128 SPOLIA ZEYLANICA. 
These cysts are widely distributed in the tissues of the oyster, 
and occur particularly in the liver, in the mantle, and along the base 
of the gills. As many as 120 have been counted in a single oyster, 
although the number varies very considerably. 
The globular cysts which occur in Placuna placenta (the window- 
pane oyster found in the backwaters of Trincomalee) are exactly 
similar to those found in the pearl oyster. In the cystic stages found 
in Placuna, Willey* also observed endogenous reproduction. In the 
examples quoted and figured by him the reproduction was poly- 
genetic, as several cysts were liberated at a time from the parent 
cyst. In the globular cysts found in the pearl oyster the endogenous 
reproduction has only as yet been observed to be monogenetic (one 
cyst being born at a time from each parent cyst), but there can be 
little doubt that, when more oyster material is available, this endo- 
genous reproduction will be found to be polygenetic, as in the larve 
inhabiting Placuna. A similar multiplication has also been noted 
in Polycercus—bladder stage of Tenia nilotica from Cursorvus 
europeus.t 
It is round the cystic stages which occur in the pearl oyster that 
the orient or cyst pearls are formed. Other pearls are also found 
in the oyster, but they have no organic nucleus. Such pearls are 
termed muscle or seed pearls. Their origin is obscure, but they 
are always found near the muscle insertions, and are believed to be 
formed round a calcospherule of excretory craw or by the sheer 
of muscles moving in different planes. 
The percentage of globular cysts in the oyster which ever become 
the nucleus of a pearl is very insignificant indeed. Occasionally 
several hundred oysters can be examined each containing 20 or 30 
cysts, and not a single pearl is to be found. This fact lends colour 
and probability to the belief that only such cysts which, for some 
unaccountable reason, die in the tissues of the oyster become 
nuclei of pearls. 
Figures of sections of decalcified pearls showing a nucleus exactly 
similar to the larva found in the tissues of the oyster are given 
by Herdman in Vol. V., ‘‘ Ceylon Reports” (Pearl Production, 
Plate II.), and there can be no doubt that this larva is the prime 
factor in pearl production, although very rarely grains of sand have 
been found in the centre of pearls. 
It has already been observed that no stage in the life-history of 
the pearl-inducing worm has as yet been obtained earlier than the 
globular cyst occurring in the oyster. This globular cyst is in many 
ways different from stages known to occur in the life-history of 
other Cestodes, such as Bothriocephalus latus or Tenia nilotica. 

ns — a ae 
* «© Report on the Window-pane Oyster of the Eastern Province,” June, 
1907. Spolia Zeylanica, Vol. V., Part XVII. 
+ Haswell and Hill: ‘* On Polycercus, a proliferating Cystic Parasite of 
the Earthworms” (Proc, Lin, Soc. N. 8. Wales (2), Vol. VIIT., 1894. 
