Bivalves
The order of the bivalves is a very large and important one. All these creatures have their shells made of two parts, or valves, which are fastened together by means of a hinge. They have no heads, and the mantle forms a kind of flap on either side of the body. They are found both in fresh and salt water. Every one knows the "fresh-water clams," or mussels, which abound in our lakes and rivers. In the central and southern parts of the United States they are exceedingly numerous and of many kinds, some rough, others smooth. All are lined with mother-of-pearl, and pretty buttons and other ornaments are made from them. Moreover, pearls are very frequently discovered inside their shells, and sometimes they are of great value.
The Pearl-Oyster
Pearls are obtained chiefly, however, from the pearl-oyster, which is found in warm seas in many parts of the world, the principal fisheries being in Ceylon, the Persian Gulf, the South Sea Islands, and off the northeast coast of Australia. They are deposited by the mantle, and it is most likely that they are really due to a grain of sand, which has lodged inside the shell and set up irritation. Indeed, it has been found that if small objects, such as tiny stones, are forced between the valves of one of these oysters, they become covered with layers of pearl in a very short time. The best mother-of-pearl is also obtained from the shells of the pearl-oyster.
Oysters
The ordinary oyster belongs to another family of bivalves, in which one part of the shell is a good deal larger than the other.
The early life of this mollusk is very curious. The spawn is known as spat, and is produced in enormous quantities. This spat looks at first like very fine gray dust, and remains for some little time within the shells of the parent. But one day in early summer the oyster opens its valves a little way, and squirts it out like a cloud into the water. For a few weeks the little oysters are able to swim, and they generally travel backward and forward with the tide. But after a while they attach themselves to some object at the bottom of the water, and there they remain without moving any more for the rest of their lives.
One would think that, since a family of oysters is so enormously large, these creatures must be the most plentiful mollusks in the sea. But by far the larger number are destroyed by other creatures before they are able to settle down; while even after that they have a great many enemies. We have already told you how mischievous starfishes are in the oyster-beds. Then borers and dog-whelks are almost equally troublesome, and besides these there is a curious kind of sponge, called the cliona, which burrows into the shells of the mollusk and gradually destroys them, sometimes actually causing them to fall to pieces.
Black Mussels
Two or three kinds of black mussels live in vast numbers on almost all coasts, clinging to rocks and submerged timber. The way in which a mussel fastens itself to its hold is very curious, for instead of turning the whole of the foot into a big sucker, as the limpet does, it spins a number of very strong threads from that part which lies nearest to the hinge; and every one of these threads is separately fastened to the support, so that the creature is moored down, as it were, by a kind of cable. These threads are known as the byssus, and hold so firmly that it is not at all easy to pull them away. Some of these mussels are good to eat, but are not as much used in the United States as in Europe.