102 PEARLS [CH. VIII 



With regard to the pearl sac which appears around 

 the worm larva in the mussel, Jameson accounted 

 wrongly for its origin in 1902. He states that "This 

 epithelium appears to arise quite independently of 

 the outer epidermis, and is no doubt due to a specific 

 stimulation on the part of the parasite, as other 

 parasites, e.g. sporocysts, cestode larvae, etc. are not 

 surrounded by such a sac." He continues by saying : 

 "At first a few cells appear which proliferate and 

 arrange themselves along the wall of the cavity.... 

 From the first these cells are basally continuous with 

 fibres of connective tissue. Their transformation into 

 the pearl sac is a gradual one, and every step can be 

 traced in sections of the parasites in situ." It was 

 the Frenchman, L. Boutan, in 1904, who continued the 

 investigation of pearl formation in the edible mussel 

 (Mytilus) and completed the story by describing hoAV 

 the pearl sac was really formed by the parasite. 



Boutan states that the parasitic larva in the 

 common mussel reaches the space between the shell 

 and the mantle of the mollusc. It then comes to lie 

 in a little pit or depression in the epithelium of the 

 mantle (diagram, fig. 9 a). 



Gradually the parasite sinks deeper and deeper 

 (fig. 9 h and c), the epithelium of the depression 

 becomes a little flask and finally a sac just com- 

 municating with the surface by a small opening. In 

 the end, the sac becomes cut ofl* altogether, and we 



