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Shell Growth Begins: Shell growth begins early in the development of the larval form of the mollusk, and in most snails the young already have two or three spiral shell whorls, known as nepionic or nuclear whorls, before they hatch from the egg capsule. In the young stages of many bivalves, the early free-swimming form, such as the spat, or young, of oysters, has two shelly valves which in a few days grow too large and heavy to permit further ;vimming. Additional growth occurs in clam -hells along the free edge of the valve, and, in the case of snail shells, along the rim of the outer lip of the aperture.
Lateral growth in trees frequently begins later than growth in height, but the period of lateral growth is longer. Rates of lateral growth likewise are low at the outset, increasing after a few weeks and then diminishing. In view of the long annual period of lateral growth in trees, deleterious environmental conditions may exercise a pronounced effect on growth. In the wood of trees of temperate regions, narrow and wide growth rings may be observed, marking years characterized by unfavorable and favorable conditions for growth.
Structure and Growth of Shells.—The shell material is produced in the form of a liquid secretion mainly from glands located along the edge of the fleshy, capelike mantle, an organ common to all mollusks. The liquid calcium carbonate becomes crystallized to form additions to the hard shell when mixed with a colloidal albumen which is also produced by the mantle. Shell deposition is not continuous, but occurs periodically during the life of the animal, sometimes every few days, weeks, or even seasonally.
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