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Show 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 convolutions depends upon the number of eggs contained (up to 100, or even more). It seems to begin on the left side, near the ovary (ov.), which is larger than the testes. It forms a series of convolutions on the left side, crosses in front of the genital opening to the right, where it forms further convolutions, passes forward as a loop to the anterior end of the body, and runs back to open out at the genital aperture. I a m not convinced that this arrangement is always strictly adhered to. The eggs in the uterus have the form depicted in m y sketch and in Levinsen's figure. They measure from #018 m m . to •023 mm., the average dimensions being "021 m m . x "018 mm., but they differ considerably in different individuals. I can add nothing new to Levinsen's observations on the yolk-gland. 1 sought in vain for this worm in Bernicla brenta Pall., Tadorna cornuta GmeL, Querquechda crecca Linn., Colymbus arcticus Linn., Larus argentatus GmeL, and Rissa 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, Fuligida ferina Linn., with infected Mytili, but without results. L. somaterke will very probably be found in the other members of the genera Somateria and GEdemia 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 differences 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 transported by currents and deposited with silt, &c. Hence mussels grown 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 Billiers, Cardium at Piel) in the immediate vicinity of the mussels, in order to ensure frequent re-infection. For I find on the coasts of the Villaine, 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 |