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 sea-beach with these pin-holes in them. The holes were made by Mr. Drill on his hunt for food. But you will now and then find shells, 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 dead, 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. LESSON XL. HOW SHELL-FISH FEED. Do the shell-fish all feed on other shell-fish? Oh, no. Some of them live on sea-weed. Some of them live by fishing. They catch, from the water, small bits of food, as small as grains of sand. The shell-fish that live on sea-weed have a long, slim tongue. It is somewhat like that of the drill. The tongue is like a tiny strap. The teeth are set on it, three or more in a row, like the points of pins. As the teeth wear out from work on the tough weed, more grow. These shell-fish walk along on their one big foot. First one side of the foot spreads out, and then the other. That pulls them along. Is it not very slow work? But what of that? All they have to do is to move about and find food. They can take all day for it. They have no house to build and no clothes to make. They creep along to a good bed of sea-weed. Then they put out the fine, file-like tongue. It cuts off flakes of sea-weed for them to eat. They are never tired of that one kind of food. Even that queer limpet, who sits on a rock and has a shell like a cap, has a head, and a foot, and a tongue that is like a rasp. And he can walk along the floor of the sea. He can climb up the rocks. The limpet has his own rock and his own hole in the rock. He goes back to his rock when he has had all that he wants to eat. [Illustration: AT LOW TIDE.] The world of the sea is as full of life as the world of the land. There is one nice little shell-fish, about as big as a pea. He lives in the sea-weed that grows on rocks. He is brown, or green, or black, or red, or dark yellow.