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.

The oyster has muscles, also. Two of them are to open his two-leafed shell, and two of them to hold it tight shut. He holds it shut so tight that it takes somebody stronger than you are, and using an oyster knife, to open it. Why is it, do you suppose, that the muscles for shutting his door are so much larger than the muscles for opening it?

That's right : Mr. Oyster is afraid of burglars. And well he may be, for there are many burglars of his own kind—mollusks—who would like to break in and eat him. Some of them have drills, and they drill a round hole in his shell in spite of locked doors. These little spiral shelled burglars belong to the snail family. And there are sponges that bore holes in the oyster's shell.

The oyster has also a much higher kind of heart than the crawfish. It is divided into two big chambers, just as yours is. One is for receiving the blood, and the other for pumping it out through the body. You can easily see this little heart when the shell of the clam or oyster is opened, and count the beats.

This heart is surrounded by a thin bag or membrane, just as it is in the higher animals ; and it is called by the same long Latin name. But as we are not studying Latin yet, we won't bother our heads with any more big words than we have to. Notice also that the oyster has kidneys, as the crawfish has not; and a greenish mass in his food tube. This greenish mass surrounds the stomach. It is called the pancreas. In higher animals there is a pancreas, and also a liver. Both the pancreas and the liver secrete a greenish liquid, or bile, which helps digestion.

You also notice that the oyster has little blood vessels, real vessels with branches, just as yours have. These branches are in his little foot—for the oyster is a "one-legged man." Why do the blood vessels branch only in his foot? Where have you the most and the largest veins? Yes, in those parts you exercise the most— your hands and feet and muscles. As you get nearer the heart these veins unite with each other forming fewer large veins; just as the branch lines of a railroad, from the smaller towns, unite with the main trunk line leading into the great city.

Now that Mr. Crawfish has improved his insides so much, we will see him put on his tail, change his legs to fins, his shell to scales, and go to swimming again.

Only, he will not swim backward this time, as the crawfish and lobster do. As a fish he will make a specialty of swimming.