SEA-FISHERIES LABORATORY. 171 



suppose that in their migration they may push some 

 epidermal cells in before them. At least this is not such 

 a violent assumption as that the connective tissue in the 

 centre of the mantle can produce an epithelial sac the cells 

 of which are indistinguishable both in structure and in 

 functions from the epidermis outside. 



In giving a preliminary account of pearl formation in 

 the Ceylon Pearl-Oyster to section D of the British 

 Association last September I took up the position that the 

 sacs enclosing the pearls were in all cases of ectodermal 

 (epidermal) origin ; and I am glad to find that Prof. A. 

 Giard, in a recent note* on the subject, takes the same 

 view, and considers that in the case of Jameson's mussel 

 there is a "passive immigration" of the epithelial cells 

 caused by the migrating parasite. 



The second point which I feel is not yet satisfactorily 

 settled, is the supposed infection of the mussel with 

 parasites by the Tapes in France and the cockle in the 

 Barrow Channel. So far as regards the latter case, 

 Jameson's conclusion is based upon the experiment of 

 placing some mussels, which he supposed to be free from 

 parasites, in a tank with French Tapes which were infected, 

 and examining the mussels from time to time until he 

 found they contained the parasites (Cercaria). Now in 

 such an experiment it is necessary to be quite sure of the 

 material used, to deal with sufficiently large numbers 

 and to have control experiments. Jameson does not seem to 

 have taken these precautions. He says of the material : — 

 " These mussels, of which I examined a number, were 

 practically without parasites. About one in every five of 

 the largest examples contained a Cercaria, one had two 

 Cercarias, and one contained a small pearl." This can 



*Comptes reudus, Soc. de Biol., Paris, 19 Dec, '03, lv., p. 1618. 



