l 3G 



CEYLON PEARL OYSTER REPORT. 



1. Size and Growth. 



There are two ways of assessing the rate of shell-growth, first the direct one of 

 keeping marked individuals under observation and noting accurately the dimensions 

 from time to time, and second the indirect method of visiting certain hanks periodically 

 and, by taking the average measurements of a number of individuals at each visit, 

 deducing the average growth from period to period. Both these plans were adopted 

 during the present investigation, and the following may be stated as a summary of 

 the conclusions arrived at. 



Growth during the first two years of life is very vigorous under healthy conditions 

 and in the presence of an abundant food supply. During the third year the rate of 

 increase is reduced, and in the fourth still further so. After that there is little or 

 no increase superficially, although the shell may grow greatly in thickness. 



During the first tbree years the marginal outgrowths or "fintrers" show great 

 development under healthy conditions. Thereafter there is less and less tendency to 

 produce these processes, the activity of the oyster's shell-growth in later years being 

 concentrated upon thickening the nacreous internal layer, largely to counteract the 

 ravages of boring enemies such as sponges and annelids. With variation in the food 

 supply, consequent upon the more or less overcrowded condition seen on several of 

 the " paars," there is marked variation in the rate of growth of the shell (fig. 44). 



Fig. 44. Pearl oysters of the same age from the Muttnvaratu and East Cheval paars. to show 

 difference in size. Traced from a photograph. 



Large numbers of measurements of oyster samples from different paars have been 

 made in the course of this enquiry. It is unnecessary to publish these in full,* so 

 we give here the results of our tables and such comparisons and conclusions as it 

 seems possible to draw. It must be stated that in computing the ages of the oldest 

 oysters now on the pearl banks, we have relied upon the accuracy of Captain Donnan's 



* I have in my hands considerable lists, tabulated by Mr. Horneu., which may possibly be inserted a^s 

 an appendix at the end of the report if it seems desirable. 



