THE CEYLON PEARL-OYSTER. 273 



below) that this epithelium is genetically related to the 

 ectoderm, then a possiV>le explanation of the difference in 

 behaviour in the encysted condition would be that those larva; 

 that carried in ectodermal cells become covered (when dead or 

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

 t, while those that were free from ectoderm become surrounded 

 by the connective-tissue cyst." 



No satisfactory instance, however, is recorded of the Cestode 

 parasite being observed surrounded by an epidermal sac. 



Again in Part III. of the Report, p. 32, Professor Herdman, 

 quoting a Report furnished to him by Mr. Hornell, says the 

 abundance or otherwise of cyst-pearls " is connected with the 

 factors which control the relative abimdance of the pearl-inducing 

 Cestode and those which conduce to its death during encystment 

 in suitable localities within the tissues — problems as yet obscure "' ; 

 while in Part V. (Pearl-Production), p. 15, he says "it is appar- 

 ently very ditHcult indeed to hit upon a stage showing the 

 commencement of the pearl-foi-mation." 



And again, in an address delivered at the Anniversary Meeting 

 of the Linnean Society of London in 1906 (17), Prof. Herdman, 

 speaking of his Ceylon work, says that it is pi'ol)ably only those 

 Cestodes that are provided with an ectodermal covering forming 

 a pearl-sac that become the nuclei of pearls. But, as stated above, 

 such an ectodermal pearl-sac has not yet been found to occur 

 around the parasite, and its occurrence is purely theoretical. 



The investigations made subsequently to the pul)lication of 

 Prof. Herdman's Reports have added little to our knowledge of 

 the subject. In 1905 Mr. Hornell published a Report on the 

 Placuna placenia pearl-fishery of Lake Tampalakamam, dated 

 June 15th, 1905 (21). In this he states (p. 5) that he dissolved 

 two Placuna pearls (out of five in his possession) and found that 

 " in each case the nucleus proved to be the dead i-emains of a 

 minute Platyhelminthian larva of the same stage and species as 

 that which forms the nucleus of cyst-pearls in Margaritifera tml- 

 tjaris." Further study I'evealed the presence of Cestode larva; in 

 the doi'sal portion of the visceral mass. He considers that these 

 ai-e identical in details of form and structure with those of J/ar- 

 garitifera vulgaris^ but expresses some doubt as to their specific 

 identity, and adds that if they prove distinct the Placuna 

 parasite will need a fresh name. Mr. Hornell further observed 

 that these larva; multiply asexually by a process of endogenous 

 budding, which he wrongly refers to as parthenogenesis. 



Mr. Hornell then goes on to make the following extraordinaiy 

 statement, which I quote as giving some indication of the con- 

 fusion of ideas which existed as to the identity of the supposed 

 pearl-producing larvae even in 1905, i. e. nearly three and a half 

 years after the beginning of the observations : — 



" The discovery of a stage in the life-history of this 

 parasite, which I am confident is homologous with the 

 PuGC. ZooL. Sec— 1912, No. XVIII. 18 



