THE CEYLON PEARL-OYSTER. 347 
M. Seurat also gave me a piece of the latero-dorsal region of 
the body of this species, with pearls im situ. This I decalcified 
and examined in oil of cloves, and afterwards sent to be sectioned. 
At the time of correcting proof the sections had not been returned. 
Examined entire in oil of cloves this specimen showed a cluster of 
about ten pearls. Most of these were like those described from 
Dr. Kelaart’s material of J. vulgaris, i. e., they had no obvious 
nuclei; one, however, had in its centre a minute hypostracum 
pearl or columnar pseudo- nucleus, about ‘1 mm. in diameter ; 
one had a tiny refractive body, about ‘03 mm. in diameter, w hich 
may have been composed of amorphous substance; and one had 
some more opaque matter which, however, contained nothing that 
could be identified as a Cestode. 
It would seem possible, in view of these observations, that 
M. Seurat may also have been led into the error of arguing that 
because the ‘T'rematode which is associated with pearls in Mytilus 
furnishes the stimulation necessary for pearl production, therefore 
the Cestode in Margaritifera plays thesame role. I hope shortly 
to receive further material from the French Pacific that will 
enable me to go into this question more fully. 
I may add that my studies on pearls from J/. maxima, and 
from the other varieties of J/, margaritifera from other localities, 
so far as these studies have gone up to the present, afford no 
evidence of the occurrence of Cestodes in the centres of pearls. 
(15) GENERAL SumMaRy. 
The following are the principal conclusions to which these 
investigations have led me :— 
(1) The evidence that the globular Cestode larvee, which sha 
Herdman regards as the cause of the formation of * fine pearls ” 
the Ceylon Pearl- Oyster, are a young stage of the worm evanbed 
by Shipley and Hornell as Tetrarhynchus unionifactor is quite 
inconclusive. I consider these worms to be more probably 
referable to the genus 7'ylocephalum (or an allied form), and have 
provisionally described them under the name of Vylocephalum 
ludificans and 7. minus, spp. nn. 
(2) The theory that these Tapeworms are the cause or a cause of 
the formation of pearls in the Ceylon Pearl-Oyster (in the sense 
in which the Trematode is the “cause” of pearls in J/ytilus, 
where the pearl-sac is normally formed as a result of the specific 
stimulation of the worm) is supported by quite insufficient 
evidence, and even their occasional occurrence in the nuclei of 
Ceylon pearls has yet to be demonstrated *. 
* Sir West Ridgeway, formerly Governor of Ceylon, Chairman of Directors of 
the Ceylon Company of Pearl Fishers, Ltd., in reply to a question put, at the annual 
meeting of Shareholders in 1909 (reprinted in the ‘ Financial News’ on December 
2\st, 1909), as to what was known with regard to the real cause and mechanism of 
pearl- formation, implied that the Directors were in possession of valuable informa- 
tion, of a secret nature, on the subject. “It was most undesirable at this moment 
that they [7. e. the Directors] should reveal the progress whicn had been made by 
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