348 DR. H. LYSLTER JAMESON ON 
It appears, in fact, as though the simultaneous presence of pearls 
and Cestodes in the Ceylon Pearl-Oyster were a case of two parallel 
diseases, comparable to the case of a dog infected simultaneously 
with tapeworms and mange, or of a man suffering at the same time 
from echinococci and scabies. And even should it be found that 
tapeworms do sometimes form the nuclei of Ceylon pearls, an 
explanation of the reason why this occurs in some cases only is 
necessary. It is, of course, possible that in certain of the Ceylon 
banks conditions may exist which cause 7'ylocephalum ludificans 
to depart from its normal habit, and acquire an ectodermal instead 
of a fibrous cyst; or it might even be found that in certain banks 
another species of T'ylocephalum (or other Cestode) occurs which, 
like the Trematode in J/ytilus, normally and habitually gives rise 
to a pearl-sac in the tissues, and which has been confused with 
Tylocephalum ludificans. But, in any case, pearls formed around 
tapeworms, if such ever occur (and this still remains to be demon- 
strated), must be so comparatively scarce that, from the economic 
standpoint, the réle of the tapeworm in pearl-production in 
Margaritifera vulgaris must be unimportant. Examination of 
such pearls from Placuna placenta and Margaritifera margaritifera 
var. cumingii as were obtainable failed to confirm the supposed 
oecurrence of Cestodes as their nuclei. 
(3) The shell of Margaritifera comprises in addition to the 
hinge-ligament the same layers as those of other typical Lamelli- 
branchs, viz. a periostracum (the outermost layer of which is 
secreted in a different manner to the remainder and is very 
much reduced in Margaritifera), prismatic substance, nacre, and 
hypostracum or muscle-attachment substance, 
In addition to these, certain pathological varieties of shell- 
substance arise when the normal rhythm of secretion is disturbed, 
the chief of which are described as amorphous repair-substance 
(which is probably simply uncalcified conchyolin), columnar repair- 
substance, and granular repair-nacre. These substances intergrade 
with normal nacre and prismatic substance, and with each other. 
The peculiar characters of these substances are the chief cause of 
the distinctive appearance of the ‘‘ pseudo-nuclei” of pearls. The 
shell-substance, except the hypostracum and the outer layer of the 
periostracum (and probably the ligament), is secreted in liquid 
form, and its structure and variation may be interpreted as the 
expression of the normal processes of the crystallisation of CaCO, 
in a colloidal medium, modified by the periodicity of the action 
of the shell-secreting tissues of the mantle. 
(4) The “Calcospherules,” which Herdman identifies as the 
nuclei of muscle-pearls, are not free concretions or “ depositions 

Mr. Southwell in his researches on this question; but he thought that gentleman 
would support him in saying that those researches had not been unsuccessful up to 
the present, and promised to be most interesting as well as satisfactory.” Mr. 
Southwell’s thoroughly frank statements of the case in his subsequent publications 
do not appear to me to reveal anything sufticiently epoch-making, from the com- 
mercial standpoint, to render such mystery uecessary ! 
[90] 
