126 CEYLON PEARL OYSTER REPORT. 



moderately large and usually uot very numerous, and which, unless abnormally 

 abundant, probably do little or no harm ; or (2) the more minute and more deadly 

 protozoon parasites, such as sporozoa, which may on occasion be present in enormous 

 quantities, and probably cause epidemic diseases. We have in various cases found 

 sporozoa in the tissues of the pearl oyster. We also know that a bed of adult oysters 

 may get into bad condition, the individuals becoming thin, discoloured and feeble ; 

 and under such circumstances rapid decimation takes place, and the bed, although not 

 yet arrived at old age, may be practically wiped out by what is clearly a parasitic 

 disease. It is highly probable that such diseased conditions are, if not the result of, 

 at least generally concomitant with, overcrowding. For example, the Muttuvaratu 

 Paar seems especially liable to dense deposits of spat, leading to overcrowding ; and 

 in our experience the oysters are more diseased and stunted and feeble on that paar 

 than anywhere else. 



(c) Over-Jishinf/. This is the exhaustion of the breeding stock of the district at a 

 time when no further supplies of young in the larval stages are being brought by 

 currents from neighbouring grounds. That, however, will comparatively rarely 

 happen, and is only likely. to be serious during the last year of a series of fisheries. 

 So long as there are oysters in their second or third years on adjoining paars, which 

 will be fished in the two succeeding years, it is safe so far as the reproduction of the 

 race is concerned to take every older oyster that can be got from the ground, as 

 those coming on, although not yet ready to be fished, are sexually mature, and may 

 be relied upon to supply spat. But in the final year of a series of fisheries, when no 

 further mature oysters remain for the following years, it is important to leave 

 sufficient stock for breeding purposes. The complete clearing of the ground, which 

 has sometimes been put forward as the ideal to aim at in a fishery, may be a short- 

 sighted and disastrous policy. 



In the future, however, if transplanting on an adequate scale is adopted, it may be 

 expected that such a state of affairs as the last fishery of a series with no younger 

 oysters growing up in the neighbourhood will be very unlikely to occur. Each 

 individual case must, however, be considered on its merits, and the Marine Biologist, 

 after an inspection of the banks, should be able to advise how the maintenance of 

 adequate breeding reserves of adult oysters may best be secured. 



VI. THE PRODUCTION OF PEARLS. 



As pearl-formation is discussed very fully in another section of the present volume, 

 it is unnecessary here to do more than add a very few sentences for the sake of com- 

 pleting this summary of results. In the Ceylon oyster there are several distinct 

 causes which lead to this unhealthy or abnormal process, the production of pearls. 

 Some pearls or pearly excrescences on the interior of the shell are due to the irritation 





