"دليل الخليج. مجلد I. القسم التاريخي. الجزء II. ج. ج. لوريمر. ١٩١٥" [‎٢‎٢‎٢‎٥] (١٢٦٢/٧٤٢)

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In the case of the Ceylon pearl oyster the cause of formation of the Causes of
earl is shown to be, as a rule, the secretion of nacreous matter within a forr ^ a Hon
cyst surrounding the body of a minute Cestode parasite, and in the case of
thg Persian Gulf oyster it is probably the same or analogous.* The
inhabitants of the Gulf still hold to the ancient superstition that the pearl
jg a drop of dew or rain, which the oyster has taken in by rising to the
surface of the sea at night, or during a shower.
To the technical classification of pearls in the Gulf we shall return Principal
later on; at present it is sufficient to observe that the natural distinctions varieties,
a re those of colour, shape, and specific gravity. With reference to colour
it may be mentioned that black pearls (by Arabs called "dead " pearls)
of high value are seldom found ; such as are obtained are usually dull and
# The following data regarding pearl prodnction are extracted from Professo 1,
W. A. Herdman's Kejoort on the Fearl Oyster Fisheries of the Gulf of Manaary
1903-06; it is possible, however, that the factors in the process noay not be in the
Persian Gulf exactly the same as they are in Ceylon waters.
The formation of a pearl in an oyster is not a natural or healthy, but on the
eontrary an abnormal and morbid occurrence. The cause of formation is not
invariable; but it is generally the intrusion of a minute parasite,—the opaque white
globular larva of a Cestode worm. This Cestode as a rule belongs to the genus
Tetmhyncus ', its length, while it inhabits the body of the oyster, ranges from
•07 to'53 miliimetres. The life-history of this parasite is not yet fully determined;
but there is reason to think that it exists at first as a free-swimming larva in the sea,
that it then makes its way into the oyster, that it subsequently passes into the file-
fish which preys upon the oyster, and that it reached its final and adult state in the
shark or ray by which in its turn the file-fish is devoured.
The Cestode on finding entrance into the body of the oyster becomes^ surrounded
by a sac, which in some cases consists of connective tissue, and in others is epithelial;
in some localities hardly an oyster is free from such cysts, and as many as 45 have
been counted in an individual specimen. The formation of a pearl generally takes
place in one of these cysts, by the nacreous matter secreted by the interior walls of th®
cyst being deposited around the larval parasite in the centre. Only about one c^st
in a hundred is pearl-bearing, and such a cyst seems always to belong to the epithelial
variety; the chemical composition of the contained pearl, it has also been ascertained,
resembles the nacreous lining of the shell, which is an epithelial product. The presence
within the body of the oyster of epithelial cells (which must be derived from the
epidermis of the mantle) is not yet fully explained; possibly the first cells of
thisldfld are carried in along with itself by the burrowing parasite. ^ Whether pearl
formation commences before the death of the parasite or after is a point still undeter
mined. Cystg, both pearlbearing and non-pearlbearing, occur in many positions
throughout the viscera and mantle of the oyster. The honour of being the first to
connect the formation of pearls with vermian parasites belongs to Dr. E. F. Kelaart
of Ceylon who died in 1859.
It should be clearly understood, however, that all pearls are not " cyst pearls
a^d that some are due to causes other than parasitic invasion. There are also
''moscle pearls, which are generally found in the muscular tissue of the oyster near
tbemsertions of the levator and pallial muscles and have as a nucleus a " calcospher-
ftle * or tiny calcareous concretio^ ; such pearls, when present, are usually numerous,
and on one occasion 193 (of which 23 were visible to the naked eye) were found at the
insertion of one levatoi* muscle. Another variety is the " ampullar pearl, formed
between the shell and the mantle or in an external pouch (ampulla) of the latter. In
tbif last kind the nucleus may be a grain of sand or other inorganic particle ; but
such is rarely the case, and, out of some hundreds of pearls of all sorts examined in
le ylon, only 3 were found to have a nucleus of this nature. Nacreons excrescences or
so-called " blister pearls " attached to the interior of the shell are due to the irritation
caused by boring animals which work through from the outside, or to the entrance
u erw \ fi ® 0 f foreign bodies between the oyster and its shell; but these can hardly
peailg. In some pearls no nucleus at all is discoverable,