200 THE FRESH-WATER MUSSEL [CH. VI 



Scotch rivers, and occasionally pearls of high value are 

 found in Unio margaritifer, which is there abundant. In 

 past times the Tay fishery was very productive and is 

 stated to have yielded pearls to the value of 10,000 in 

 the three years 17611764. 



The origin of pearls in the pearl oyster Margaritifera 

 vulgaris (Avicula fucata) has been traced to a similar 

 source. Kelaart first, in 1859, connected the formation of 

 pearls with parasitic worms. Recently Prof. Herdman 1 

 has made an investigation of the Ceylon pearl fisheries 

 and finds that nematodes, trematodes and cestodes are 

 all concerned, while rarely a grain of sand forms the 

 " nucleus " in pearl formation. The most common cause of 

 pearls he finds to be a larval cestode of a Tetrarhynchus 

 form. In its earliest stages the larva is said to be free 

 swimming: from the oyster it probably passes into the 

 file-fish (Batistes sp.), or into some species of Trygon 

 which prey upon the oysters. 



Before closing this chapter it will be well to review 

 a few noteworthy features in the life-history of the mussel. 

 When compared with that of marine Lamellibranchs the 

 life-cycle is remarkable on two accounts; firstly, the 

 prolonged period of attachment to the parent, and the 

 suppression of the free-swimming veliger stage, secondly, 

 the post-embryonic life as a parasite upon the skin of a 

 fish. In the majority of marine molluscs dispersal is 

 effected during the veliger stage, the powers of locomotion 

 of the adults being in most cases feeble or altogether 

 absent. The minute transparent veliger swims in the 



1 Nature, vol. LXVII. April 30, 1903. 



