We call these creatures shell-fish, but the name is a very bad one. No fish makes a shell, and these creatures are not fishes at all, but far lower in the scale of life. A fish is an animal that has a backbone and a skeleton that lies inside its body. The bodies of the creatures that produce the seashells are soft, and have neither a backbone nor any other bones.
These kinds of creatures existed in the sea long before the fishes were evolved. When they die, their bodies are gradually dissolved away, but the empty shell that was made by them remains. It is now much lighter than it was, for its inhabitant is not there to fix it to a rock or seaweed, and so it is cast up by the waves on the shore where we find it.
Sometimes, when we dig far inland, we come across many minute seashells deep down in the Earth. These shells prove to us that at one time, long, long ago, the sea used to cover that place. As the little creatures died, their shells dropped to the bed of the sea and were gradually covered by layer upon layer of mud, until that which was previously the sea-bed at length became dry land.
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