1902.] ORIGIN OF PEARLS. 157 



Oercarise, and one contained a small pearl. It is apparently- 

 difficult for infection to take place, except on the bottom, owing 

 to the absence of swimming- organs in the parasite. Hence the 

 absence of Cercarise in these examples. 



Eleven days after they were placed in the tank I examined two 

 of these Mytili, and found that the first contained one and the 

 second two Cercarise. These Oercarife were recently immigrated 

 examples, as they were small, rather transparent, and not yet 

 surrounded by sacs. 



I then transferred the experiment to Brighton, where Mr. W. 

 Wells, the Marine Superintendent at the Aquarium, kindly kept 

 the molluscs in a tank in his private ofiice. 



On the 18th of November, 1901, two months after the specimens 

 were placed in this tank, I examined six of the mussels. Of these 

 one contained six Cercarise, another four, two had each three 

 parasites, one contained two, and one was still uninfected. 



When the experiment was transferred to Brighton I added 

 about two dozen mussels that had been in the Brighton Aquarium 

 for two years. I examined six such mussels before introducing 

 the others, and found that none of them contained live Oercaria^, 

 though four of them had one small pearl apiece. 



On 5th April, 1902, 1 took up a sample of 10 mussels from this 

 tank, comprising five of the specimens originally taken from the 

 pier at Piel, and five of those that had been transferred from 

 another tank at Brighton. 



The following table sufiices to show that in both cases infection 

 had taken place : — 



of 



(6) Specimens transferred from other tank at Brighton. 

 No. 6. Contained 19 Oercarise. 

 No. 7. „ 3 Oercarise. 



No. 8. „ 2 Oercarise. 



No. 9. „ 1 (dead) Oercaria. 



No. 10. Still uninfected. 



The Adult Leucithodendrium, 



Although I have not had an opportunity of making a direct 

 feeding experiment upon Somateria or (Edemia, there is hardly any 

 doubt that the parasite that causes the formation of the pearl- 

 sac, and consequently of the pearl, in Mytilus edulis is the larva 

 of Leucithodendrium somaterice (Levinsen), originally described 

 from the Eider Duck [Somateria moUissima Linn.) in Greenland, 



