THE CEYLON PEARL-OYSTER. 347 



M. Seurat also gave me a piece of the latero-dorsal region of 

 the body of tliis species, with pearls in 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 

 !ib(jiit ten pearls. Most of these were like those described from 

 Dr. Kelaart's material of M. 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, which 

 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 Trematode which is associated with pearls in Mytilus 

 furnishes the stimulation necessary for pearl pi-oduction, therefore 

 the Cestode in Maryaritiferap\a.ys the same 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 M. maxima, imd 

 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 Oestodes 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 larvpe, which Prof. 

 Herdman regards as the cau.se of the formation of "fine pearls" in 

 the Ceylon Pearl-Oyster, are a young stage of the worm described 

 by Shipley and Hornell as Tetrarhynchus unionifactor is quite 

 inconclusive. I consider these worms to be more probably 

 referable to the genus Tylucejjhalum (or an allied form), and have 

 provisionally described them under the name of Tylocephalitvi 

 ludificans and T. 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 Mytilus, 

 where the peavl-sac is normally formed as a result of the specific 

 stimulation of the worm) is supported by quite insufiicient 

 evidence, and even their occasional occurrence in the nuclei of 

 Ceylon pearls has yet to be demonstrated *. 



* Sir West Ridgeway. formerly Governor of Ceylon, Cliairman 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 

 21st, 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 f i. e. the Directorsi should reveal the progress which had been made by 



