Naturalists generally accepted these conclusions, that pearls originate in pathological secretions formed, either as the result of the intrusion of hard substances, or by the encysting or covering of ova or other objects of internal origin; and there was no important cleavage of opinion until the development of the parasitic theory, as a result of the researches of the Italian naturalist Filippi, and those following his line of investigations. This theory is not severely in conflict with those of Réaumur, Linnæus, Home, etc., but relates principally to the identity of the irritating or stimulating substance which forms the nucleus of the pearl. In examining a species of fresh-water mussel, the _Anodonta cygnea_, occurring in ponds near Turin, and especially the many small pearly formations therein, Filippi observed that these were associated with the presence of a trematode or parasitic worm, which he named _Distomum duplicatum_, and which appears to be closely allied to the parasite which causes the fatal “rot” or distemper in sheep. Under the microscope, the smallest and presumably the youngest of these pearls showed organic nuclei which appeared undoubtedly to be the remnants of the trematode. In Anodonta from other regions, which were not infested with the distoma, pearls were very rarely found by Filippi. In a paper,[55] published in 1852, containing a summary of his observations, he concluded that a leading, if not the principal, cause of pearl-formation in those mussels was the parasite above noted; and in later papers[56] he included such other forms as _Atax ypsilophorus_ within the list of parasitic agencies which might excite the pearl-forming secretions, comparing their action to that of the formation of plant-galls. [Illustration: Mexican pearl-oyster (_Margaritifera margaritifera mazatlanica_) with adherent pearl ] [Illustration: Group of encysted pearls in shell of Australian pearl-oyster (_Margaritifera maxima_) ] American Museum of Natural History [Illustration: Mexican pearl-oyster (_Margaritifera margaritifera mazatlanica_) with encysted fish ] American Museum of Natural History [Illustration: Group of encysted pearls (Oriental) ] [Illustration: Reverse of same group, showing outline of the individual pearls ]