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 

 larvee 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- 



