H AVE you not all heard the song, "Rock-a-by, Baby, in the Tree-top"? What babies live in tree-tops? You will say, "Bird, wasp, bee, and spider babies swing in the trees."
Do you know that there are small cradles that rock all day long on the waves?
Up and down, in the sun, on the water, rock the cradles of many shell-fish.
What are shell-fish?
They are soft sea-animals that live in hard shells.
But you must know that these are not true fish. A true fish is an animal that lives in the water, and has a back-bone. The backbone of a fish is very much like your backbone.
All fish can swim. Most of them have fins and scales. Very many of them have long, slim, smooth bodies, that will glide easily through the water.
All of you can see fish, in the ponds, lakes, or brooks near your home. You often have them to eat on your table.
If you live in the city, you can go to the place where fish are sold and look at them. In some other book I may tell you a little about the true fishes.
In this book I shall now tell you a very little about what are called "shell-fish."
This is not a very good name for them, but we will use it, because you will hear it from many people, and will often see it used in books.
The right name for these shell-fish is a hard word, which means "soft body" or "soft thing." That suits them very well, for they are all soft bodies; they have no bones.
There are also in the water soft-bodied things that have no shells to cover them. In the next book we shall tell you of some of them.
The soft things that live in shells are mostly round or wedge shape. Their shells serve them for houses to live in, for ships to sail in, for coats to cover them, for bones to keep their soft bodies in shape.
The shells of these soft things are of many forms. Some are all in one piece, like a twist or curl. Some have two parts, like the covers of a book. These two parts are held by a hinge. And some shells are made in many pieces or scales.
There are three kinds, or orders, of shellfish. One kind has a head on its foot. Another has a head much like that of the snail. Still another kind has no head. Well! That is a queer thing, to have no head!
Let us learn first about the shell-fish with a head and a foot.
There are many kinds of shell-fish of this order. They differ in size, color, shape, and way of life. But if we learn about one, we shall have an idea of them all.
You know that the hermit crab steals a shell to live in. It is often a long shell, like a curl. That is the sort of shell that shellfish with heads live in. It is a shell all in one piece.
These shells are very hard and thick. Why is that? The fish in them is soft. It has no bones.
If these soft things had no hard shells, they could not live. The waves would kill them. The crabs, fish, and other animals in the sea, would eat them at once.
Let us see how a shell-fish is made. Have you a shell to look at? If not you can look at these shells in the picture. The conch, or winkle, is the largest shell-fish you will be likely to find. His body is soft but tough. It runs to a point.
That back part takes fast hold of the post in the shell, so that Mr. Conch will not drop out. On one side of his body he has a hook like a thumb. That is to pull him back into his shell when he wishes to hide.
The front end of the conch is wide and thick. Here we find his mouth. Near his mouth he has two feelers, such as insects have, to touch things. Where the feelers join his head he has two eyes.
His foot is flat, and is as big as all the rest of his body. It is just the size of the open part of his shell. Why is that?
The shoe on his foot is hard, like horn. When he draws back into his shell, that shoe is his door. It fits close. It shuts him in safe in his shell.