THE CEYLON PEARL-OYSTER. 329 
Pl. XL, fig. 26 shows a pearl having for its nucleus a double 
hypostracum-pearl, the two constituents being apparently 
separated by a mass of amorphous substance. Other examples 
were examined which showed intergradations between double 
and single hypostracum-pearls, 
In fig. 27, from the same preparation as the above, there is a 
central cavity, containing a few granules, and surrounded by 
amorphous and columnar substances followed by nacre and finally 
by hypostracum. In another case in the same preparation the 
same state of affairs was shown, but there were further layers of 
nacre outside the hypostracum, resulting in a fair-sized pearl. 
These cases are of interest as indicating that the attachment of 
muscle-fibres to a pearl may arise secondarily, in the course of 
its growth. In some cases the columnar substance is stratified, 
as in the pearl at the bottom left-hand corner of fig. 19, 
Pl. XXXVIIT. andin fig. 49, Pl. XLIV. In Pl. XL. fig. 28, which 
is the left-hand pseudo-nucleus of the double pearl shown in 
fig. 49, there is at one pole a distinct transition from the columnar 
matter of the pseudo-nucleus to normal nacre (nac.’), just as 
occurs in the substance of the pearls figured at Pl. X LI. fig. 34 
and Pl, XLIII. fig. 43. This pseudo-nucleus is followed by 
normal nacre (naec.), after which there is a layer of brittle horny 
matter (am.), which I regard as analogous to amorphous repair- 
substance ; this is followed by another columnar layer (col."'), 
imperfect on one side, and this again by nacre. 
A preparation from the second unlabelled specimen in the 
British Museum showed, side by side, all kinds of intergradations 
between the various kinds of nuclei and pseudo-nuclei of muscle- 
pearls described above. 
[regret that | am as yet unable to trace the origin of the 
muscle-pearls beyond the cyst-like sac, lined with hypostracum 
or another form of. shell-substance. One or two preparations 
show cavities lined with a few cells, which may represent the 
first stages of pearl-sacs. Moreover, here and there, the relations 
of the epidermis, at the place where the muscle-attachment 
epithelium gives place to ordinary mantle-epithelium, suggest 
that these cysts arise by direct invagination at thisregion. What 
may be the mechanism which gives rise to this invagination I 
cannot say, there are certain indications that suggest that it may 
be associated with the presence of parasitic Protozoa, an idea 
suggested by me in 1902 (25, p. 162); but until I have been able 
to examine better-preserved material, it would be unwise to 
hazard yet another theory of pearl-production. At any rate, it 
is clear that, if pearls in J/. vulgaris ave of parasitic origin (and 
T adhere to the view that the distribution of pearl-producing 
examples of this and most other molluscs can at present best be 
explained on the parasitic hypothesis), the immediate cause of the 
pearl is not the mechanical irritation caused by the body of the 
parasite, but rather the toxie properties of its secretions, which 
lead to the pathological changes (formation of the tumours that 
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