266 DR. H. LYSTER JAMESON ON 



rise to peaii-formation. The ectoderm, in such cases, 

 would probably also be damaged, and cells may be carried in 

 Avith 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 confii-med 

 by the present writer. 



(ii.) Boring Animals. 



While recognising that pearly excrescences or " blisters " are 

 mainly due to borers such as Leucoclore and Clioue, 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. 



(iii.) 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 

 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 

 mollusc. Some pearls are certainly formed around intrusive 

 Nematodes. We have a complete cyst peaid, free and 

 unattached, of which the nucleus is a coiled Cheiracanthas 

 uncinaPas, 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 loithout a Nucleus. 



Prof. Herdman points out that both in the case of the Ceylon 

 Pearl-Oyster and Mytilus 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 number of parenchyma-pearls, the presence 

 of a nucleus of foreign origin is quite vmnecessary, 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 attiibute the conditions where 



