1 
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 ‘apes 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 (Cerearia). Now in 
such an experiment it is necessary to be quite sure of the 
material used, to deal with sufticiently 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 tive of 
the largest examples contained a Cercaria, one had two 
Cercarie, and one contained a small pearl.’ This can 
*Comptes rendus, Soc. de Biol., Paris, 19 Dec: 703; 1v.,1p: 1618. 
