THE CEYLON PEARL-OYSTER. 2h 
states that these cysts which form around the parasites become 
the nuclei of pearls, and a decalcified pearl shows an organic 
nucleus in the centre surrounded by concentric layers of conckyolin, 
the whole having a diameter of about a millimetre, and the nucleus 
being a scolex *225 mm. long and easily recognisable as that of 
T'ylocephalum. 
It appears that in this case also the parasites may be present 
in great numbers without pearls being found (Seurat (35), 1904, 
p- 295). 
Here, again, examination of the scanty material available (see 
p- 346) has yielded no confirmation of the presence of Cestodes in 
the pearls of J. margaritifera var. cumingit. Iam endeavouring 
to obtain further material from the Eastern Pacific, in order to 
extend my observations on this species. 
I may say that from the first time I read Professor Herdman’s 
Reports and papers on the subject I was sceptical as to the 
relationship of the Cestode to pearl-formation. 
Indeed, before Prof. Herdman’s departure for Ceylon, on 
examining Dr. Kelaart’s material at the British Museum, which 
Mr. KE. A. Smith kindly placed at my disposal, I had detected the 
existence of these Cestode iarvee (which Kelaart seems to have 
regarded as “eggs of Entozoa”) in their connective-tissue cysts in 
the Ceylon pearl-oyster, and after having examined the larvee, and 
also having decalcified pearls from the same oyster, dismissed the 
Cestode as probably not concerned in pearl- formation. 
My chief grounds for doubting the Cestode theory were the 
following :— 
(a) The absence of evidence that the Cestode ever occurred in 
an epidermal sac, and the fact that it was almost invariably sur- 
rounded by a fibrous capsule or cyst which does not oceur around 
the pearl. 
(6) The large proportion of the Cestodes that showed no sign 
of becoming pearl-nuclei, pointing to the conclusion that pearl- 
formation does not necessarily, or even normally, follow from 
infection. Thus, in a footnote to p. 12 of Part V. of his Report, 
Prof. Herdman says : 
“In comparing these statistics |7. e. of numbers of parasites 
and of pearls in Mytilus] with those of the Ceylon pearl- 
oyster, one is struck by the wholly different ratio borne 
by pearls to parasites in thetwo cases. In the mussels, 
pearls are far more numerous than the living parasites. In 
our Ceylon oyster, parasites may be exceedingly abundant * ; 
while pearls (cyst-pearls) are relatively very rare, probably 
not more than one to a hundred parasites.” 
* Mr. Southwell (42), p. 128, says; *‘ As many as 120 have been counted in asingle 
oyster ”’; and further down on the same page: ‘‘ Occasionally several hundred oysters 
can be examined, each containing 20 or 30 cysts, and not a single pearl is to be 
found,” 
18* 
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