Normally, an oyster lies with its two valves slightly ajar. Note the rubbery ligament inside the hinge. When the oyster hears a noise or sees a shadow that suggests danger, the adductor muscle contracts, pinching the ligament and shutting the shell tight. It cannot stay closed long at a time.
There is a constant current of water flowing into the front of the shell, between the mantle and the body, bathing the gills. This vigorous current is produced by microscopic cilia (hairs) that cover the gill surfaces, and have the strange property of moving rhythmically with oar-like stroke, all acting in unison. The water passes through small pores into the tubular substance of the gills, where the blood is oxygenated.
Food particles, with which the seawater is laden, are wafted along between the gills, but they do not enter the pores. They accumulate in windrows just outside, and reaching the lips, they are urged on by labial cilia into the mouth. Thus an oyster feeds as it breathes.
The oyster's eyes are obscure pigment spots in the mantle margins. Feeling, a well-developed sense, is also located in the mantle. Knots of nervous matter connected by threads constitute the oyster's simple nervous system. This is all a headless, foot less mollusk needs.
The "beard," which certain oyster recipes require to be removed, is the gills. The "heart," so-called, is the adductor muscle.
Life History.— In early summer an oyster's gill chambers become gorged with a milky fluid. The females contain ova, the males milt. These two reproductive elements are discharged when ripe, and their unison (the fertilization of the ova) occurs in the water, which soon swarms with fertile eggs. It is estimated that a large oyster can produce sixty million eggs. The average is probably twenty-five million or less.
The swimming embryo is a somewhat spherical body, one 425 The Oysters five-hundredth of an inch in diameter, with tufts of cilia in bands around it. The whirling millions of young "fry" give the water a cloudy appearance, and the oyster-grower is nervous with apprehension. The important thing is the settling of these free youngsters. It occurs after a day or two. If the weather is warm, the surface of the water smooth, and the bottom strewn with shells, a good "set" may be expected. But a cold spell kills millions, and a boisterous sea drifts millions out into deep water.
The "spat" once settled is fixed for life. It may survive or
perish, but it is utterly passive. Glued to its support, the oyster assumes its characteristic form, though scarcely larger than a pin head.
The growth of a shell demands a constant supply of lime in solution. The disintegrating of dead shells furnishes this lime food in abundance. As if by instinct the spat chooses for its resting place a rough, limy surface. Old beds furnish plenty of shells. Living and dead oyster shells are coated with spat each season.
Besides the lime so essential for shell formation, an oyster's food consists of animal and plant organisms of minute size that breed in flocculent mud. The young larva of crabs and sponges, and mollusks, including oyster spawn and fry, are added to infu sorians and diatoms. Clouds of these microscopic forms rise in the water to settle again as daylight fades. Coming and going they pay toll to the oysters.
Roughly speaking, each year of an oyster's life adds an inch to the length of its shell. June spat is as big as a finger-nail in the late autumn. The shell lengthens for about ten years. After that it grows in thickness. The full development of the repro ductive powers is reached at four years, though spawning begins much earlier. The average oyster's natural life is about ten years. In easy circumstances undisturbed oysters have acquired size and thickness that warrant the estimate of an extreme age limit of fifty years. Layers of the shell indicate different periods of growth, but these do not tally with years. Growth is generally confined to the summer months. Winter is spent in comparative hibernation. Summer may be divided into periods of quiescence and growth, registered by several thin layers of shell. The rate depends upon the length of the growing season and the abundance of the required food elements.
426 The Oysters Oyster Culture.— The oyster industry in this country began with the wasteful exploitation of extensive natural beds found in Chesapeake Bay and about Long Island Sound. That day soon passed. Scientific research into the life history of an oyster is the foundation upon which our belated and still imperfect system of oyster culture rests. The oyster-grower pores over the latest Government bulletin on the subject. The oyster crop repays scientific cultivation ; new beds are planted to supply the ever growing demand. Methods are being improved every year.