i THE CEYLON PEARL-OYSTER. 331 
Cestodes may possibly occur in the centres of these pearls at 
times, as Professor Herdman maintains, just as grains of sand 
and other organic and inorganic particles certainly do; but this 
remains to be proved, and my material has afforded no evidence 
in favour of this much-quoted theory. And, even if it should 
be proved that Cestodes do occasionally occur as the “ nuclei” of 
‘eylon pearls, the real causes which underle the development 
of the pearl-sae (which is the essential factor in pearl-production) 
have still to be discovered. The characters of the centres of 
parenchyma-pearls can best be understood by reference to the 
next section of the paper, which is largely descriptive of them, 
though many of the pearls referred to therein are unquestionably 
‘*muscle-pearls,” 
(14) Descriptions or THE CENTRES OF INDIVIDUAL PEARLS. 
In the absence of evidence connecting them with muscle- 
pearls I propose to treat the four pearls from the Persian Gulf, 
and the twenty-one pearls purchased for me in Ceylon by the 
Ceylon Company of Pearl Fishers, Ltd., as parenchyma-pearls, 
and to describe along with them eight of the pearls in Prof. 
Herdman’s collection of slides which are not obviously muscle- 
pearls. These descriptions will be followed by an account of a 
number of samples and collections of pearls from J/. vulgaris 
from Ceylon and other localities. 
A. Material from the Persian Gulf. 
It may be useful to begin with the material from the Persian 
tulf. This, so far as it has been examined—for some of my 
specimens still remain unopened,—comprised three pearl-bearing 
examples, containing four small pearls. All these pearls 
appeared to be of the kind classified by Prof. Herdman as “ cyst- 
pearls” and by the present writer as ‘* parenchyma-pearls,” as 
they seem to have arisen quite independently of, and away from, 
the muscle-insertions. 
I append a short description of the conditions in each of these 
cases. 
Preparation LXIV A contained a solitary pearl, about 2mm. 
in diameter and slightly lenticular in shape, situate in the left 
mantle-lobe, on a level with the anterior end of the attachment 
of the outer gill-lamella. Unfortunately, owing to imperfect 
penetration in the embedding process, this pearl did not present 
a complete series in section. It lay wholly in the subepidermal 
parenchyma, which was sharply cut off from the underlying 
visceral connective-tissue by a layer of musculature, under which 
were the tubules of the ovary. The pseudo-nucleus was a radially 
calcified concentrically laminated body, about 3 mm. in diameter, 
with a few darkly staining granules in the centre. 
The second specimen, Preparation LXIV 3, contained another 
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