266 DR. H. LYSTER JAMESON ON 
rise to pearl-formation. The ectoderm, in such cases, 
would probably also be damaged, and cells may be carried in 
with the inorganic particles.” 
As shown below, the presence of grains of sand and other foreign 
particles in the nuclei of some Ceylon pearls has been confirmed 
by the present writer. 
(i1.) Boring Animals. 
While recognising that pearly exerescences or “ blisters ” are 
mainly due to borers such as Lewcodore and Clione, Herdman and 
Hornell say (Report V. p. 28) that 
‘in exceptional cases a free pearl may be formed in this 
way.” 
No specific instances, however, are cited, nor is any explanation 
suggested as to what would be the mechanism in such cases. 
(iil.) Parasites other than Cestodes. 
In his lecture at the Royal Institution, referred to above, 
Prof. Herdman said: 
“We shall I think be able to show in our final report that 
Cestodes, Trematodes and Nematodes are all concerned in 
’ : . 
pearl-formation.”’ 
At the same time he recognised the “ larval Cestode of the 
Tetrarhynchus form” as the most important cause. Again, in the 
Y LY tohite) 
Report (V. p. 29), Herdman and Hornell say : 
‘A fuller experience is causing us to incline to the view 
that various parasites may act as pearl nuclei, even in the same 
molluse. Some pearls are certainly formed around intrusive 
Nematodes. We have a complete cyst pearl, free and 
unattached, of which the nucleus is a coiled Cheiracanthus 
uncinatus, on which the pearl deposit is not sufficiently thick 
and opaque to obscure the coils so as to render identification 
difficult.” 
This pearl does not appear to be in Prof. Herdman’s collection. 
(iv.) Pearls without a Nucleus. 
Prof. Herdman points out that both in the case of the Ceylon 
Pearl-Oyster and J/ytilus some pearls have no trace of a nucleus 
(Report V. p. 18). He figures one such pearl from Mytilus, 
magnified 100 times. The existence of pearls without nuclei 
was recorded by Harley (11) in 1889. The observations set 
out below show that in the Ceylon Pearl-Oyster, both in muscle- 
pearls and in a great nuinber of parenchyma-pearls, the presence 
of a nucleus of foreign origin is quite unnecessary, and point to 
the conclusion that the origin of the pearl-sac is usually due to 
stimulation other than that caused mechanically by such bodies. 
So far as Mytilus is concerned, I attribute the conditions where 
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