PEARLS IN MUSSELS 7 



about looking for a suitable host. Within this host it 

 would come to rest and begin budding off numerous 

 secondary larvae, in which stage it may assume con- 

 siderable size and becomes known as a sporocyst. No 

 one, however, has seen the eggs hatch, or the free- 

 swimming larva ; but Mr. Jameson produces evidence 

 to show that the sporocyst stage occurs in two other 

 common molluscs viz.,- in a clam (Tapes decussatus) 

 and in the common cockle (Cardium edule). The 

 former mollusc abounds in the black gravelly clay 

 which forms the bottom of the mussel-beds at Billiers; 

 and every specimen out of nearly two hundred ex- 

 amples investigated by Mr. Jameson was found to be 

 infested with sporocysts containing larvae closely re- 

 sembling those which act as pearl-nuclei in the edible 

 mussel. Exactly similar sporocysts were found in 

 about fifty per cent, of the common cockles examined 

 in the Barrow channel, where the species Tapes decus- 

 satus does not occur. 



Within the sporocyst certain secondary larvae are 

 formed, as is habitual with the flukes. These secondary 

 larvae are the cercariae ; and it is in this stage that the 

 animal makes its way into the pearl-mussel and ulti- 

 mately forms the nucleus of a pearl. Precisely how it 

 leaves the sporocyst and the first host i.e., the Tapes 

 or Cardium is not known. Certain experiments made 

 by Jameson, who placed mussels which he thought 

 were free from parasites in a tank with some infected 

 Tapes, are not quite conclusive, and have been ably 

 criticized by Professor Herdman. It is true that, 

 when examined later, the mussels were well infected ; 

 but it was not definitely shown that they were not in- 

 fected at the start; and further, the numbers used were 

 too small to justify a very positive conclusion. Still, 

 on the whole, it may safely be said that life-history of 

 the organism which forms the pearls in Mytilus edulis 

 probably involves three hosts : the scoter, which con- 



