26 CEYLON PEARL OYSTER REPORT. 



in size the well-being of the older generation was affected, a result that showed 

 itself first by arrest of the growth of the shell and then, later, by starvation, lowered 

 powers of vitality and reproduction, and wide-spread mortality. 



Regarding the arrest of growth, we noted that the older oysters on the Mid-west 

 Cheval measured 69"42 millims. X 64"54 millims. in March, 1902, when they were 

 2j to 2f years old, while survivors on less thickly populated parts of the bank were 

 only 72 - 30 millims. X 65"25 millims. in March, 1904, an increase in size of but 

 2 '8 8 millims. in height by 071 millim. in length in 2 years, a growth so slight as 

 to justify the conclusion that oysters from the Western Cheval practically attain 

 their maximum height and length upon reaching the age of 3 years, increasing 

 thereafter in no appreciable extent save in thickness, which is due to deposits of 

 nacre upon the inner surface of the valves. 



In the case of the Muttuvaratu, the Karativo and the Dutch Modragam paars, the 

 disappearance of the oysters has been total, caused not so much by the competition 

 introduced by the presence of myriads of vigorous young though this also existed 

 and no doubt had some effect as by the overcrowding and diseased condition set up 

 by the immense number of adults. This has had such baneful effect that when 

 Mr. Hornell saw them in March of last year they showed marked deterioration upon 

 their condition 4 months previously, while their average size showed actually a 

 decrease in height and length (58 - 84 millims. X 54 - 32 millims. in November, 1902, as 

 against 57 '54 millims. X 54'UO millims, in March, 1903). These oysters are in a 

 shrunken, poorly nourished condition, and out of 227 examined, 25, or over 11 per 

 cent., were affected with the "yellow" disease a malady marking the imminence of 

 wide-spread mortality, and characterised by the invasion of most of the tissues by 

 immense numbers of leucocytes crowded with yellow granules. 



That the Muttuvaratu oysters should have been entirely cleared away, while a 

 relatively large number of the same generation is left upon the Western Cheval, is 

 undoubtedly due to the overcrowding having been less acute and less wide-spread 

 in the latter case. 



Subsidiary Causes. The general enemies of the pearl oyster were present as usual, 

 such as star-fish, boring molluscs and the smaller oyster-eating fishes (Batistes, &c). 

 The two former, the star-fishes especially, do much harm, and their reduction in 

 number is one of the benefits to be derived from dredging on a large scale. The 

 smaller oyster-eating fishes have not been excessively numerous upon the Cheval and 

 are not at present a destructive agency, seeing that the oysters are now too large and 

 too strong to be crushed by the small jaws of these fishes. Their ravages are only 

 to be feared when a bank is covered with spat ; then these fishes gather from all 

 cpaarters and, if the deposit is limited and the nature of the bottom sufficiently 

 smooth, the attack may involve partial or even entire annihilation of the bed. The 

 boring-annelid, Polydora sp., was markedly abundant on the South-west Cheval, 

 but comparatively few bad cases could be traced to its tunnelling. Far otherwise 



