The Cockle
This is another very well-known bivalve. Its heart-shaped shells, covered with low ridges, you must know by sight. It is one of the burrowing mollusks, spending its life buried in sandy mud. It is especially common at the mouths of large rivers, where enormous quantities are collected to serve as human food. And its large muscular foot is not only used in digging, but also enables it to leap to a considerable height. It is to this family that the quahog or hard clam of our markets belongs.
Razor-shells
These, too, are inhabitants of the mud, and if you want to find their burrows all that you have to do is to visit a patch of sandy mud when the tide is out, and stand quietly watching it. Before long you are sure to see a little jet of water spurt out of the mud to a height of three or four inches. Now this water has been squirted out of the siphon-tubes of a razor-shell, and if you walk to the spot, treading very carefully, you will find a tiny hole in the mud. This is the entrance to the burrow, and if you want to get the animal out, the best way to do so is to drop a little salt down the hole. For it is a very strange fact that although the razor cannot live in mud at the bottom of fresh water, it does not like pure salt at all, and is sure to come up to the surface and try to get rid of it. But if you fail to seize it at once it will retreat to the very bottom of its burrow, and no amount of salt will persuade it to come up again. The soft clam, which is sold in our markets in such enormous quantities, is a near relative of the razor.
The Piddock
One of the most wonderful of all the bivalves is the piddock, as it is a boring mollusk, living buried in the solid chalk or limestone. If you should examine the rocks which are left bare at low water along the shore of the Mediterranean, or some other warm sea, you would often find that they are pierced by numbers of rather large round holes. These are the entrances to the burrows of piddocks; and if you could split the rock open you would find several of these creatures lying in their tunnels.
Sometimes, when they are boring, their burrows become choked up behind them with the material which they have scraped away. Then they just squirt out a jet of water from their siphon-tubes, and so wash the passage clear.
It is really owing to the work of the piddocks that chalk and limestone cliffs are so much cut away by the sea. The waves by themselves can do very little in this way. For when they wash up against the face of the cliff they leave the spores of seaweeds behind them; and these very soon grow and cover the whole surface with a mantle of living green, which almost entirely prevents the cliff from being worn away. But the piddocks drive their burrows into the rock just below the surface of the water, boring backward and forward till it is completely honeycombed by their tunnels, which only have just the thinnest of walls left between them. Then the sea washes into the burrows, and breaks these walls down, so that the whole foundation of the cliff is cut away. Very soon, of course, there is a landslip, and hundreds of tons of chalk or limestone, as the case may be, come falling down. Then the piddocks begin working again a little farther back, and the process is repeated; and so on over and over again.
On many parts of the south coast of England long stretches of rocks run ever so far out into the sea, and are only partly left bare at low water. Those rocks were once the bases of cliffs, which the piddocks and the waves together have cut away. And it even seems almost certain that the Strait of Dover was cut in this manner, and that if it had not been for the labors of the piddocks, carried on day after day for thousands upon thousands of years, Great Britain even now would not be an island, but would still form part of the continent of Europe, as we know that it did in ages long gone by!
The Teredo