Page:Encyclopædia Britannica, Ninth Edition, v. 18.djvu/119

OYSTER 107 The generative organs of the oyster consist of a system of branching cavities on each side of the body lying immediately beneath the surface. All the cavities of a side are ultimately in communication with an efferent duct opening on the surface of the body a little above the line of attachment of the gills. The genital opening on each side is situated in a depression of the surface into which the renal organ also opens. The genital products are derived from the cells which line the cavities of the genital organs. The researches of Hoek have shown that in the same oyster the genital organs at one time produce ova, at another spermatozoa, and that consequently the oyster does not fertilize itself. Hov many times the alternation of sex may take place in a season is not known. It must be borne in mind that in what follows the species of the European coasts, Ostrea edulis, is under consideration. The ova are fertilized in the genital duct, and before their escape have undergone the earliest stages of segmentation. After escaping from the genital aperture they find their way into the infra-branchial part of the mantle cavity of the parent, probably by passing through the supra-branchial chamber to the posterior extremity of the gills, and then being conducted by the inhalent current caused by the cilia of the gills into the infra-branchial chamber. In the latter they accumulate, being held together and fastened to the gills by a white viscid secretion. The mass of ova thus contained in the oyster is spoken of by oyster fishers as " white spat," and an oyster containing them is said to be " sick." While in this position the ova go through the series of changes figured in vol. xvi. p. 638 (fig. 6). At the end of a fortnight the white spat has become dark- coloured from the appearance of coloured patches in the developing embryos. The embryos having then reached the condition of " trochospheres " escape from the mantle cavity and swim about freely near the surface of the water among the multitude of other creatures, larval and adult, which swarm there. The larva? are extremely minute, about -j-i^ inch long and of glassy transparency, except in one or two spots which are dark brown. From the trochosphere stage the free larvse pass into that of " veligers." How long they remain free is not known ; Prof. Huxley kept them in a glass vessel in this condition for a week. Ultimately they sink to the bottom and fix themselves to shells, stones, or other objects, and rapidly take on the appearance of minute oysters, forming white disks 2^5- inch in diameter. The appearance of these minute oysters constitutes what the fishermen call a " fall of spat." The experiment by which Hoek conclusively proved the change of sex in the oyster was as follows. In an oyster containing white spat microscopic examination of the genital organs shows nothing but a few unexpelled ova. An oyster in this condition was kept in an aquarium by itself for a fortnight, and after that period its genital organs were found to contain multitudes of spermatozoa in all stages of development. The breeding season of the European oyster lasts from May to September. The rate of growth of the young oyster is, roughly speaking, an inch of diameter in a year, but after it has attained a breadth of 3 inches its growth is much slower. Prof. Mobius is of opinion that oysters over twenty years of age are rare, and that most of the adult Schleswig oysters are seven to ten years old. The development of the American oyster, 0. virginiana, and of the Portuguese oyster, 0. angulata, is very similar to that of 0. cdulis, except that there is no period of incubation within the mantle cavity of the parent in the case of these two species. Hence it is that so-called artificial fertilization is possible ; that is to say, the fertilization may be allowed to take place in a tank or aqua rium in which the conditions are under control. But if it is possible to procure a supply of spat from the American oyster by keeping the swarms of larvae in confinement, it ought to be pos sible in the case of the European oyster. All that would be necessary would be to take a number of mature oysters containing white spat and lay them down in tanks till the larva? escape. This would be merely carrying oyster culture a step further back, and instead of collecting the newly fixed oysters, to obtain the free larvae in numbers and so insure a fall of spat independently of the uncertainty of natural conditions. Natural beds of oyster occur on stony and shelly bottoms at depths varying from 3 to 20 fathoms. In nature the beds are liable to variations, and, although Prof. Huxley is somewhat sceptical on this point, it seems that they are easily brought into an unproduc tive condition by over-dredging. Oysters do not flourish in water containing less than 3 per cent, salt ; and hence they are absent from the Baltic. The chief enemies of oysters are the dog-whelk, Purpura lapillus, and the whelk-tingle, Murex erinaceus,vf}rich bore through the shells. Starfishes swallow oysters whole. Cliona, the boring sponge, destroys the shells and so injures the oyster; the boring annelid Lcucodorc also excavates the shell. The wandering life of the larvae makes it uncertain whether any of the progeny of a given oyster-bed will settle within its area and so keep up its numbers. It is known from the history of the Liimfjord beds that the larvae may settle 5 miles from their place of birth. The genus Ostrea has a world-wide distribution, in tropical and temperate seas ; seventy species have been distinguished. Its nearest allies are Anomia among living forms, Gfryphgga among fossils. For the so-called Pearl-Oyster see PEARL. (J. T. C.). Oyster Indust ;//. The oyster industry of the world is seated chiefly in the United States and France. Great Britain has still a few natural beds remaining, and a number of well-con ducted establishments for oyster culture. Canada, Holland, Italy, Germany, Belgium, Spain, Portugal, Denmark, Norway, and Russia have also oyster industries, which are comparatively insignificant, and in the case of the two countries last named, hardly worthy of consideration in a statistical statement. Recent and accurate statistics are lacking except in two or three instances. A brief review by countries in the order of their importance is here pre sented. United States. This is by far the most extensive of the fishery industries of the country, yielding products three times as valuable as those of the cod fishery and six times those of the whale fishery. In 1880 it employed 52,805 persons, and yielded 22,195,370 bushels, worth to the fishermen $9,034,861. On 13,047,922 bushels there is a rise of value in passing from producers to market, which amounts to $4,368,991, and results either from replanting or from packing in tin cans. The value of the capital invested in the industry is returned as 10,583,295. There are employed 4155 vessels, valued at $3,528,700, and 11,930 boats. The actual fishermen number 38,249, the shoresmen 14,556. Fully 80 per cent, of the total yield is obtained from the waters of Chesapeake Bay. 1 France. The oyster industry of France employed in 1881 29.431 2 men, women, and children in the parks, beds, and preserves. The number of such establishments upon the public domain was 32,364, with an area of 19,891 acres, and 970 establishments upon private property, with an area of 926 acres. From these 374,985,770 oysters were dredged during the season of 1880-81, from September 1 to June 15, worth 2,061,753 francs, while the total number of oysters disposed of during this period amounted to 680,372,750, worth 17,951,114 francs. This total includes the oysters dredged in the sea as well as those gathered from the arti ficial breeding-grounds or parks. Great Britain. A brief discussion of the British oyster fisheries may be found under FISHERIES, vol. ix. p. 265. A recent estimate 3 gives the total value of the oysters obtained from British seas at 2,000,000, worth 2d. each, or, perhaps, 240,000,000 in all. An extensive import trade is carried on with the United States, which has grown up within the past decade, as is shown by the following statement 4 of import values: 1874, $41,419; 1 The statistical summary prepared for the Fisheries Division of the Tenth Census by Mr Ernest Ingersoll shows the details, by States, of the oyster industry of the whole country. 2 Bouchon-Braudely stated in 1877 that the industry of oyster culture in France supported a maritime population of 200,000. It is difficult to reconcile this statement with the official statistics. 3 That of Mr James G. Bertram in Brit. Quart. Rev. for January 1883. 4 Derived from the records of the United States Treasury.