LINNEAN SOCIETY OE LONDON. 27 



3. The calcareous corpuscles in the walls of the vesicle. 



4. The division of the (? muscular) tissue, in the floor of the 



invagination into several masses which probably repre- 

 sent bothria. 



The possession of all these characters together definitely stamps 

 the organisms as larval Cestodes. 



The majority of these Cestode larvoe in the tissues of the Oyster 

 do not die to become entombed in the costly sarcophagus which we 

 know as a pearl. Probably it is only those that are provided with 

 an ectodermal covering forming a pearl-sac that become sacrificed 

 for the profit of man and adornment of women. The rest grow to 

 some extent in the pearl-oyster, and then await, encysted in the 

 tissues but alive, their legitimate further development in the next 

 host when their sheltering mollusc is devoured by a fish. In such 

 cysts and around such parasites we find no epithelial sac, and as a 

 consequence there can be no deposition of pearly matter. Whether 

 or not it is the case that only dead parasites supply the stimulus 

 necessary to induce pearl-formation, and whether, as Griard has 

 suggested, the parasites may be infested and killed by a species of 

 the protozoon Glugea, so that that Sporozoon comes to be eventually 

 responsible for the pearl, I am not yet prepared to say — the Cejdon 

 material has yielded no fresh evidence bearing upon that point. It 

 seems clear, however, that the epithelial sac is always associated 

 with pearl-formation, and that, in the absence of the epithelium, 

 only a thick-walled connective-tissue cyst is produced. If we adopt 

 the view (stated above) that this epithelium is genetically related 

 to the ectoderm, then a possible explanation of the ditfereuce in 

 behaviour in the encysted condition would be that those larvce that 

 carried in ectodermal cells before them became covered (when dead 

 or while still alive) by a pearl-sac and embedded in a pearl, while 

 those that were free from ectoderm became surrounded by the con- 

 nective-tissue cyst, and remained alive to perpetuate the race by 

 reaching a final host. 



In the first account I gave of these Ceylon parasites, it was sug- 

 gested that the next stage after that found in the pearl-oyster 

 occurred in a species of Balistes (which, is sometimes found feeding 

 on the oysters), and that the adult worm inhabited one of the large 

 Elasmobranch fishes (Sting-iiaysand Eagle-Eays) which frequent the 

 Pearl Banks. Shipley has now identified as the adult of our Tetra- 

 rhynchus iinionifactor a parasite that we found in the Great Kay, 

 Rhinoi)terajavanlca*,thQ "Walwadi tirikkai" of the Tamils, M'hich 

 is known to feed sometimes upon pearl-oysters and sometimes upon 

 lish. No fresh light has been thrown upon the possible occurrence 

 of an intermediate (late immature) stage in the Ballstes (which 

 eats the oysters and in its turn is eaten by the large rays) ; and 

 although that intermediate host may not be absolutely necessary 



* See the Section on Parasites by Shipley and Hornell in the forthcoming 

 Part V. of the Ceylon Eeport. 



