1902.] ORIGIN OF PEARLS. 143 



As it was not possible for me 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 my obsei-vations. This 

 mollusc produces peails in many localities on the coasts of Europe, 

 but it is only on certain beds that peails are abundantly foi-med. 

 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 ai-e 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 ai-e 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 Yillaine, there is a colony of peai-l-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 investigatmg the causes of 

 peail-foi-mation and the conditions foi- 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 peai'ls. 



I visited BUliers 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. somaterice 

 (Levinsen), which in the mature condition inhabits the intestine 

 of the Eider Duck. I found larvae, very similar to these, in Sporo- 

 cysts in Tajies decussatics, 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, bvit that in this case 

 Cardium edule 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 VUlaine 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 Distoimiin {Leucithodendriitm) 

 somaterice. 



For histological work, pearls were decalcified in situ in the 

 tissues and then sectioned. Others were decalcified, cleaned, and 



