A pearl is a lustrous concretion produced by certain bivalve molluscs (muscles and oysters) and valued as a gem. Pearls consist almost entirely of nacre, which is the substance forming the inner layers of the mollusc shells. Nacre, known as mother-of-pearl, is composed primarily of aragonite crystals. The pearl is an abnormalgrowth resulting from the invasion of the body of the mollusc by a minute parasitic worm (it was previously believed to be invasion by a grain of sand or similar which caused the pearl to be produced), which cannot escape and irritates the mollusc which retaliates by coating the irritant with layer upon layer of nacreous material. Both marine and freshwater molluscs produce pearls, but the most valuable varieties originate in the pearl oyster of the Persian Gulf. The most highly prized pearls are spherical. When a pearl that has been cut from the shell presents a hemispherical surface, it is sometimes called a bouton pearl. If a solid pearl has an irregular shape, having grown over a rough object, it is known as a baroque pearl. In the jewellerytrade, pearls are commonly known as pear, bell, or drop, according to the shape. Pearl coloration varies widely, the most prized shades being white, black, rose, and cream. River pearls are produced by freshwater mussels in various parts of the world. China is the principal trader in river pearls. Natural, spherical pearls have been cultured successfully since 1920. In this process a mother-of-pearl bead, from three-quarters to nine-tenths of the diameter of the desired product, is introduced into the
pearl oyster. Over a period of years the oyster deposits layers of nacre around the bead. Cultured pearls are not easily distinguished from genuine pearls except by an expert. The technique of producing spherical cultured pearls was developed in Japan, and the culturing of pearls is a major Japanese industry - a small bead of mother of pearl being inserted into the oyster which grows the pearl around the bead. Artificial pearls, in contrast to cultured pearls, are entirely man-made, largely of glass. Research Pearl
The American Bittern (Botaurus lentiginosus) is a medium-large heron-like bird of the order Grallae, family Ardeidae, with a brown back, brown- streaked front, black cheek stripe and blackish primary feathers. It is an uncommon winter visitor and resident in freshwater marshes of southern California. Research American Bittern
Angelfish is any of a number of unrelated fishes. The freshwater angelfish, genus Pterophyllum, of South America, is a tall, side-to-side flattened fish with a striped body, up to 26 centimetres long, but usually smaller in captivity. The angelfish or monkfish of the genus Squatina is a bottom-living shark up to 1.8m long with a body flattened from top to bottom. The marine angelfishes, Pomacanthus and others, are long narrow-bodied fish with spiny fins, often brilliantly coloured, up to 60cm long, living around coral reefs in the tropics. Research Angelfish
Bass is the name of a number of fishes of several genera, but originally belonging to a genus of sea-fishes (Labrax) of the perch family, distinguished from the true perches by having the tongue covered by small teeth and the preoperculum smooth. Labrax lupus, the only British species, called also sea-dace, and from its voracity sea-wolf, resembles somewhat the salmon in shape, and is much esteemed for the table, weighing about 15 Ibs. Labrax linedtus (Roccus linedtus), or striped bass, an American species, weighing from 25 to 30 Ibs., is much used for food, and is also known as rock-fish. Both species occasionally ascend rivers, and attempts have been made to cultivate British bass in fresh-water ponds with success. Two species of black bass (Micropterus salmoides and Micropterus dolomieu), American freshwater fishes, are excellent as food and give fine sport to the angler. The former is often called the large-mouthed black bass, from the size of its mouth. Both make nests and take great care of their eggs and young. The Centropristis nigricans, an American sea-fish of the perch family, and weighing 2 to 3 Ibs., is known as the sea-bass.
The Brook Lamprey (Lampreta planeri) is a European freshwaterfish of the family Petromyzonidae. The Brook Lamprey is serpentine in shape, growing to about 25 cm in length, with a dark bluish grey or greyish green back and sides and a silvery white belly. The Brook Lamprey lives in small sub-montane and mountain rivers and streams, and is non-parasitic feeding on organic debris while young, and not feeding when adult but instead spawning and then dieing some days later. Research Brook Lamprey
The bullhead or Miller's Thumb (Cottus gobio) is a small freshwaterfish, about 10 cm long with a large broad head, and sharp spines on the gill covers. The armed bull-head is the Aspidophorus europoeus, found in the Baltic and northern seas; the six-horned bull-head (Cottus hexacornis) is a North American species. In America this name is given to a species of Pimelodus, called also Oat-fish and Horned-pout. Research Bullhead
The burbot or eelpout (Lota vulgaris) is a fish of the cod family, which is confined to freshwater, and is widely though locally distributed throughout Europe. It is recognisable by its long tapering body, the small head with a barbell on the chin, the elongation of the second dorsal and anal fins and the small scales. In some parts of Europe it is greatly esteemed as a food. Research Burbot
The Climbing Perch (Anabas scandens) is a freshwaterfish about 20 centimetres long, with a compressed body and a long spiny dorsal fin. It is found in India and countries to the east, and can travel long distances on land, breathing air by means of a bony labyrinth richly supplied with blood- vessels and situated in the upper part of the bronchial chamber. Research Climbing Perch
The Probert Encyclopaedia was designed, edited and programed by
Matt and Leela Probert
Abbreviations
Research Pearl
