142 DR. H. LYSTER JAMESON ON THE [Mar. 4, 



variance with my experience; and apparently presupposes that 

 the Trematocle can sui-vive complete calcification, which would 

 indeed be a very remarkable biological phenomenon. According 

 to Dubois it is only cei-tain pearls that, by the death of the 

 Bistomtcm, escape this annual disintegration and so reach greater 

 dimensions. 



Yon Hessling (1858), it seems, was the first to ascertain that 

 the peail is formed inside an epithelial sac, and he emphasized 

 the importance of this structure. He regarded the sac as being- 

 derived from the blood-cells. 



This sac has been noted by Diguet (1899), who suggests that 

 it may be due to the stimulation of a parasite. I can find no 

 support for Diguet's view, that the formation of the pearl in this 

 sac proceeds on difierent lines to those on which the substance of 

 the shell is deposited. 



The "vesicle or bag of the ovum" figured by Home (1826, 

 pi. xiii.) may also be this sac. 



Before entering upon an account of my own observations, I 

 wish to express my thanks to Mr. H. H. Arnold Bemrose for 

 kindly preparing the microphotographs which accompany this 

 paper ; to Baron Louis d'Hamonville for much valuable inform- 

 ation concerning the pearl-bearing mussels of Billiers ; to 

 Mr. A. Scott, of the Lancashire Sea- Fish Hatcheries, for supplying 

 me with abundant material from the Piel mussel-beds ; and to 

 Mr. W. Wells, Marine Superintendent at the Brighton Aqviarium, 

 for conducting experiments for me. 



The distribution of pearl- producing individuals of Margaritifera 

 margaritifera L., M. maxima Jameson, Pinna nigrina Lam., 

 Hippopus hippopus L., and Triclacna gigas Lam., in New Guinea 

 and Torres Straits, suggested to me that pearls were the result 

 of a specific pathological condition, and that the circximstances 

 necessary to ensure infection were present only in certain areas, 

 often of small extent. I soon convinced myself, by a study of 

 material that I brought home with me, that Trematodes formed 

 the nuclei of some of the pearls in each of the above-named 

 species, but that others contained nothing more than a few 

 yellowish granules in the centre. The same results were obtained 

 with specimens of Mytilus edulis, sent me from Lancashire by my 

 friend Mr. James Johnstone. In all cases where the pearls had 

 been preserved in situ in the tissues, they were found to be 

 enclosed in a sac composed of an epithelivim physiologically and 

 histologically identical with the outer shell-secreting epidermis of 

 the mantle. This observation at once accounted for the similarity 

 in structu.re between the layers of the shell and those of which a 

 pearl is composed. The obvious conclusion was that this sac is the 

 direct, and the Trematode the indirect, cause of pearl-production, 

 and that the key to the problem of the origin of pearls might be 

 obtained by investigating the origin of the sac and its relations 

 to the Trematode. 



