Fully as remarkable as the observations of Dr. Jameson are the results claimed by Professor Dubois in experimenting with a species of pearl-oyster (_M. vulgaris_) from the Gulf of Gabes on the coast of Tunis, where they are almost devoid of pearls, a thousand or more shells yielding on an average only one pearl. Conveying these to the coast of France in 1903, he there associated them with a species of trematode-infested mussel (_Mytilus gallo-provincialis_), and after a short period they became so infested that every three oysters yielded an average of two pearls.[65] This claim has not been without criticism; but who ever knew scientists to agree?
In the pearl-oyster of the Gambier Islands (_M. margaritifera cumingi_), Dr. L. G. Seurat found that the origin of pearls was due to irritation caused by the embryo of a worm of the genus _Tylocephalum_, the life of which is completed in the eagle-ray, a fish which feeds on the pearl-oyster.[66]
In 1903, Prof. W. A. Herdman, who, at the instance of the colonial government, and with the assistance of Mr. James Hornell, examined the pearl-oyster resources of Ceylon, announced: “We have found, as Kelaart did, that in the Ceylon pearl-oyster there are several different kinds of worms commonly occurring as parasites, and we shall, I think, be able to show that Cestodes, Trematodes, and Nematodes may all be concerned in pearl formation. Unlike the case of the European mussels, however, we find that in Ceylon the most important cause is a larval Cestode of the Tetrarhynchus form.”[67]
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.