CEYLON PEAEL BANKS. 213 



Since these experiments were made , with the object of proving 

 that a bony fish did not enter into the life-cycle of the pearl-inducing 

 parasite, but that only the pearl oyster and a ray were concerned, 

 two conditions were necessary : — 



(1) That the alimentary canal of each fish should have been 



quite free from cestodes at the begimiing of the experi- 

 ment. Southwell attempted to arrive at this condition 

 of things by treating the fishes with extract of male fern 

 and castor oil. 



(2) That the fishes and oysters should have been absolutely 



isolated from other organisms, such as bony fishes. 

 Southwell believed this had been achieved by sur- 

 rounding the enclosure with expanding metal having 

 a four-inch mesh. 

 With regard to the first condition, the only test made to prove 

 the efficacy of the purgative used — ^namely, by killing a fish three 

 days after treatment and examining the intestinal contents — 

 revealed the presence of a few cestodes. Furthermore, in explaining 

 the presence of the large number of specimens of Phyllobothroides in 

 his second experiment, Southwell says, "it would seem probable 

 that these cestodes Avere present when the fish were placed in the 

 enclosure." If this possibility be admitted, the value of the whole 

 experiment is nullified. 



The second condition m as not strictly observed. Southwell admits 

 this, and says, " the food supply consisted entirely of oysters, save 

 possibly such small fish as were able to pass through the expanded 

 metal." After seeing the nursery in question, the wTiter was of 

 the opmion that the enclosure was not proof against the inroads of 

 other fishes. 



Mr. Southwell admits the unsatisfactory results of the experiments, 

 but still adheres to his view that there are only tAvo hosts , a view 

 which may probably prove correct. 



The conclusion that one is forced to is that further investigations 

 are required before the question of pearl production may said to be 

 satisfactorily solved.* If Southwell's experiments are reliable, it 

 would appear that the pearl oyster has larvae of at least four kinds 

 of tapeworms, viz., Tetrarhynchus unionif actor , Tetr. herdmani ., 

 Phyllobothroides hutsoni, and Phyllobothroides kerJchafui. This is not 

 inconceivable, but, if true, it would raise the interestuxg question. 

 if the cestode origin of pearls be accepted, of the formation of pearls 

 from the larvae of all these different cestodes, and not only from 

 Tetr. unionif actor. 



* Dr. Jameson's Paper on pearl production had not been received when this 

 wari written. Dr. Jameson's exaiuination of a largn number of pearls failed to 

 detect the presence of a larval tapeworm in the nucleus, and on these grounds 

 he rejects Herdman's theory. The Paper produces only negative evidence, 

 and Dr. Jameson is not in a position to offer an alternative explanation. The 

 whole question will need re-investigating. 



