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."1
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.2
The
spherical larvae 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 larvae. 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
"calcosphe-
1 "Pearl Oyster Fisheries of the Gulf of centa Pearl Fishery of Lake Tampalaka-Manaar," London, 1903, Vol. I, p. II. mam," Colombo, 1906.
2 Hornell, "Report on the Placuna pla-
