150 



DR. H. LYSTER JAMESON ON THE 



[Mar. 4, 



residual material in the sac, vacate it, and settle down in the 

 immediate vicinity (PI. XVII. fig. 15). 



The residvial matter in the first sac forms the nucleus of a pearl, 

 and if the Trematode dies another is formed in the new one beside 

 it. If these pearls grow and iuse a double pearl is formed, the 

 nucleus of one half being obviously a Trematode, that of the other 

 being merely granular matter. 



I have traced three stages in the formation of such a pearl — the 

 first, in which a Cercaria is found in a sac with a vacated sac close 

 by ; the second, in which a small pearl is close to the live Cercaria ; 

 and the third, in which two or more small pearls lie close together, 

 only one having a Trematode for nucleus. 



Dubois (1901) suggests that the death of the Distoma may 

 sometimes be determined by Sporozoa, some members of which 

 group are known to attack Trematodes. In one of the specimens 

 that I sectioned there was a parasitic protozoon embedded in the 

 tissues. If such parasites were to occur fi-equently they would 

 of course facilitate and intensify the production of pearls. But 

 they are not essential, any more than the presence of the dead 

 Distoma in the sac is necessary for pearl-formation. 



Text-fig. 22. 

 ext.ep. con. nu. s 



mt.ep. 



A Pearl about to become attached to the Shell. 



nu., nucleus of pearl ; int.ep., internal ciliated epidermis of mantle ; ext.ep., external 

 epidermis of mantle ; con., conchyolin basis of pearl ; s., epithelium of pearl-sac ; 

 e.t., connective tissue. X 50. 



A pearl may increase in size until its diameter is considerably 

 greater than the thickness of the mantle, so that it protrudes 

 visibly. It may even break through the ciliated epidermis ; for 

 valuable pearls have been found in the branchial chamber and 

 also outside the shells. 



