Little is known as to the longevity of molluscs, but from the size of some of the bivalves, and from the partitions in the shell of the large nautilus, some of them must have many years of life. An Oyster Culture Company in England have verified oysters up to twenty years. At the age of two years oysters begin to give out spat, which assumes a calcareous covering in about a fortnight, and by the time they are a year old, the shells are about 1 inches across. Neither has it been ascertained for how long the oyster continues to produce. Semicircular rings on the convex hack of the oyster shell indicate repetitions of growth, but seemingly not annual growth. On one occasion the spat of one oyster was counted, and found to number 1,740,000, but the quan tities of spat are not continuous. In 1867, 1858, and 1859, the oysters on the British coast produced spat in great abundance, and it is supposed that this was in some relation with the warmer temperature of the water. The great enemy of the oyster is the star-fish. It spreads itself over the.convex surface of the shell, and by some mechanical process or chemical action bores through to the animal within. The dog-whelk, and also Murex erinaceus or sting-winkle, also destroy many by boring through the shell ; and barnacles likewise destroy much of the young spat. The starfish swallow the smaller bivalve entire, and dissolve the animal out of its shell.
Defence.—To protect themselves from enemies, many gasteropods withdraw within their covering shell, and close the opening by a horny or Shelly lid. Most spiral shells are provided with this means of sheltering themselves. Bivalve shells, as the oyster and mussel, close their valves.• The cephalo pods are provided with an ink-bag, from which it ejects a black pigment to cloud the water and facilitate its escape. This was formerly used for writing, and in the preparation of sepia, but Indian ink and sepia are now made of lamp-smoke or of prepared charcoal. In cold and temperate climates, the molluscs are subject to hybernation ; also aestivation when the heat is great. In these changes they cover themselves with leaves, or bide in the mud; the limneidm frequently glide beneath the surface of the water, and hybernate or mstivate in the mud. There are various other defences. The olives or rice-shell Molluscs are very active animals, and can turn over when laid on their back ; as the tide retires, they may be seen gliding about or burrowing in the sands. Conus aulicus, one of the cones (Conidm), sometimes bites when handled. The solen or razor-fish, if taken from its burrow, can quickly rebury itself.
Atimentaiy.Molluscs are eaten by most races. Amongst these are the loligo, the oyster, mussel, cockle, periwinkle, the baliotis, scallops, and whelks ; the arms of the cuttlefish are eaten by the Neapolitans and Chinese.
Helix pomatia, the grand escargot or apple snail, is eaten in France during Lent. They are reared and fattened in snail gardens. This was a culture of the ancient Romans.
Fusus antiquus (Chrysodomus antiquus), the red whelk, is largely dredged in Britain for food. Applied to the ear, a sound as of the sea may always be heard in it, from which it is named the roaring buckie. The common whelk of Britain,
Buccinum undatum, is eaten by the people, and used as bait by fishermen. It is dredged for, and is also taken in baskets, baited with dead fish.
Another edible mollusc is the common peri winkle, Littorina littorea. Besides these, the mussel scallop, Pecteu maximus, which is served up broiled, and the razor-fish (Solen maximus), the cockle (Cardium edule), the limpet (Patella vulgata), are all eaten. The mussels are full grown in a year; and from some unascertained cause they are at times extremely deleterious. They are largely used for The poor of India use as food species of Cyclas, Cerithium, Nerita, and Corbicula, one species found' within tidal influence, and two others in fresh water. Also Velorita cyprinoides, Gray, of the family Cyrenidm, Unio, Limnea stagnalis, Ampullaria glauca, L., Planorbis Indicus, P. Coro mandelicus, and Paludina Bengalensis.
Ommastrephes sagittatus is the principa food of the dolphin and cachalots, as well as of the albatross and larger petrels, and is exten• sively used in the cod fishery off Newfoundland Sailors call them sea arrows, also flying squids from their habit of leaping out of the water often to fall on the ship's deck. The pteropoch swarm in all seas. In high latitudes they are th( principal food of the whale, and also of many birds. The mussel, the limpet, the whelk, an the calamary are all used as bait in the fisheries.
Economy.—Many shells consist of two layers an upper or outer, which is rough, and more co less developed, and an inner layer, called nacre which is smooth. The nacre consists of a series of delicate layers one over the other in an imbri• cated manner, with their edges presenting a series of lines with waved margins. The brilliancy the colours reflected from this iridescent surfacl depends on the thinness of the laminae or layer( of the nacre ; where the layers are thick, like those in the oyster, only a dull white appearance is presented, but, as in the ear-shell and mother. of-pearl shell, the iridescent colours reflected fron the thin layers are beautiful ; they are used for the ornamentation of papier-mache work, and for other ornamental purposes. As seen in cabinets, the outer rough layer is always removed, and the hard layer below polished. The beautiful pearl so largely used for ornament, the name of which is synonymous with purity, is the produce of various species of molluscs. The nacre is secreted by part of the mollusc known as the mantle, and pearl is the result of the nacreous matter being. secreted by the mantle around some nucleus lyinj loose in the body of the shell, generally a grain 01 sand or some loose particle of nacre. The largess pearls are obtained near Ceylon in the Persia" Gulf, and in the Persian Gulf on the coast of Arabia, from the pearl oyster, the Avicula mar. garitifera, in eastern seas from A. fimbriata of California. The shells are opened, and the pearl: picked out, and the shells are used for their nacre which is called mother-of-pearl, almost as valuabh as the pearl, being, with that of the Haliotis splen. dens, extensively employed in ornamenting papier. macho work.