64 ESSENTIALS OF ZOOLOGY 



Jameson has demonstrated the part played by Distoma 

 margaritarum in the formation of pearls of the common bait 

 mussel, Mytilus edulis, and Boutan emphasised the fact that 

 the pearl is always produced in a pouch formed by the outer 

 mantle epithelium. It is believed that oriental pearls are 

 provoked by a Distoma, perhaps the same as that of the 

 common mussel, or one closely allied to it. Birds are the final 

 hosts of this parasite, and the mussel is probably the second 

 of two invertebrate hosts. Herdman and Hornell found that 

 the pearls of Ceylon resulted from a cestode parasite in the 

 larval state, and probably other larvae may be found to be 

 encysted in mussels with pearl formation. Methods of culture 

 have been extended to the encouragement and protection of 

 pearl fisheries in the East Indian region, and with notable 

 success in Japan. Encouraging success has followed also the 

 artificial stimulation of the mantle in the formation of blister 

 pearls or half pearls by trepanning the shell and introducing 

 artificial nuclei. Perfect pearls have been produced artifici- 

 ally in Japan, and they are apparently little different from 

 the costly fine pearls of the Eastern fisheries. 1 



The bilateral symmetry expressed by the valves of the 

 shell is likewise characteristic of the animal protected by 

 them. The mantle lobes, which depend sub-equally on each 

 side, are lateral expansions of the dorsal region of the body. 

 The outer ectodermal layer which is applied to the inner face 

 of the shell is firmly attached thereto along the pallial line. 

 Beyond this, it ends in a ventral border extending from the 

 region of the anterior to the posterior adductor muscles. The 

 inner epithelium of the mantle is ciliated and is continuous 

 with the ciliated ectodermal layer of the gills and foot. The 

 two folds of the mantle fuse posteriorly and divide the opening 

 into a ventral inhalent and a dorsal exhalent opening. The 

 mantle is thickened in the region of the respective openings. 

 The margin of the lower opening is produced into tentacular 

 processes, and it is limited at its ventral angle by the 

 approximation of the borders of the mantle. The visceral 

 mass of the mussel is situated dorsally and medianly, and 



1 1902, Jameson, Proc. Zool. Soc. ; 1903, Herdman, The Pearl Fisheries of 

 Ceylon ; 1904, Boutan, Arch, de Zool. exper. et gener., 4me Ser., T. g. 



