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 ====XI. Why the Crawfish Crawled into a Shell==== If I were to ask you "What is the best part of an oyster, or a clam?" You would say: "The inside, of course."

And you would be right; but you might not mean just what I meant. There are probably no oysters in your aquarium, but you could easily have snails. Snails, clams and oysters all belong to the mollusk family. "Mollusk" is from a Latin word meaning soft. All of these animals are soft bodied.

While the mollusks are so simple on the outside, as compared with the crawfish, they have a good many more parts on the inside The oyster is like a watch in more ways than one. It's easy to see that he has a hard case; and also that this case opens and shuts with a hinge. He also has a little heart that goes like a watch: "Tick, tock; tick, tock." So does your heart. The oyster also has two tubes, through which he breathes his food and air out of the water, and these tubes—or rather the water in them—goes back and forth, back and forth, like the pendulum of the big hall clock.

Through one of these tubes he takes the water into himself. He passes it out through the other. The water flows over his gills or lungs, and so he gets his fresh air. And with these same gills he gets his food. The gills are full of little holes, and the holes are surrounded by little paddles—just as we have seen in the sponges and the crawfish, and in some of the plants. These paddles collect the one-celled plants and animals out of the water, and pass them on to the oyster's mouth. His mouth, which is just above his foot, has four lips.

While the lobster has such a big stomach—his three-roomed stomach—the oyster has a small stomach. There are at least two good reasons for this. One is that his life is so much quieter than the lobster's. He doesn't need to eat so much. Another is that, as he has so much better inside parts to digest with, he neither needs those outside claws to tear up his food, nor a big three-roomed stomach to keep straining out the large pieces and digesting them. There are no large pieces in his food, because he lives on little one-celled plants and animals.

By dropping a little red ink from a medicine dropper into the water, near the opening of his two tubes, you can see the clam in your home aquarium breathing the water into himself and out again.