Genre with Oysters. Oyster is the common name for a number of different families of salt-water bivalve molluscs that live in marine or brackish habitats. In some species, the valves are highly calcified, and many are somewhat irregular in shape. Many, but not all oysters are in the superfamily Ostreoidea. Some types of oysters are commonly consumed, and in some locales are regarded as a delicacy. Some types of pearl oysters are harvested for the pearl produced within the mantle. Windowpane oysters are harvested for their translucent shells, which are used to make various kinds of decorative objects. The word oyster comes from Old French, and first appeared in English during the 14th century. The French derived from the Latin, the feminine form of, which is the latinisation of the Ancient Greek oyster. Compare bone. True oysters are members of the family Ostreidae. This family includes the edible oysters, which mainly belong to the genera Ostrea, Crassostrea, Ostreola, Magallana, and Saccostrea. Examples include the European flat oyster, eastern oyster, Olympia oyster, Pacific oyster, and the Sydney rock oyster. Ostreidae evolved in the Early Triassic epoch: The genus Liostrea grew on the shells of living ammonoids. Almost all shell-bearing mollusks can secrete pearls, yet most are not very valuable. Pearls can form in both saltwater and freshwater environments. Pearl oysters are not closely related to true oysters, being members of a distinct family, the feathered oysters. Both cultured pearls and natural pearls can be extracted from pearl oysters, though other molluscs, such as the freshwater mussels, also yield pearls of commercial value. The largest pearl-bearing oyster is the marine Pinctada maxima, which is roughly the size of a dinner plate. Not all individual oysters produce pearls naturally. In nature, pearl oysters produce pearls by covering a minute invasive object with nacre. Over the years, the irritating object is covered with enough layers of nacre to become a pearl. The many different types, colours and shapes of pearls depend on the natural pigment of the nacre, and the shape of the original irritant. Pearl farmers can culture a pearl by placing a nucleus, usually a piece of polished mussel shell, inside the oyster. In three to seven years, the oyster can produce a perfect pearl. Since the beginning of the 20th century, when several researchers discovered how to produce artificial pearls, the cultured pearl market has far outgrown the natural pearl market. A number of bivalve molluscs also have common names that include the word oyster, usually because they either taste like or look somewhat like true oysters, or because they yield noticeable pearls. In the Philippines, a local thorny oyster species known as Tikod amo is a favorite seafood source in the southern part of the country. Because of its good flavor, it commands high prices. Oysters breathe primarily via gills. In addition to their gills, oysters can exchange gases across their mantles, which are lined with many small, thin-walled blood vessels. A small, three-chambered heart, lying under the adductor muscle, pumps colorless blood to all parts of the body. At the same time, two kidneys, located on the underside of the muscle, remove waste products from the blood. Their nervous system includes two pairs of nerve cords and three pairs of ganglia. There is no evidence that oysters have a brain. While some oysters have two sexes, their reproductive organs contain both eggs and sperm. Because of this, it is technically possible for an oyster to fertilize its own eggs. The gonads surround the digestive organs, and are made up of sex cells, branching tubules, and connective tissue. Once her millions of eggs are fertilized, the female discharges them into the water. The larvae develop in about six hours and exist suspended in the water column as veliger larvae for two to three weeks before settling on a bed and reaching sexual maturity within a year. Oysters are filter feeders, drawing water in over their gills through the beating of cilia.