THE PARASITES OF THE PEARL OYSTER. 9] 



half-dozen individuals have been met with during the course of the present 

 investigation. 



The local distribution of M. margaritifercB is noteworthy. Its Cercariae swarm in 

 the stunted pearl oysters of the Muttuvaratu Paar ; but in those from the Eastern 

 ChevaJ they are very rare. Rocky ground predominates in the former locality, sand 

 in the latter. As a consequence the mollusean, annelidan and fish constituents of 

 the fauna show considerable divergence, and this, in turn, influences the numbers of 

 the two unknown animals which lodge respectively the sporocyst and the adult form 

 of this Trematode, and thus produces the erratic local distribution noted. 



These Cercarian larvse are usually found in the gills of the pearl oyster ; there they 

 occur frequently in considerable numbers. In a fragment of gill-lamella from a 

 Muttuvaratu individual, made up of L2 filaments, as many as 6 of these Trematodes 

 were found. To a much smaller degree they infest the mantle. In the other organs 

 they .seldom appear. 



Their abundance is strangely variable even in oysters from the same "paar. In 

 those from the Muttuvaratu, the majority contain from 20 to 40 each; occasional 

 individuals are, however, found infested by an extremely limited number, aud in a 

 few instances there is a total absence of the parasite. On the other baud, in the 

 large well-grown oysters from the Eastern Cheval, during the past eighteen months it 

 was exceptional to find even a single one. 



To give an account of the life-history of the Trematode and to discover the hosts 

 of the other stages in its life-cycle, it will be needful to anchor on the Muttuvaratu 

 Paar for a sufficiently long period to permit of an exhaustive examination of the 

 principal organisms that live there in association with the oysters. A lengthy visit 

 to this particular paar is also required in order to determine whether this Trematode 

 has any importance as a pearl-inducing factor, although, so far as present evidence 

 goes, it is strongly against such a presumption every one of the dozen cyst-pearls 

 obtained from this bank which have been decalcified has yielded a Cestode larva as 

 nucleus. 



The stage of Muttua margaritifercB, which is met with in the pearl oyster, is a 

 quiescent Cercarian form of an advanced character, the alimentary canal and the 

 copulatory organs being fully developed. What appear to be testes are also present, 

 but no tract; ol ovary was detected. 



The larva lies coiled or, rather, doubled upon itself (Plate III., fig. 54) within a thin 

 membranous adventitious capsule. It frequently changes position, rotating slowly 

 \\ ithin its prison. 



When liberated by the rupture of the investing sac, the body is seen to be 

 gracefully lanceolate in outline when viewed from either the dorsal or the ventral 

 aspect. Anteriorly it is somewhat truncate, posteriorly it narrows rapidly, ending in 

 a blunt rounded angle (Plate III., fig. 55). In length it is remarkably uniform, 

 0"75 millim. to 0'9 millim. covering all the individuals measured. 



N 2 



