THE CEYLOX PEARL-OYSTER. • 275 



states that these cysts which foi-iu aiound the parasites become 

 the nuclei of pearls, and a decalcified pearl shows an organic 

 nucleus in the centre surrounded by concentric layers of conchyolin, 

 the whole having a diameter of about a millimetre, and the nucleus 

 being a scolex -225 mm. long and easily recognisable as that of 

 Tylocephalum. 



It appears that in this case also the parasites may be present 

 in great numbers without pearls being found (Seurat (35), 1904, 

 p. 295). 



Here, again, examination of the scanty material available (see 

 p. 346) hns yielded no confirmation of the presence of Cestodes in 

 thepeai'ls of M. viargaritifera var. auninyii. I am endeavouring 

 to obtain further material from the Eastern Pacific, in order to 

 extend my observations on this species. 



I may say that from the first time I read Professor Herdman's 

 Reports and papei'S on the subject I was sceptical as to the 

 relationship of the Cestode to pearl-formation. 



Indeed, before Prof. Herdman's departure for Ceylon, on 

 examining Dr. Kelaart's material at the British Museum, which 

 Mr. E. A. Smith kindly placed at my disposal, I had detected the 

 existence of these Cestode larvae (which Kelaart seems to have 

 regarded as " eggs of Entozoa ") in their connective-tissue cysts in 

 the Ceylon pearl-oyster, and after having examined the larva;, and 

 also having decalcified pearls from the same oyster, dismissed the 

 Cestode as probably not concerned in pearl -formation. 



My chief grounds for doubting the Cestode theory were the 

 following : — 



(a) The absence of evidence that the Cestode ever occurred in 

 an epidermal sac, and the fact that it was almost invariably sur- 

 rounded by a fibrous capsule or cyst which does not occur ai'ound 

 the pearl. 



(b) The large proportion of the Cestodes that show^ed no sign 

 of becoming pearl-nuclei, pointing to the conclusion that pearl- 

 formation does not necessarily, or even normally, follow from 

 infection. Thus, in a footnote to p. 12 of Part V. of his Report, 

 Prof. Herdman says : 



" In comparing these statistics [i. e. of numbers of parasites 

 and of pearls in Mytihis] with those of the Ceylon peail- 

 oyster, one is struck by the wholly different ratio borne 

 by pearls to parasites in the two cases. In the mussels, 

 pearls are far more numerous than the living parasites. In 

 our Ceylon oyster, parasites may be exceedingly abundant* ; 

 while pearls (cyst-pearls) are relatively very rare, probably 

 not moi'C than one to a hundred parasites." 



* Mr. Southwell (42), p. 128, says ; " As many as 120 have been counted in a single 

 oyster " ; and further down on the same page : " Occasionally several hundred oysters 

 can be examined, each containing- 20 or 30 cysts, and not a single pearl is to be 

 found.'' 



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