12 CEYLON PEARL OYSTER REPORT. 



pearl banks and prey upon the oysters. We have not yet succeeded in finding the 

 adult worm, but it will probably prove to infest the Hays or other large Elasmobranchs 

 which may devour Batistes. 



We have also found the pearl-provoking Cestode larva in Pinna where pearls are 

 sometimes found but we have no reason to suppose that Pinna finds a place in the 

 same life-cycle with the pearl oyster. Nor do Ave think that any other Mollusc or 

 Invertebrate is implicated. The stages in the life-cycle, as regards environment, are 

 probably as follows : 



1. Free embryos in the sea ; 



2. Encapsuled in the pearl oyster ; 



3. Later larval stage in the File-fishes ; 



4. Adult in large Elasmobranchs. 



We suppose, then, that the adult Worm from the body of the Shark or Ray sets 

 free its numerous young embryos into the sea ; and there we pick up the history, for 

 amongst our Vermean embryos in the tow-net we have some caught on the Muttu- 

 varatu Paar, in November, 1902, which we consider to be the younger stage of our 

 Tetra/rhynchus larva. Such free-swimming embryos on an oyster bank will readily 

 gain access to the body of the oysters. They will be carried in with the current of 

 water, and may either pass into the alimentary canal with the microscopic food 

 particles or get lodged in the gills. We have found these in the gills, and have also 

 found very young stages in the mantle ; while later stages of larger size are common 

 in the liver and in the connective-tissue of the body-wall generally. In some samples 

 of oysters examined scarcely any individuals are free from the encysted parasites, 

 and in one 45 cysts were counted. In the case of oysters so infested which are 

 eaten by the File-fishes and from the frequency with which we, and others, have 

 found the broken-up shells in the stomach there can be no doubt that these fishes do 

 eat the oysters the Cestode larva? are transferred to the body of the new host ; 

 and Mr. Hoknell has found them in quantity in the alimentary canal and peritoneum 

 of specimens belonging to the two common species Batistes rnitis and B. stettatus, from 

 several localities. The Cestodes are now in a later stage, and are clearly recognisable 

 as Tetrarhynclms ; but are not yet adult and have no reproductive organs. Whether 

 they become mature in time in the body of the File-fish or, as we think more probable, 

 in a larger animal winch feeds upon that fish, such as one of the large Elasmobranchs 

 common in these seas, we cannot yet say ; but possibly that point may be settled 

 before the Report is finished. 



In those oysters, on the other hand, which do not become the prey of the File-fish 

 the larval Cestodes have no opportunity of fulfilling their destiny as parasites, but die 

 and become encapsuled in the layers of pearly nacre deposited by the living tissues of 

 the oyster upon the source of irritation. What proportion of the parasites become 

 the nuclei of pearls we are not able to say, but many, we feel sure, neither complete 

 their life-history nor yet remain long enough encapsuled in the bodies of adult oysters 



