In his investigation of the Placuna oyster in 1905, Mr. James Hornell found that the origin of pearls was due to minute larva of the same stage and species as that which causes the pearls in the Gulf of Manar oyster.[68] The spherical larvæ of this tapeworm sometimes occur in great abundance, and there is evidence of forty having been found in a single pearl-oyster. Mr. Hornell states that the living worm does not induce pearl formation, this occurring only when death overtakes it while in certain parts of the oyster. As a consequence, pearls are more numerous in oysters which have been long infected, where the worms are older and more liable to die. This parasitic worm has been traced from the pearl-oyster to the trigger-fishes, which eat the pearl-oysters, and thence into certain large fish-eating rays, where it becomes sexually mature and produces embryos which enter the pearl-oyster and begin a new cycle of life-phases. It seems, therefore, that the latest conclusions of science appear entirely favorable to the parasitic theory as explaining at least one, and probably the most important, of the causes for the formation of pearls; and that some truth exists in the statement that the most beautiful pearl is only the brilliant sarcophagus of a worm. This morphological change is not peculiar to mollusks, for in most animal bodies a cyst is formed about in-wandering larvæ. Fortunately for lovers of the beautiful, in the pearl-oysters the character of the cyst-wall follows that of the interior lining of the shell, and not only simulates, but far surpasses it in luster. While the theory that pearls are caused by the intrusion of some unusual substance has the evidence of actual demonstration in many instances, and is unquestionably true to a large extent, yet microscopic examination of some pearls suggests the theory that a foreign substance is not always essential to their formation, and that they may originate in calcareous concretions of minute size, termed “calcospherules.” As regards their origin, Professor Herdman classifies pearls into three sorts: (1) “Ampullar pearls,” which are not formed within closed sacs of the shell-secreting epithelium like the others, but lie in pockets or ampullæ of the epidermis. The nuclei may be sand-grains or any other foreign particles introduced through breaking or perforation of the shell. (2) “Muscle pearls,” which are analogous to gallstones, formed around calcospherules at or near the insertion of the muscles. And (3) “Cyst pearls,” in which concentric layers of nacre are deposited on cysts containing parasitic worms in the connective tissue of the mantle and within the soft tissues of the body.[69]