Filippo de filippi of italy vigorously researched the

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Filippo de Filippi of Italy vigorously researched the fresh-water mussel Anodonta cygneafrom 1852 to 1859, finding a relationship between the trematode parasite Distomum duplicatum and pearls, concluding that parasites caused pearl formation, and proposing a parasite origin theory (de Filippi, 1852, 1859). Later, he also pointed out the participa-tion of parasites such as the water mite Atax ypsilophorus. The relationship between pearl and parasite was also suggested by Friedrich Küchenmeister, T. von Hessling, Karl Möebius, Edward Frederik Kelaart, and A. Villa (Küchenmeister, 1856; von Hessling, 1856; Möbius, 1858; Villa, 1860; Herdman, 1903; Shiplev and Hornell, 1904). Robert Garner later added his support to the parasite origin theory (Garner, 1872), and in 1898 Der Cavaliere Comba also held forth parasites as possible contributors to pearl for-mation in oysters. Furthermore, Raphael Dubois (Dubois, 1901), H. Lyster Jameson (Jameson, 1902), Louis Boutan (Boutan, 1903), and M. Alfred Giard (Giard, 1903) supported the idea of parasites as the cause of pearl formation. More-over, on the basis of their research between 1901 and 1906, M. L. G. Seurat, W. A. Herdman, and J. Hornell stated that pearl formation was caused by parasites (Nishikawa, 1907; Dakin, 1913; Fujita, 1923; Ogushi, 1938; Matsui, 1958, 1965a).Many researchers from the 19th to the 20th century like-wise supported the parasite origin theory as explaining the cause of pearl formation. Thus, many academics concluded that the larva, egg, or remains of parasites such as trema-todes and tapeworms prompted pearl formation. This was a firmly established theory.However, there were some observations that were not explained by the parasite theory. In 1859, E. F. Kelaart found many parasites in pearl oysters from Sri Lanka (then Ceylon), and accepted a role for parasites in pearl forma-tion. However, he also suggested that there were multiple causes of pearl formation, finding an egg inside a pearl taken from the gonad, and discovering that a minute silicic diatom test stimulated the mantle and became a pearl nucleus. At the same time, the egg origin theory and sand grain theory attracted much criticism (Kunz and Stevenson, 1908a). These studies, however, were obscured by the large volume of parasite origin studies, and did not draw attention.In contrast, the pearl sac that T. von Hessling found in the middle of 19th century and that L. Diguet identified as important in pearl formation received much attention at the beginning of the 20th century. In 1902, H. Lyster Jameson of England stated that a parasite could stimulate the forma-tion of the pearl sac, that the pearl sac was derived from epi-thelial cells of the mantle, and that the epithelial cells in the pearl sac were in a distinct disease state (Jameson, 1902). In 1903, L. Boutan of France also lent his support to the par-asite origin theory, stating that parasites were the cause of pearl formation (Boutan, 1903). However, in 1904, he con-cluded from experiments with M.edulisthat when a parasite entered the space between mantle and shell, and subse-quently moved into the recess in the cell layer of the mantle,

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