Page:Popular Science Monthly Volume 20.djvu/158

146 that of the sea, but the density is not important, except within wide limits, though the specific gravity should not fall below 1·01 at 60° Fahr., distilled water being represented by 1·00. The main necessity is that the water should contain a sufficient amount of lime to furnish the animal with the principal constituent of its shell.

The mature marketable oyster is so well known that it is unnecessary to describe it; but the following will assist the reader, who is not acquainted with the anatomy of the animal, to a correct appreciation of the facts stated and terms used in subsequent pages:

That part of the oyster usually known as the heart is a muscle, called the abductor muscle; its office is to keep the valves (or shell) closed, and prevent the ingress of deleterious matter.

The two valves are hinged at the round, blunt end of the shell, and between this hinge and the abductor muscle lies the body of the oyster or visceral mass, which is made tip of the light-colored reproductive organs and the dark-colored digestive ones, packed together in one continuous mass. The mouth of the oyster is that part nearest the hinge, and what is usually called the "beard" of the animal is known as the "gills."

The oyster lies on its side in the shell, and the minute animal and vegetable organisms contained in the water, and which form the food of the animal, are passed along between the gills to the mouth by the action of myriads of small vibrating hairs, called cilia. These cilia cover the surface of the gills, and, by a rapid and simultaneous motion in one direction and a slow one in the opposite, cause a strong current to set into the lips of the valves, bringing in not only what is suitable for food, but many other minute organisms and particles which thus come in contact with the gills and what they may hold.

The European oyster (Ostrea edulis) and the American oyster (Ostrea Virginiana) are varieties of the same family, and, though differing in several particulars, are not so dissimilar but that the conditions favorable to the growth and life of one may be considered as equally so for the other. With each variety the formation of the generative matter is gradual, and the spawning-season of both is during the early summer months, its advent depending probably upon the temperature, the higher temperature hastening and the lower retarding that event. No particular temperature can be assigned, as it depends greatly upon the locality, but departures from the normal temperature of the spring and summer months will have the effect described. In the same way an increased or diminished density of the water, whether due to change of temperature or to the addition of water of greater or less specific gravity than that usually surrounding the animals, has probably a similar accelerating or reverse effect upon the spawning, though as yet we can not speak with certainty upon this point.

Generally, both in Europe and America, the spawning-season may be said to be from June 1st until August 15th, though variations of