146 CEYLON PEARL OYSTER REPORT. 



thicker by deposits of nacre on the inside. The distance apart of the successive 

 marginal ridges is a good guide to the vigour of the oysters, healthy conditions being 

 indicated when the distances are considerable, and overcrowding or scarcity of food 

 when they are massed thickly together, especially at the margin. Many of the 

 Muttuvaratu oysters show this latter appearance to a marked degree, the edges of 

 the shell, instead of being thin and delicate as they are on the South-east Cheval, 

 being comparatively thick and formed of a number of very slight layers of growth, 

 which do not imbricate, but are massed at the same level, so as to show like the edges 

 of a pack of cards. 



The connection of shell-repair with occasional pearl formation depends upon the 

 fact that the pearl oyster has to withstand the attacks of various animals sponges, 

 worms, molluscs which bore into the shell. In attempting to repair such lavages 

 the oyster thickens the nacreous layer, and in some cases piles up pearly excrescences 

 on the interior of the shell which may be separated as pearls of an inferior quality. 

 It is probable, also, that when the shell is damaged fine particles, either splinters of 

 nacre or foreign bodies, gaining access to the interior, may serve as the nuclei of free 

 pearls. Finally, it must be remembered that shell-growth and pearl-growth are 

 similar and comparable processes. In both cases limy salts in an organic matrix are 

 deposited by the living tissues in the one process on the outside of the body, and in 

 the other around some internal particle, which in the case of the finest pearls is the 

 Cestode larval Tetrarhynchus. I shall return to these matters in the later section of 

 this Iteport, which will treat of pearl formation. 



