“The oyster is a genus of lamellibranchiate molluscs of the section with a single adductor muscle. The shell consists of two unequal and somewhat irregularly-shaped valves of laminated and closely foliated structure, and the hinge is without tooth or ridge, the valves being held together by a ligament lodged in a little cavity in each. The animal is in its organisation among the lowest and simplest of lamellibranchiate molluscs. It has no foot, and, except when very young, no power of locomotion, or organ of any kind adapted to that purpose. Its food consists of animalcules, and also of minute vegetable particles, brought to it by the water, a continual current of which is directed towards the mouth by the action of the gills. The gills are seen in four rows when the valves of the shell are separated, a little within the fringed edge of the mantle. In the most central part is the adductor muscle; and between the adductor muscle and the liver is the heart, which may be recognised by the brown colour of its auricle. The mouth—for, as in the other lamellibranchiata, there is no head—is situated beneath a kind of hood formed by the union of the two edges of the mantle near the hinge. It is jawless and toothless. The ovaries are very large during the season of reproduction, which extends over certain months when oysters are out of season for the table. Oysters are hermaphrodite.”[82]
[82] Chambers’ ‘Encyclopædia.’
The fecundity of the oyster is amazing. Leeuwenhoek estimated that an oyster, when full of spawn, contained from 3000 to 4000 of its offspring, and it has also been computed that one oyster alone produces nearly a million and a quarter of eggs. The eggs are hatched and the young produced within the shell and mantle of the parent, where they continue floating or swimming about in the vicinity of the gills in a creamy-looking kind of mucus or fluid until expelled. Their expulsion is preceded by a change of appearance in the fluid to a brownish or muddy colour; a circumstance that may possibly indicate an
alteration of composition in the liquid unfavorable to the infant oyster, and thus lead to its departure. Their departure or expulsion from all the parent molluscs of the oyster-bank or bed takes place at the same time.
When they leave the parent shell the young oysters, which in this condition are called spat, are not more than 1⁄120th of an inch in length; and two millions of them when closely packed do not occupy a space of more than a cubic inch. Thus cast adrift they are carried away by currents, their multitudinous numbers being considerably diminished by their falling a prey to numerous fish, as well as from their frequent inability to find a suitable resting-place. This obtained, the young oyster or spat attaches itself to it, and makes it the permanent home on which it eats, grows, and breeds, and, debarred of locomotion, passes its existence unless, of course, removed by external causes. Pending its obtaining a suitable locality the young oyster is provided with a powerful swimming apparatus which, it has been surmised, becomes absorbed or otherwise disappears when its function is rendered unnecessary by the stationary life of the oyster after it has secured a habitat.
The objects to which it attaches itself are numerous. The Ostrea parasitica, a species of oyster found in warm climates, fixes itself to the roots and branches of trees growing within reach of and washed by the tide. Again, in some of the southern states of North America, large oyster-beds, which are sometimes of such magnitude as to form buttresses against the force of the tides and winds, originate from the habit of young oysters attaching themselves to the shells of old ones. Similarly the banks of some of the rivers of Georgia, which run up some few miles inland from the sea, are composed of masses of living oysters attached to each other. These banks, which are so massive as to make a channel for the river, are known as racoon banks, because this animal is one amongst others which frequents them for the sake of devouring the oysters.... In some of the French parcs, or artificial oyster-beds, the young oysters attach themselves to large unglazed tiles, or to faggots or other solid bodies which are placed in suitable situations for the purpose; in the English artificial beds hurdles are frequently employed, upon which the spat become deposited. It appears the young oysters select dark objects, such as slate or black stones, in preference to bodies of a lighter colour to fix themselves to, and that they choose, where practicable, the inner side of the object, or that portion of it away from the light. After a time the young oysters are removed from the breeding beds, placed in the fattening beds, from whence they are removed when they have attained a sufficient size, and sent upon the market. In England oysters are not regarded as fit to be eaten until they
are at least three years old; whereas in France they are served up to table about a year earlier. The chief enemy of the young oyster is a species of whelk, known in France as the bigourneau, dog whelk, or piercer. These creatures, which are found in immense quantities in the celebrated oyster-beds at Arcachon, near Bordeaux, cause great destruction amongst the bivalves. Part of their anatomy consists of a boring apparatus, with which they pierce the shell of the oyster; whatever of the dead oyster is left by the whelk is devoured by the crabs, which creep into the aperture in the shell made by the former.
We have already alluded to the abundance of oysters in parts of Georgia, where, we may add, they are not only confined to the alluvial shores of the rivers, but are also found in large numbers amongst the long grass of the adjoining low lands.
In these districts it is by no means an uncommon practice for the inhabitants to improvise a meal by picking up a bunch of oysters and roasting them over a fire kindled on the spot. In many of these localities the oysters occur in quantities so immense that a vessel of 100 tons might be loaded within three times her own length.[83]
[83] Chambers’ ‘Encyclopædia.’