286 Professor W. A. Hcrdman [March 27, 



French writers have made similar observations in the case of Donax 

 and other Lamellibranchs ; and Dubois (1901) has more recently 

 ascribed the production of pearls in mussels on the French coast, to 

 the presence of the larva of Distomum margaritarum. Jameson 

 (1902) then followed with a more detailed account of the relations 

 between the pearls in Mytilus and the Distomid larvae, which he 

 identifies as Distomum (Bracliycoeliimi) somaterise (Levinson). Jame- 

 son's observations were made on mussels obtained partly at Billiers 

 (Morbihan), a locality at which Dubois had also worked, and partly 

 at the Lancashire Sea-Fisheries marine laboratory at Piel in the 

 Barrow Channel. Finally, Dubois has just published a further note * 

 in which, referring to the causation of pearls in Mytilus, he says 

 (p. 178) : " En somme ce que ce dernier [Garner] avait vu en Angle- 

 terre en 1871, je I'ai retrouve en Bretagne en 1901. Quelques jours 

 apres mon depart de Billiers, M. Lyster Jameson, de Londres, est 

 venu dans la memo localite et a confirme le fait observe par Garner 

 et par moi." But Jameson has done rather more than that. He has 

 shown that it is probable (his own words are " there is hardly any 

 doubt") that the parasite causing the pearl-formation in our common 

 mussel (not in the Ceylon " pearl oyster ") is the larva of Distomum 

 somaterise, from the eider duck and the scoter. He also believes that 

 the larva inhabits Taj)es or the cockle as a first host before getting 

 into the mussel. 



We have found, as Kelaart did, that in the Ceylon pearl oyster 

 there are several difi'erent kinds of worms commonly occurring as 

 parasites, and we shall I think be able to show in our final report 

 that Cestodes, Trematodes and Nematodes are all concerned in pearl 

 formation. Unlike the case of the European mussels, however, we 

 find so far, that in Ceylon the most important cause is a larval 

 Cestode of the Tetrarhynchus form. Mr. Hornell has traced a 

 considerable part of the life history of this parasite, from an early 

 free-swimming stage to a late larval condition in the file fish (Balistes 

 mitis) which frequents the pearl banks and preys upon the oysters. 

 We have not yet succeeded in finding the adult, but it will probably 

 prove to infest the sharks or other large Elasmobranchs which 

 devour Balistes. 



It is only due to my excellent assistant, Mr. James Hornell, to 

 state that our observations on j^earl formation are mainly due to 

 him. During the comparatively limited time (under three months) 

 that I had on the banks 1 was mainly occupied with what seemed 

 the more important question of the life-conditions of the oyster, in 

 view of the frequent depletion of particular grounds. 



It is important to note that those interesting pearl-formation 

 parasites are not ouly widely distributed over the Manaar banks, 

 but also on other parts of the coast of Ceylon. Mr. Hornell has 



♦ Comptes RenduB Acad. d. Sci., 19th Jan., 1903, 



