278 DR. H. LYSTER JAMESON ON 



Pearls, as desci-ibed by Professor Herdman and Mr. Hornell, the 

 position is briefly this : — 



1) Ceylon Pearl-Oysters were found to contain large 

 numbers of Cestode parasites Avhich occurred simul- 

 taneously with pearls, but which did not necessarily 

 result in the formation of pearls. 



(2) The nuclei of the majority of " cyst-pearls " were 

 thought to be identified as consisting of the remains of 

 these parasites, though Mr. Hornell's figures of such 

 nuclei are capable of other interpretation. 



(3) No satisfactory evidence was adduced of the Cestode 

 having acquired a surrounding epidermal sac, such as is 

 normally formed around the pearl-producing Trematode 

 in Mytilus, though Prof. Herdman admits that this sac 

 is essential for pearl-production. The first stages in the 

 supposed process are therefore purely hypothetical and 

 imsupported by obsei-vation, besides pre-supposing an 

 abnormal departure from the parasite's usual habit. 



The evidence in favour of the theory is mainly that the more 

 highly infected the oysters are with these particular Cestodes, the 

 richer they are in pearls. Thus it was observed in 1904 (Report 

 III. p. 32) that the oj^sters from the North-West Cheval, besides 

 being the most extensively infected with Cestode-cysts were also 

 the richest in cyst-pearls. And, again, Mr. Southwell records 

 (40, p. 194), that 



" the infection of the very old oysters [with tapeworm -cysts] 

 found on the Kondatchi Paar in 1908 was remarkably low, 

 and, as bearing practical proof that infection and pearl yield 

 are intimately connected, it is interesting to note that the 

 pearl yield also was remarkably low, the valuation only 

 working out at about Rs. 18 per 1,000 oysters." 

 These facts might be explained, however, on the assumption 

 that the conditions favourable to pearl-production are also favour- 

 able to Cestode infection. 



Characters, Identity, and Life- Histories of the Cestode Parasites 

 of Margaritifera vulgaris. 



Apparently the fii-st announcement of the supposed relation 

 between the Cestode and pearl-production was made by Prof. 

 Herdman at his Royal Institution lecture on March 27th, 1903 

 (14). In this he says that he and Mr. Hornell have px-oved so 

 far " that in Ceylon the most important cause is a larval Cestode 

 of t\\Q Tetrarhynchns form.'' Again, in the Report of the British 

 Association, Southport, 1903, p. 695, Prof. Herdman says: " The 

 parasite in the case of the majoi'ity of the cyst-pearls of Ceylon 

 is the larva of a Cestode which appears to be new, and will 

 bo described under the name of ' Tetrarhynclais unionifactor ' '"' ; 

 and the pearl-inducing parasite is referred to throughout Prof. 

 Herdman's Report under this name. 



