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Show 1902.] ORIGIN OF PEARLS. 143 As it was not possible for m e to return to the habitat of the true Pearl-Oysters, I selected the Common Mussel (Mytilus edulis) as a suitable species upon which to begin m y observations. This mollusc produces pearls in many localities on the coasts of Europe, but it is only on certain beds that pearls are abundantly formed. The most favourable places seem to be estuaries or land-locked channels. In such situations pearls may be found in almost every example, except those which are attached to stakes or floating objects, and so raised off the bottom. The pearls produced by the Common Mussel are, like the nacreous lining of the shell, lacking in lustre. They are generally white or silvery, but blue and brown examples are not uncommon. They have no value as gems, though, strange to say, a market seems to have existed for them in the first half of the last century ("D. C," 1830). They are mostly formed in the subcutaneous tissue of the dorsal body-wall or in the mantle-lobes. When they occupy a more deep-seated position they have probably been secondarily displaced. In the little harbour of Billiers (Morbihan), situated on the estuary of the Villaine, there is a colony of pearl-bearing mussels, that has been described by d'Hamonville (1894). After reading d'Hamonville's account, I was struck with the idea that this colony should offer special facilities for investigating the causes of pearl-formation and the conditions for infection. D'Hamonville found that although the mussel is abundant all round the coast, pearls are only produced in the harbour itself, the beds being, at most, only a few acres in extent. Here almost every shell, if not too young, contains pearls. I visited Billiers in August 1901 and again in December of the same year, and had no difficulty in finding the parasites and tracing the part played by them in pearl-formation. They were the larvae of a Distomid belonging to the subgenus Leucitho-dendrium (Loos), and very closely resembling L. somaterke (Levinsen), which in the mature condition inhabits the intestine of the Eider Duck. I found larvae, very similar to these, in Sporo-cysts in Tapes decussatus, and subsequently proved the infection of Mytilus experimentally from these Sporocysts. In September of the same year I visited Piel, Lancashire, and found that there also pearls are caused by the same parasite, but that in this case Cardium eclule acts as " first host" for the Sporocyst. Finally, in December 1901, when I revisited Billiers, I examined five specimens of the Common Scoter or Black Duck, (Edemia nigra L., which is notorious in the Villaine for its depredations on the mussel-beds, and is locally called, on account of its habit of feeding on Mytilus, "Cane mouliere." Every one of these specimens was teeming with Distomum (Leucithodendrium) somateriai. ,,.,.,. •, • ,.r For histological work, pearls were decalcified in situ in the tissues and then sectioned. Others were decalcified, cleaned, and |