The Mollusca form a very large group of Invertebrate animals; they live on land as well as in the water, but the aquatic species are much more numerous than the terrestrial, and the deep-sea dredgings are constantly bringing to light new forms. Some of the shells which protect their soft bodies, and are formed by the animals themselves, are marvels of beauty, and many of them are secured from injury by a waterproof coating. A number of extinct animals, such as Ammonites and Belemnites, belong to this group--their shells may be seen in any good museum; those of the Belemnites, as their name implies, are shaped like a dart; those of the Ammonites, like that of the beautiful Nautilus of our times; but the fisherfolk of Whitby, where they are found in numbers, say they are "snakes turned to stone." But as we have been speaking so much of sea-creatures, I think we will now leave the oysters, cockles, mussels, and razor-fish, and choose the familiar garden-snail as our specimen of the Mollusca, or Soft-bodied Family. I fancy you need no introduction to that snug little householder. Often, however, as you have touched his soft horns, you possibly do not know that the very house in which you first made his acquaintance has been his habitation ever since; for young snails come from the egg with the shell upon their backs, and they never quit that first house for a larger one, for as they grow, their shell-house grows too. Look at this empty snail shell, and say whether God has not given a beautiful coat of mail to protect a creature without a bone in its body, and so sensitive that "Give but his horns the slightest touch, His self-collecting power is such, He shrinks into his house, with much Displeasure." But _how_ does the house grow large so as to suit the growing tenant? Most shells are made from a part of the animal called the mantle, and increase round the rim; if the snail's house is broken, its slime will harden over the injured part and repair it. Then, when the cold weather comes, and the snail prepares to bury itself underground for several months, and take its winter nap, it makes a strong cement of earth and slime, with which it builds up the open part of its shell--but, wonderful to think of, the clever little mason leaves, as it were, one brick out of the wall, and thus there is a tiny opening, too small to let in the water, but large enough to admit air sufficient to keep him alive during his long sleep.