In the month of May the mother oyster produces a very large number of eggs—sometimes as many as eight or nine hundred thousand! These are called “oyster spat,” and for several weeks she keeps them in her gills. Then one day she suddenly opens her valves and squirts them out into the water, where they look like a little cloud of the finest possible dust. For a short time after these eggs hatch the baby oysters swim about, and travel backwards and forwards as the tide rises and falls. After a while, however, they sink down and fasten themselves to some object at the bottom of the sea; and when once they have done this they never move again. They always lie upon their left sides, with the smaller and flatter of the two valves uppermost; and there they remain for five years at least before they reach their full size.
Oysters feed, too, in a very odd way. You know, perhaps, that inside the shell of an oyster there is a tufted organ which we call the “beard.” This consists of the gills. Hidden away underneath these is the mouth; and the gills do not merely suck out the air which has been dissolved in the water, as those of other animals do, but sift out every little tiny scrap of decaying matter which the oyster can use for food as well. So an oyster’s gills enable it to breathe and to catch its dinner at the same time!
PLATE XIII
THE SADDLE OYSTER (2)
This is a very curious oyster; for in its flat lower valve, just below the hinge, is a large oval hole. Through this hole passes a strong band of muscle, to which is fastened a kind of shelly knob which looks just like a button. By means of this the animal fastens itself down to some object at the bottom of the sea; and very often indeed it is found attached to the shells of other molluscs, looking something like the saddle on the back of a horse. That is why it is called the “saddle oyster.”
Another curious fact about this creature is that very often its shape completely alters as it grows older. While it is quite small it looks very much like an ordinary oyster. But as time goes on it generally takes the form of the object on which it rests. So you might easily find half-a-dozen shells of the saddle oyster, not one of which would be shaped like any of the others.
1. THE OYSTER.2. THE SADDLE OYSTER.
3. THE COCKLE.