GENERAL SUMMARY AND RECOMMENDATIONS. 



123 



Fig. 12. Pearl-oyster shell honeycombed by 

 Cliona mwgaritiferce, Dendy. 



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 

 < 1/iona 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 cousecpiently 

 Cliona marga/ritiferm may be regarded as 

 facilitating the transference of the pearl- 

 inducing parasite from the oyster to its 

 ultimate host. 



Another boring enemy is the small Poly- 



cluete worm Polydora (or Leueodore) hornelli. It is questionable, however, whether 



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



lamellibrauchs, worms, algae, &c, 



I lore in the pearl oyster's shell 



which is sometimes a veritable 



microcosm containing represen- 

 tatives of nearly every group of 



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 



Fig. 13. Pearl oyster shells 

 enveloped in Corals ; re- 

 duced to about one-half 

 natural size. Other ex- 

 amples were shown in 

 tig. 38, p. 114, in Part 1. 



