INTRODUCTION'. 13 



to become pearls of value. There are many potential and incipient pearls in young 

 oysters which, even if transferred to the body of a fish, contain only dead larvae, and 

 so are lost from the parasite population ; while the death of their host prevents them 

 from causing pearl-formation. It is not sufficient for the oyster to be infected by the 

 Tetrarhynchus, it must also live, retaining its parasite, until such time as it can 

 produce sufficient deposit of the calcareous secretion to form a true pearl. In the 

 case of the Ceylon pearl oyster very little increase in size of the shell by additions to 

 the ventral margin takes place after an age of three and a half or at most four years 

 has been reached. But the shell after this period thickens greatly by the deposit of 

 nacreous material in its interior and especially in the neighbourhood of the hinge at 

 the dorsal edge. This is the time of rapid pearl-formation. The oyster's tissues are 

 then in the condition which leads to the secretion and deposition of limy material 

 either externally by additions to the thickness of the shell, or internally as successive 

 coats deposited upon any particle such as a dead parasite or, it may be, a sand- 

 grain, a Diatom frustule, a fragment of nacre, ova, or excreta which has given rise 

 to the necessary stimulation. 



There is general agreement amongst those who have seen most of the Ceylon 

 oyster on its native banks that the animal does not, as a rule, live beyond six or seven 

 years. Captain Donnan's opinion is that " the oysters may be profitably fished at 

 the age of four years, and that they are in their prime at five years, and may be kept 

 till that age if circumstances will permit of it ; but if they are kept till the sixth year 

 they are almost certain to be found dead." From the figures he gives in his reports 

 it seems that the age of the oysters at fisheries where we have reliable data has been 

 nearly always under five years. The pearl oyster should be fished when it measures 

 3 inches in diameter at right angles to the hinge line, has no soft growing edge to 

 the shell, and shows a deep V-shaped groove between the valves at the hinge. 



Although it is not correct to say that there are no pearls in young oysters, for we 

 have found some of moderate size in quite small oysters, still there is no doubt that 

 rapid and widespread pearl-formation begins only when the animal is about three and 

 a half years old and has ceased to grow a thin spreading ventral margin to the shell. 

 During the year or eighteen months following this is the time when it is most 

 profitable to fish the oysters up and open them for the sake of their pearls. Although 

 some oysters contain more pearls than others, and some paars have on occasions been 

 characterized by richness in pearl-production, the Cestode parasites are apparently 

 very widely spread and generally distributed. All oyster communities of the proper 

 age contain pearls, and it would probably be impossihle to transplant young oysters 

 to any ground in the Gulf of Manaar where they would not become to some extent 

 infected by the pearl Cestode. The frequent thinning out of young oysters, and the 

 transporting as required from one ground to another, which we shall recommend at 

 the conclusion of this Report, will, however, have the effect of causing a more even 

 distribution of the parasites, since some oysters will he taken from more highly 



