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Show 1902.] ORIGIN OF PEARLS. 141 It has had many supporters, and still maintains a prominent position in zoological text-books and popular compilations. It is doubtless largely due to a confusion of true pearls with " blisters " or pearly excrescences on the shell. There is no recorded instance of an undoubted sand-grain having been found in a pearl, although hundreds have been examined. All attempts to produce pearls by introducing such bodies into the tissues or between the shell and mantle have led, at best, only to the formation of " blisters." Such methods of obtaining the latter have long been known to the Chinese, and have repeatedly been applied in other countries. Chemnitz, Beckmann, and others (1791) regarded Linnaeus's " secret process " as merely boring the shells. However, no subsequent boring experiments have yielded anything but blisters, and the popular notion of Linnaeus's modus operandi is little more than a guess. A great step in the right direction was made when Filippi, in 1852, discovered the connection between pearls and the presence of Distomum duplicatum in Anodonta. Filippi regarded these Trematodes as encysted. In his later papers he allowed other forms such as Atax ypsilophorus to be occasional causes of pearl-formation. He recognized that the action of these parasites was specific, and compared it to the formation of plant-galls. Kiichenmeister (1856) associated pearls in Margaritana margaritifera with the larvae of Atax ypsilophorus van Beneden, which occur in the mantle, enclosed in cysts secreted by the mollusc. He held that other parasites, as well as bodies of internal origin, might also cause pearls. Mobius (1857) found Trematode remains in pearls from the Pearl-Oyster of the West Coast of America (probably Margaritifera margaritifera L., var. mazatlanica Hanley *). Kelaart (1859) held that parasites played an important part in pearl-formation in Margaritifera vulgaris (Schumacher) in Ceylon, but did not associate any definite organism with it, although he found several species living in the Pearl-Oyster. Thurston (1894) confirmed the existence of platyhelminthan parasites in the same species, but did not assert that they had anything to do with pearl-production. Garner (1871) found that pearls in Mytilus edidis and Margaritana margaritifera were due to Distomids, against which the molluscs protected themselves by coating them with calcium-carbonate. Comba (1898), who claims to have discovered a method of producing free pearls by artificial means, says (p. 6) that the cause is " un parassito il quale viene dal mollusco awi-luppato di strati di una bava che indurendosi forma la perla formando cosi una pustola ed una pallina che cresce in grossezza." Dubois (1901) found in Mytilus edidis that the production of pearls was due to Distomid larva?, to which (without description) he applied the name Distomum margaritarum. His account of the " desagregation " of formed pearls, and the liberation, to repeat their life-cycle, of the parasites that form their nuclei, is quite at i For revised nomenclature of the Pearl-Oysters, see Jameson, 1901. |