[ vi ] 



cause lesions in the body, and (2) because some of them provide the centres ot 

 stimulation which give rise to pearl-production. The last is by far the more 

 important, and at an early period of our work in Ceylon it engaged the attention of 

 Mr. Hornell and myself. 



On the Cheval Paar, in March, 1902, we satisfied ourselves that the "orient" 

 pearl, free in the tissues of the pearl oyster, is deposited around a cyst containing 

 a Cestode larva, and preliminary notices to this effect were published in my Royal 

 Institution lecture of March 27, 1903,* and at the Southport meeting of the British 

 Association in September, 1903. 



The Cestode larvae were found in several stages, and the later ones, at least, clearly 

 belonged to the genus Tetrarhynchus. Mr. Hornell then found later Tetrarhynchid 

 larvae in the bodies of File fishes (Batistes) which, we were able to show, sometimes 

 devour the pearl oysters. Finally, Mr. Hornkll's discovery at Trincomalee in 

 November, 1903, of an adult Tetrarhynchus in the intestine of a Sting-ray (Tieniura 

 melanospilos, Blkr.), which had freshly-swallowed Balistes in its stomach, led us to 

 suppose that the Cestode which passed its youngest stage in the pearl oyster became 

 transferred as a later larva to the File-fish and attained maturity in the Sting-ray. 

 This view was expressed tentatively in the Introduction to Part I. and more 

 definitely in a letter to 'Nature' of November, 1903. Mr. Shipley, however, who 

 kindly consented to collaborate with Mr. Hornell in working up these and the other 

 parasites for the joint report which appears in this volume, from a further microscopic 

 examination of the specimens sent home by Mr. Hornell, has come to the conclusion 

 that these various larvae differ too much in their minute characters to be placed as 

 stages in the one life-history. He regards them as separate animals, and although it 

 is highly probable that the sequence of hosts Pearl-oyster, File-fish, Sting-ray 

 will prove to be very much as was indicated in Part I., still the pearl-producing 

 parasite has apparently not yet been traced through all its stages to the adult 

 condition : further field-work still lies before Mr. Hornell. 



Our original statement that the nucleus in the case of the "orient" pearls is a 

 Cestode larva holds good, and it is interesting to find that this observation has been 

 independently corroborated by M. G. Seurat, working alone in his laboratory 

 at Rikitea, in the island of Mangareva (Gambier Archipelago). The oyster on which 

 Seurat worked was a Meleagrina, and the Cestode parasite he found is, according to 

 GlARD,t an Acrobothrium, or some allied form. It is possible that some of our Ceylon 

 Pearl Oyster parasites may also belong to the genus Acrobothrium, although the 

 more advanced ones are certainly Tetrarhynchids. 



It is probable that Mr. Shipley and Mr. Hornell will be able to contribute a 

 further paper on these parasites in the last volume of this Report. 



* See also Nature ' for April 30, vol. LXVIL, p. 620. 



t 'Comptes Rendus Sou. Biol.,' Paris, Nov. 6, 1903, vol. LV., p. 1222. 



