The rocks, and the wooden piles of the pier, are often covered with the hard shells known as Barnacles, or Acorn Shells. If you slip on them with bare feet their sharp edges cut you. Each Acorn Shell is a little house. Have you ever caught a glimpse of the animal living inside?
If you will look very carefully, you will see that the Acorn Shell is made of three-sided pieces, closely joined. There is a little door at the top, kept tightly closed until the tide comes up and covers the rocks. Then watch, and you will see a bunch of tiny feathers appear through a slit in the door. This means that the animal is hungry, and has put its twelve legs out of doors to catch a dinner!
This is strange, but true! The Barnacle is always upside down in its home, and its twelve feathery legs are thrust out of the door at the top. They make a fine net, in which minute animals are caught and brought into the mouth below. This funny creature actually kicks its food into its mouth! If you own a magnifying glass, you can see this for yourself at the seaside.
You will not be able to see the mouth, however, which is inside the shell. It is fitted with moving parts, and feelers, like the mouth of a crab. Also, the Barnacle has a good set of teeth to grind its food. It has no real eyes, having no use for them. Of what use are eyes to an animal standing on its head in a small dark shell! Now and then it casts its coat (like the Crab and Shrimp). The old coat is rolled up and thrown away outside the door.
Now comes the strangest thing of all. As a baby, the Barnacle is a free swimming creature. It has three pairs of legs, a tail, a useful mouth, and one eye. After kicking about in the sea for some time, and changing its skin, it changes its shape entirely. It now looks more like a tiny mussel. It has two little "shells," two eyes, legs, and feelers. Now its swimming days are nearly over, and it must settle down. It gives up eating, and roves about looking and feeling for a place to settle on.
Finding a suitable spot, the little animal stands on its head. Then a kind of glue is formed, which fixes it for life to that place, head down. The two shells and the two eyes are now thrown off. The Barnacle quickly builds up a shelly house, and, after a life of adventure and change, becomes a fixed Barnacle for the rest of its days.
For many years people knew little of this strange animal. All its wonderful changes, and the way its body is made, tell us plainly that the Barnacle is actually first cousin to the Crab, Lobster, Shrimp and Prawn! It belongs to the class known as the Crustacea; but, for some reason or other, it has chosen to live its grown-up life fixed to a rock.
EXERCISES
1. How does the Shrimp swim?
2. Of what use are Shrimps and Prawns in the sea?