W HEN the drill gets on the back of an oyster, what can the oyster do? Nothing. The poor oyster cannot help himself. Does he hear hour after hour the file of the drill on his shell? Yes.
He knows the drill will get in and kill him, but all that he can do is to keep still and wait.
The oyster is not the only kind of shell-fish that the drill eats. When the drill goes after the poor shell-fish that have no heads, he eats them at his ease.
They cannot help themselves. They do not know how to get away from Mr. Drill. The shell-fish that have no heads live in shells made of two parts, like the covers of a book. The two parts are held to each other by a hinge. The oyster has such a shell.
It is a bad thing, it seems, to have no head. Without a head who can take care of himself?
Let us see Mr. Drill try a fight with a shell-fish that has a head. Now he meets his match!
He goes to the top of the shell. He makes fast, and begins—file, file, file. The fish inside hears him. "O, are you there, Mr. Drill?"
What do you think the shell-fish does? He draws his body out of the way, and builds up a nice little wall! So, when Mr. Drill gets his hole made, and puts in his tongue—no fish, only a hard wall! Then Mr. Drill also moves along.
He picks out a good place. Once more he goes to work—file, file, file. "O, here you are, Mr. Drill!" And the shell-fish with a head once more pulls his body out of the way, and makes a new wall.
Then Mr. Drill has the same luck as before. Sometimes he gets tired of the war and goes off. Now and then, as he too has a head, he finds a spot where there is no room for the wall. There he makes his hole and sucks out the animal.
You will find very many of the shells on the seabeach with these pinholes in them. The holes were made by Mr. Drill on his hunt for food.
But you will now and then find shells, such as the thick clam shell, full of holes, like a network. This is not done by Mr. Drill.
Shells and bones are made of two kinds of stuff. One is lime, which is hard like stone. The other is not so hard; it is more like dry glue.
These shells with so many holes are old shells, long empty, and the glue part has gone out of them.
How did it get out? It was bored out by a kind of sponge. Only the lime part is left, like a fine net.
When bones or shells have only the lime part left, they will break and crack like glass. If they have too little lime, they will bend.
For all Mr. Drill has a head, he is not so wise as at first he seemed to be.
He will sit down and make a hole in an old dead shell where no fish lives. Now and then he makes a hole in an old shell, long ago turned into stone. He will spend two days on such a shell as this!
Did you know that bones and shells and plants sometimes turn to stone?
You will some day learn about that strange fact.