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oceanic vent An aquatic biome associated with hydrothermal vents located in deep-sea, midoceanic ridges; these warm vents are of volcanic origin.

octopus The soft, bag-shaped body of the octopus pulses like a bellows. Eight long, slender arms, lined with cuplike suckers and connected by thin membranes, reach out in all directions. Over the animal flow waves of color gray, brown, pink, blue, green, perhaps even an angry red if it is suddenly frightened. It may move over the bottom of the aquarium tank on the tips of its arms as delicately poised and graceful as a ballet dancer. One has the impression of a soft, flexible creature in constant, controlled motion. About 50 different kinds of octopuses are distributed throughout the oceans of the world. Those off the east and west coasts of North America have bodies only three or four inches long, with short arms about as long as the body. Other kinds reach a total length of nine or ten feet and may weigh 70 pounds.

The octopus dwells on the ocean bottom in shallow waters where it crawls about on its arms, searching in every crevice for its favorite food of shrimps, crabs, and mussels. It is a skillful hunter and attacks such large prey as sharks and dogfish. It stalks a victim until it can drop down on it from above. Then wrapping the arms around it, and with suckers firmly attached, it drags the prey into its powerful jaws. It is the terror of pearl divers in Asian waters, where it has been known to hold a man down until he drowns. If the octopus is losing the battle it shoots out a cloud of black ink and hastily retreats. It may lose one or more arms in the fight, but they are soon replaced. Its worst enemies are the savage conger and moray eels.

It is an antisocial creature, living alone in a rocky den. Sometimes it digs its own den, moving stones of considerable size and closing the entrance with stones when it retreats inside. The female lays her eggs either singly or in grapelike clusters, depending on the species. She guards them for as long as 50 days until they hatch and then leaves them.

omnivorous Referring to an animal that eats both plant and animal life.

onion . It is thought that the onion gets its name from the Latin unus, meaning "one." The onion has been used as a cure for a variety of ailments, but it is best known for its use in cooking. A biennial bulb-bearing plant, the onion has hollow leaves that project from a short stem found at the base of the bulb. It has a fibrous root system. Onions are grown in fertile moist soil from either small black seeds or sets. Sets are small bulbs or a clove of a separable bulb or bulbels, which appear in the flower cluster of some onions. Scallions also called green onions or table onions and pearl onions small white onions are pulled before maturity. Ripe onions are kept longer in the ground before harvesting. Onions are often dried, dehydrated, boiled, or pickled. The onion's pungent, sulphur-rich oils cause tearing when they mix with moisture in the eyes. The common onion (Allium cepa) belongs to the lily family Liliaceae. Plants related to the onion are shallots, chives, leeks, and garlic.

orchid Extraordinary prices have been paid for a single rare specimen of the remarkable orchid family of plants. These flowers are so exquisite in their delicate colorings of rose, lilac, yellow, white, and green, so pleasing in their fragrance of violet or orrisroot, and so graceful in many of their forms that they have become the favorites of flower lovers around the world. Large numbers of orchids are imported annually from South America, the East Indies, the Philippines, and Australia. In fact, to prevent their total extinction, some South American governments have been forced to stop all orchid collecting.The orchid family, or Orchidaceae, is generally regarded as the largest family of flowering plants in terms of number of species, which is estimated at some 17,000 to 30,000. They are found nearly worldwide and are particularly abundant in the tropics

ovulation -induced Ovulation that is triggered by copulation

owl Night-flying raptors, or Strigiformes are made up of two families, the barn owls and the typical owls, vary in size from the large eagle owl to the tiny Scops Owl. However, the similarities among owls are more remarkable than are the differences. In general, owls have very keen hearing, eyesight that is extremely sensitive to any movement in dim light, eyes situated in the front of the head, soft feathers that allow for "silent" flight (an aid in hunting), facial discs, and heads that can rotate 270 (three quarters of a circle). Owls eat small birds, rodents, and, rarely, carrion. They bolt their food down, ejecting the undigestible parts in the form of pellets. There seems to be no regularity in their breeding seasons. They typically lay two to seven eggs in hollow trees, in other birds' nests, or in human structures. The female owls are usually larger than are the males.

oyster One of the most nourishing of all sea foods, the oyster is a bivalve a shellfish having two shells of the mollusk group. They live in quiet bays and river mouths and are found along the shores of all temperate and tropical oceans.Oysters belong to the families Ostreidae and Aviculidae. The hundred species of oyster vary widely in size, shape, and habits. The inedible oysters from which fine pearls are obtained belong primarily to the genus Meleagrina. The edible oyster never produces a valuable pearl because its inner lining is chalky, lacking the mother-of-pearl that gives the true pearl its beauty.

Oysters breed in the summer. The female can produce almost half a billion eggs in a single season, but only about one egg in four million will reach maturity. Within five to ten hours of fertilization the egg produces a free-swimming larva (collectively called spat) that swims by means of a crown of rapidly moving hairs, or cilia. At the end of two weeks the larva attaches itself to a stationary object. This process is known as setting or striking. Only a small percentage of larvae make a successful strike. The young oyster grows rapidly, edible oysters reaching market size in three to five years. The oyster shell is usually oval in shape, grayish white, and coarsely ridged. There are two halves, or valves, that open and close at the hinge. The lower half, which attaches to a submerged object, is larger and rather flat; the upper valve is arched. The soft body of the oyster is attached to the shell's interior by a stout muscle that extends from one valve to the other and enables the oyster to close its shell tightly. A fold, called the mantle, completely lines the shell and secretes new shell. Large flaps form the gills, which are covered by very fine hairs. When the shell is open the hairs move rapidly, causing oxygen-laden water to flow over the gills. The current also pulls in food particles that strain through the gills and into the mouth and digestive tract. A large oyster may filter more than a barrel of water in a day.

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