7 8 ECONOMIC ZOOLOGY AND ENTOMOLOGY 



usually certain fish that feed on the molluscs. In these the 

 adult parasite lives, and some of the young embryos that are 

 set free in the water gain access to the gills, liver or mantle of 

 the molluscs. Here many go on with their development until 

 the mollusc is eaten by a fish and the life cycle is begun again. 

 In some instances, however, the irritation due to the presence 

 of the parasite causes a calcareous secretion, like that forming 

 the shell, to be deposited around the parasite. This forms the 

 nucleus of a pearl which grows by additional layers being 

 deposited around it, the luster depending on the kind of mollusc 

 in which it is found. "The most beautiful pearl is only the 

 brilliant sarcophagus of a worm." 



