Oysters are gathered from natural beds or from artificial grounds. The oyster breeders place movable tiles or frames for the spat to fall on. When the young have become affixed to these “stools” they are frequently carried away to develop in a different locality. The oysters are finally fattened in sea ponds or inlets that contain a large diatom population. At Marennes, on the west coast of France, the water in which the oysters are grown contains a particular blue diatom. After feeding upon these diatoms, the beard of the oyster becomes stained a bluish-green colour—the well-known “Marennes vertes” oysters. A natural oyster bed is formed on stony ground free from mud and sand, so that the oyster, after becoming attached to a stone, is completely surrounded by clear sea-water. Oysters do not flourish in water containing less salt than ordinary sea-water. Thus, there are no oysters in the Baltic Sea. The chief enemies of the oyster are the dog-whelk that bores through the shell, and the starfish that pulls the valves apart and attacks the oyster inside. The oyster is widely distributed in tropical and temperate seas all over the world. The approximate value of the annual oyster crop of the world is £4,000,000, representing a crop of 10 billion oysters. In Europe up to 75 per cent of the oysters are reared from spat in artificial beds—not more than 7 per cent being “native.” In the United States, however, over 40 per cent are still obtained from natural beds. The simplest form of oyster culture is the preservation of the natural bed. These beds are easily destroyed or made unproductive by over-dredging. Colonies are broken up. Other animals are admitted. Breeding oysters are covered up by stones and shells, and suffocated. Ridges suitable for the development of the spat are broken down. After the beds have been properly protected and preserved the next stage is to extend the area of the natural beds. This involves a knowledge of the conditions of depth, temperature, salinity and character of bottom that are necessary to the successful growth of the oyster. Finally the productivity of an oyster “park” and the quality of its produce can be greatly improved by providing artificial “stools” for the reception and development of the spat. Many substances can be used for this purpose. The Romans used earthenware tiles, and similar tiles are used to this day in France. Brushwood, trees, stones and stakes, and old oyster shells (cultch) are also used. The earthenware tiles used in France are hollowed on one side to receive the spat, and are coated with lime to facilitate the removal of the oysters when they are a year old. They are then from 1/2 to 1 inch in diameter, and are picked off the stools and placed on stands where they are thinned out from time to time as they grow.