we might call it a nose--will stretch out to a fine
point; and it contains a rasping tongue even harder than that of the
Periwinkle. He sets to work. Moving the rasp up and down, he drills a
neat round hole in the shell of the animal he is attacking. No shell is
safe from him; and no tool could make a neater hole.
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