Page:Popular Science Monthly Volume 6.djvu/23

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13
THE NATURAL HISTORY OF THE OYSTER.

cowry or Cypræa, if an adult shell, you will find that, while the entire shell is covered with a shining enamel, there is along the whole length of the back of the shell "a line of pale color." The animal extends both folds of the mantle outside of itself and over the shell, and that line is where the lips of the folds meet. This mantle has much to do with obtaining food. The oyster opens its shells about a quarter of an inch apart. These membranes, that make the mantles from both sides of the animal, meet just at the opening of the shell. They are fringed at their edges with rows of tiny cilia, or soft fleshy hairs of extreme delicacy. This pallial fringe in an eminent degree serves the oyster as organs of touch. Probably this sense, though distributed somewhat over the entire surface of the body, is along this fringe exquisitely acute. The English call this fringe the oyster's beard. It is protruded just a little out of the shell; and these cilia, almost numberless, keeping up their rapid movements in the water, make as it were two parallel vibrating curves, which beget a sort of aquatic vacuum inside the shell, into which the water flows, as in a diminutive whirlpool. The stream thus affected brings with it the algae spores and animalcules which constitute the oyster's food. But where is the oyster's mouth? Speaking popularly, it is away back near the hinge of the shell, as shown in the cut. To this point the current flows. Now, it must not be supposed that all is fish that comes to the oyster's net. Far from it. Hence this mollusk has eclectic functions. Doubtless a sharp spicule of a sponge may occasionally get into the mouth, even as a bone splinter can get by accident into a human throat. The word "tentacles," in the cut, refers to certain organs, that might be called labial or lip fingers. These, it will be noticed, have immediate relation to the mouth. They are the organs for discriminating food—functionally they are manipulating lips. The stomach is not shown in the cut, being overlaid by the other organs. The intestine, at least a part of it, is exposed, and its extremity is really the anus or vent. So much, then, is apparent, that the oyster possesses an alimentary system of some complexity.

A series of plates or plaited frills, lies on the mantle, if indeed it is not a specialized portion of that organ. These plaits are the branchiæ or gills. In the respiratory system of an oyster these branchiæ or gills are precisely the same to it as are the gills to a fish, or our lungs to us. Through these gills the water is passed. After imparting to the blood the oxygen taken from the air which the water contained, that water, now laden with carbonic-acid gas, is expelled at the respiratory aperture, or ex-current orifice, the dark spot in the figure immediately under the end of the intestine, which we have already said is the anus or vent, whence this refuse water, like a cleansing stream, passes directly out of the shell. This contrivance is certainly very beautiful. It is in fact a miniature sewer carrying off promptly and quickly the excrements as fast as they are made.