These are common on rocky parts of the coast, and you may find them crawling about on the weed-covered rocks in thousands when the tide is out. They have tooth-ribbons just like that of the borer, but they do not use them in the same way, for they feed only upon seaweeds. And they are remarkable for having the foot divided by a kind of groove, which runs right down the middle. When a periwinkle crawls, it moves first one side of this foot forward, and then the other side, so that although it has no legs it may really almost be said to walk.
The Cowry
One of the prettiest of the gastropod shells, is that of the cowry, in some parts of Africa used as money. It would seem strange to earn one's living just by picking up money on the sea-shore, wouldn't it? And perhaps you might think that every one who lived near those parts of the coast where cowries are found must be very well off. But then sixteen hundred of these shells are only worth about a quarter of a dollar, so that you would have to hunt for a very long while and stoop a great many times in order to obtain sufficient even to buy food. And it must be very awkward to have to carry several sacks of money when one goes out marketing! Many of them, however, are extremely beautiful.
Limpets
Commoner still are the limpets, which you may find in thousands clinging to the rocks that are left bare when the tide goes out. They fasten themselves down by means of the broad, fleshy foot, which acts as a big sucker. And so firmly do they hold that it is almost impossible to pull them away.
After a time, the edges of a limpet's shell cut a circular groove in the rock to which it clings, so that even the sea-birds cannot drive their beaks underneath and force it from its hold. And though, when the tide is up, the mollusk will wander to a distance of two or even three feet in search of food, it always seems to return to its resting-place before the retreating waves again leave the rock uncovered.
Amphineurans
This order of mollusks contains the curious creatures which are known as chitons. These may be described as sea-armadillos, for they are covered with a kind of shelly armor, consisting of a series of plates, and can roll themselves up into balls, in order to protect themselves from the attacks of their enemies.
One of these mollusks is called the prickly chiton, because it is covered all over with sharp spines, like a hedgehog. It grows to a length of nearly six inches. But long before it reaches its full size the spines are rubbed off, so that a large example of this creature is nearly always perfectly bare. The chitons live among muddy rocks at low-water mark, and are not common outside the tropics or in shallow water.
The order of the amphineurans is quite a small one, and so is that of the scaphopods, which consists only of the tooth-shells, which are very common on the sandy coasts of the Northern Pacific, and look rather like very tiny elephants' tusks. The Indians of the Puget Sound region used to string them as ornaments, and valued them highly.