THE PARASITES OF THE PEARL OYSTER. 79 



The ectoparasites of the pearl oyster consist principally of shell-boring animals 

 which tunnel more or less extensively into the nacreous portion of the valves. Of 

 these, the only one whose depredations are of economic importance is the boring 

 sponge Cliona niaiyart/ij'ertr, Dknmy. The other two, the Polychgete worm 

 Leucodore sp., and the Lamellibranch mollusc Lithodomus sp. (date-shell), are never so 

 numerous as to constitute a serious danger to the oyster population of the pearl 

 1 i.inks. 



I. CESTODES OF THE PEARL OYSTER. 

 I. Cestode Larv.e in the Pearl Oyster, Margaritifera vulgaris, Schtjm. 



The cestode larvse of the pearl oyster pass through several stages in the tissues of 

 the host, that is if they escape being entombed within the centre of a pearl. 

 Economically these unpleasant little creatures are of supreme importance to the 

 Ceylon pearl fishery, as their presence in the oyster causes the formation of the finest 

 quality of pearl and those with the highest lustre. 



These larvae first attracted attention during the second cruise'" of the " Lady 

 Havelock," on March 6th, 1902, and the following clays, when numbers of the early 

 globular stage were dissected out from the livers of oysters dredged from the West 

 Cheval Paar. Subsequently, during the investigation carried out at the Galle 

 Biological Laboratory, a second and more advanced stage of a Tetrarhynchus larva 

 was found in the same animal. Details of the morphology and histology were 

 then worked out, and the relationship which the larvre bear to pearl formation was 

 investigated. 



It is usual for a Tetrarhynchus to enter its first host as an embryo enclosed in an 

 egg-shell. As VaullegeardI says, ' ; L'oeuf doit etre avale" par un animal marin ; il 

 ne se developpe que si cet etre lui fournit un milieu convenable, mais les Tetra- 

 rhynques paraissent peu tenir a une espece ;" the drawings made by one of us 

 (Plate I., fig. 1) of a free-swimming larva taken in the plankton at the north end of 

 the Muttuvaratu Paar seems to point to the fact that the embryo of the pearl- 

 forming parasites may leave the egg-shell before entering their first host, and this 

 must be so if the organisms in question are truly the young of the Tetrarhynclnis ; 

 this point, however, requires corroboration. 



The earlier and more globular-shaped of the two stages met with in the pearl 

 oyster is by far the more numerous, as may naturally be expected. It accounts 

 for all save one per cent, of the total cases found. The liver, the gills (Plate I., 

 figs. 3 and 4), and the connective tissue of the mantle (Plate I., fig. 2) are the 

 organs chiefly favoured ; the muscles are practically free, while the gonads yield 

 comparatively few. 



* See " Narrative," this Report, Part I., p. 70. 



t ' Mem. Soc. Normandie,' XIX., 1897 to 1899, p. 353. 



