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44
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THE BOOK OF THE PEARL
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the
first larval stage, and the third is the edible mussel, in which the
second larval stage of the parasite stimulates the formation of pearls.
At the Brighton Aquarium and the Fish Hatchery at Kiel, Dr. Jameson
claims to have succeeded in artificially inoculating perfectly healthy
mussels with these parasites by associating them with infested
mollusks, and thereby producing small pearls.
From
Dr. Jameson's interesting paper we abridge the following account of
the manner in which the pearls are developed. The trematode enters Mytilus edulis as
a tailless cercaria,and at first may of ten be found between the mantle
and the shell. The larvse, after a while, enter the connective tissue
of the mantle, where they come to rest, assuming a spherical form,
visible to the naked eye as little yellowish spots about one half
millimeter in diameter. At first the worm occupies only a space lined
by connective-tissue fibrils, but soon the tissues of the host give
rise to an epithelial layer, which lines the space and ultimately
becomes the pearl-sac. If the trematode larva completes its maximum
possible term of life, it dies, and the tissues of the body break down
to form a structureless mass which retains the form of the parasite,
owing to the rigid cuticle. In this mass arise one or more centers of
calcification, and the precipitation of carbonate of lime goes on until
the whole larva is converted into a nodule with calcospheritic
structure. The granular matter surrounding the worm, if present, also
undergoes calcification. The epithelium of the sac then begins to shed
a cuticle of conchiolin, and from this point the growth of the pearl
probably takes place on the same lines and at the same rate as the
thickening of the shell.1
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.2 This claim has not been without criticism ; but who ever knew scientists to agree ?
In the pearl-oyster of the Gambier Islands (M. margaritifera cu-mingi), Dr. L. G. Seurat found that the origin of pearls was due to irritation caused by the embryo of a worm of the genus Tylocepha-lum, the life of which is completed in the eagle-ray, a fish which feeds on the pearl-oyster.3
1 "Proceedings of the Zoological Society of ! Seurat, "Observation sur l'évolution de
London," 1902, pp. 148-150. l'Huître perlière des Tuamotu et des Gara-
a "Comptes Rendus de l'Académie des Seien- bier," 1904. ces," Paris, 1903, Vol. CXXXVII, pp. 611-613.
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