160 DR. H. LYSTER JAMESON ON THE [Mar. 4 



and, as a rule, a little to the right of the sucker. At its hinder 

 end it receives the vasa deferentia (v.d.). The testes (te.) are a 

 little more comspicuous than in the larva. The uterus (ut.) opens 

 out just in front of the penis. The arrangement of its convolu- 

 tions depends upon the number of eggs contained (up to 100, or 

 even more). It seems to begin on the left side, near the ovary 

 (ovX which is larger than the testes. It forms a series of con- 

 volutions on the left side, crosses in front of the genital opening 

 to the right, where it forms further convolutions, passes forwai-d 

 as a loop to the anterior end of the body, and runs back to open 

 out at the genital aperture. I am not convinced that this arrange- 

 ment is always strictly adhered to. 



The eggs in the uterus have the form depicted in my sketch 

 and in Levinsen's figure. They measure from -018 mm. to 

 •023 mm., the average dimensions being '021 mm. x '018 mm., 

 but they differ considerably in difierent individuals. I can add 

 nothing new to Levinsen's observations on the yolk-gland. 



I sought in vain for this worm in Bernicla brenta Pall., 

 Tadorna cornuia Gmel., Querquedida crecca Linn., Coli/mbus 

 arcticus Linn,, Larus argentatus Gmel., and Bissa tridactyla 

 Linn. I have had no opportunity of examining other allied 

 birds. 



Being unable to secure live Eiders or Scoters, I tried feeding a 

 male Pochard, Fuligula ferina Linn., with infected Mytili, but 

 without results. 



L. somaterice will very probably be found in the other members 

 of the genera Somateria and CEdemia when looked for. 



The Conditions essential to Pearl-production. 



The characters and life-history of the parasite suffice to account 

 for the anomalous distribution of pearl-bearing mussels, and, by 

 analogy, throw light on the cause of the difierences in the number 

 of pearls produced by the true Pearl- Oysters on various pearling 

 and shelling grounds. 



In order to be abundantly infected Mytilus must be on the 

 bottom, for the tailless Cercaria or " Cercariceum " is dependent 

 upon its limited creeping powers, and the chance of being trans- 

 ported by currents and deposited with silt, &c. 



Hence mussels gi'own on stakes, like those on Piel pier, 

 although right in the middle of the beds of pearl-bearing 

 individuals, are practically uninfected. 



Secondly, there must be an abundance of the first host {Tapes 

 at BiUiers, Cardium at Piel) in the immediate vicinity of the 

 mussels, in order to ensm-e frequent re-infection. For I find on 

 the coasts of the Yillaine, where Tapes is scarce, the proportion of 

 infected Mytili is small. Moreover, on the Roosebeck Scar, outside 

 the Barrow Channel, where Cardium is not found, pearls do not 

 occur frequently. 



Thirdly, the beds must be near the feeding-grounds of the 



