GENERAL SUMMARY AND RECOMMENDATIONS. 



123 



in all directions, rendering it so rotten that it can no longer hold together, and so 



falls an easy prey to any assailant. Many 

 pearl oysters have their valves penetrated by 

 Cliona to some extent, and in some beds a 

 considerable proportion are as much affected 

 as the example shown in fig. 12. This is a 

 disease of adult life. Young shells never 

 contain Cliona, and the older the affected 

 oyster is the worse does it get. It will be 

 noticed that the ravages of this sponge 

 have a bearing on pearl formation. The 

 more friable shells are eaten more readily 

 by the voracious fishes, and consequently 

 Cliona margaritiferce may be regarded as 

 facilitating the transference of the pearl- 

 iriducing parasite from the oyster to its 

 ultimate host. 



Another boring enemy is the small Poly- 



Fig. 12. Pearl-oyster shell honeycombed by 

 Cliona margaritiferw, DENDY. 



chaete worm Poly dor a (or Leucodore) hornelli. It is questionable, however, whether 

 this really does serious harm, except indirectly, in the case of the Ceylon pearl 

 oyster. It no doubt, by its 

 burrows between the layers of 

 the shell, helps in disintegration; 

 it lets in mud and sand-grains, 

 and it is sometimes the cause of 

 nacreous thickenings or blisters 

 in the interior. It is not, how- 

 ever, of anything like the same 

 importance as Cliona and the 

 Gastropods, and cannot, taken 

 by itself, be considered a cause 

 of death. A few other organisms, 

 lamellibranchs, worms, algae, &c., 



bore in the pearl oyster's shell Fig. 13. Pearl oyster shells 



enveloped in Corals ; re- 



duced to about one - half 

 natural size. Other ex- 



amples were shown 

 fi 38 p 



which is sometimes a veritable 



microcosm containing represen- 



,,. , 



tatives ot nearly every group of 



J 



the Invertebrata but none of 

 them do serious harm, and they 

 need not be considered further. 



Similarly, the associated animals on the outside of the shell in most cases cause 



R 2 



were sown in 

 j n p ar t I. 



