This shell is often more than a foot in diameter. But if you were to see it when it is first taken out of the sea you would never think that it was a shell at all. Indeed it looks much more like a big shapeless lump of blubber, for the animal covers it entirely with its muscular mantle, so that the shell itself is completely concealed.
Very little is known of the habits of the chambered nautilus, for it lives at the bottom of the sea, at a depth of two or three hundred fathoms. It is found in various parts of the Indian and Pacific oceans.
Gastropods
A great many well-known creatures belong to this large group, first upon the list being the slugs. We need not describe these animals, but perhaps you will be surprised to hear that they have shells! These shells are very small, however, and are entirely covered over by the mantle, so that they cannot be seen unless the body is dissected.
Slugs have the most wonderful power of stretching out and drawing up their bodies. You may see one of these creatures crawling about on a damp evening, and measuring fully five inches in length. But at the slightest touch it begins to contract, and in a few seconds it is just a shapeless lump, scarcely half as long as it was before. The odd little tentacles are drawn back into the head, and the head is drawn back into the body so that if you did not happen to know what it was you might easily mistake it for a pebble.
On the right-hand side of a slug's body, as it crawls along, you will notice a rather large and almost round hole. This is the entrance to the breathing-organs, which lie just behind the head and underneath the mantle.
During the daytime slugs remain in hiding, lying behind the loose bark of dead trees, or under logs and large stones, or in heaps of decaying leaves. And if the weather is very hot and dry they do not come out even at night, for they very soon die if they are deprived of moisture. But on warm, damp evenings they travel for long distances in search of food, which is almost entirely of a vegetable character. In Europe every gardener knows what injury they do to gardens there, but in America the slugs are practically harmless.
A good many different kinds of slugs are found in Great Britain. The largest of all is the great gray slug, which often grows to a length of more than six inches. Then the black slug is very common in many parts of the country. It is not always black, however, for one may often find examples which are brown, or yellow, or gray, or even white. The milky slug, which has a thick creamy slime, is plentiful everywhere. And sometimes one may dig up a very curious slug—testacella—which feeds on earthworms, and follows them down to the very bottom of their burrows. When the weather is cold, this slug makes a kind of cocoon of earth and slime, and lies fast asleep inside it, often for many months at a time.
Snails
In many ways snails are very much like slugs, but they have a shell large enough to contain the entire body when the animal withdraws inside it. Several hundred different kinds of snails are found in North America, and many more in other parts of the world, varying in size from that of a small pinhead to that of a big walnut. Some are exceedingly numerous, others so rare and singular in their living-places that they are highly prized by conchologists. All snails lay eggs, usually in damp soil; and if you will turn over an old log in the woods in summer, you will be almost certain to find some of the minute shining globules. When winter draws near all the snails go into hiding, and they have a most curious way of closing the entrances to their shells by making little doors across them, composed partly of slime and partly of very small fragments of earth. This is in order to prevent the frosty air from getting in and killing them. But it would never do, of course, to keep all the air out, for in that case they would be unable to breathe. So they always leave a tiny hole in the middle of each door, through which just enough air can pass to prevent them from being suffocated.