106 PEARLS [CH. 



condition of the famous pearl banks of the Gulf 

 of Manaar, Avhich, notwithstanding numerous bad 

 years, have yielded much profit to the Government 

 of Ceylon. The results of their investigations^ are 

 extremely valuable, and it must be remembered 

 that the primary object of the expedition was to 

 report upon the conditions of the pearl banks, the 

 distribution of the beds of pearl oysters and the 

 influences affecting their life and prosperity. 



However, in addition to this work, Herdman and 

 Hornell also examined many pearl oysters and such 

 pearls as were obtainable for the purpose of deter- 

 mining the cause of their formation. 



Obviously they were well acquainted with the 

 numerous theories of their predecessors when they 

 first went out to Ceylon, and consequently when in 

 March 1902 they noticed numerous white larvae 

 (fig. 10) parasitic in the "liver" of the pearl oyster, 

 they devoted some attention to their occurrence and 

 nature, and came to the conclusion that these para- 

 sites were the young of cestode worms (the most 

 familiar cestode worms are the common tapeworms 

 of man, the dog, etc.), and subsequent work showed 

 that some at least of them were the larvae of a worm 

 that occurs frequently in Elasmobranch fishes and is 

 called Tetrarhynchiis (fig. 11). 



1 Given in a Eeport to the Ceylon Government, published by the 

 Eoyal Society, in 1903-6. 



