Page 72 - Records of Bahrain (3) (ii)_Neat

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Records oj Bahrain

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In the case of the Ceylon pearl oyster the cause of formation of the Causes of
pearl is shown to be, as a rule, the secretion of nacreous matter within a formalicnf
cyst surrounding the body of a minute Cestode parasite, and in the case of
the Persian Gulf oyster it is probably the same or analogous.* The
inhabitants of the Gulf still hold to the ancient superstition that the pearl
is a drop of dew or ram, which tho oystor has taken in by rising to tho
surface of the sea at night, or during a shower,
lo the technical classification of pearls in the Gulf we shall return Principal
latei on ; at present it is sufficient to observe that the natural distinctions vaviotica,
arc those of colour, shape, and specific gravity. With reference to colour
it may be mentioned that black pearls (by Arabs called “dead ,J pearls)
of high value are seldom found ; such as are obtained are usually dull and
' * ^t° following data regarding pearl production are extracted from Professo1'
A. Herd mini's Report on the Pearl Oyster Fisheries of the Gulf of Manaar,
1903*00; it is possible, however, that the factors in tho process may not bo in tho
Persian Gulf exactly the eamo as they are in Ceylon waters.
The formation of a pearl in an oyster is not a natural or healthy, but on the
contrary an abnormal and morbid occurronco. Tho cause of formation in not
invariable ; but it is generally the intrusion of a minuto parasite,—tho opaque white
globular larva of a Cestodo worm. This Ccstodo as a rulo belongs to tho genus
Tetrarhyncus -, its length, wliilo it inhabits tho body of tho oyster, ranges from
'07 to '53'millimetres. The life-history of this parasite is not yet fully determined;
but thero is reason to think that it exists at first as a free-swimming larva in the sen,
that it then makes its wav into tho oyster, that it subsequently passes into the file-
fish which preys upon the oyster, and that it rennhoa its final and adult stato in tho
shark or ray by which iu its turn the Ale-fish is devoured.
The Cestodo on finding cntranco into the body of the oyster bccomos surrounded
by a sac, which in some cases consists of ronnectivo tissue, and in others is epithelial!
in some localities hardly an oyster is freo from such cysts, and as many ns 45 hav®
been counted in an individual spocimon. The formation of a pearl generally takes
place in one of these cysts, by tho nacreous matter socreted by the interior wall9 of tho
cyst being deposited around the larval parasite in tho centre. Only about one c^st
in a hundred is pearl-bearing, and such a cy9l seems always to belong to the epithelial
variety ; the chemical composition of the contained pearl, it has also been ascertained,
roeemhios the naorcous lining of the shell, which is an epithelial product. Tho presoncc
within the body of the oyster of epithelial cells (which must be derived from tho
epidermis of the mantle) is not yet fully explained: possibly the first colls of
this kind are carried in along with itself by tho burrowing parasite. Whether pearl
formation commences before the death of tho parasite or after is a point still undeter­
mined. Cysts, both pearlbearing and non-peavlhearing, occur in many positions
throughout tho viscera and mantle of the oyster. The honour of being the first to
oonnect the formation of pearls with vermian parasitos belongs to Dr. E. P. Kcloart
of Ceylon who died in 1859.
It should be clearly understood, however, that all pearls aro not " cyst pearls
and that some are due to causes other than parasitic invasion. Thero aro also
" muscle pearls, ** which are generally found in tho muscular tissuo of the oyster near
the insertions of the levator and pallial muscles and havo as a nucleus a " calcospbcr*
ule " or tiny calcareous concreliob ; such pearls, when present, are usually numerous,
and ou one occasion 193 (of which 23 were visible to the naked oye) were found at the
insertion of one levator muscle. Another variety is the “ ampullar ” ponrl, formed
between the shell and the mantle or in an external pouch (ampulla) of the latter. In
this last kind the nucleus may be a grain of sand or other inorgnmo particle ; but
such is rarely tho case, and, out of some hundreds of pearls of all sorts oxaminoa m
Ceylon, only 3 were found to have a nucleus of this nature. Naorcous oxcrcsceoces or
sc-called “ blister pearls " attached to the interior of tl o shell aro duo to the irritation
caused by boring animals which work through from the out^do, or to the ontranoo
otherwise of foreign bodies between the oyster and its shell ; but those can hardly
he considcied peaile. In some pearls no nucleus at all is discoverable.
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