174 



GENERAL ZOOLOGY 



of pearls in the common edible sea-water mussel, My'tilus 

 ed'ulis (in Fig. 126). A trematode parasitic worm (Fig. Ill) 

 in the larval stage creeps between the valve and the mantle- 

 fold. In its wanderings it starts to bore through the mantle- 

 fold. The tissue is soft, but the parasite may rest for a while 

 among the loosely connected cells. In that case it is imme- 

 diately surrounded by a minute fold of the outer epithelium 

 (surface-cells) of the mantle. About the trematode larva as 

 a center the epithelium (Fig. 90, 2) begins to secrete a sub- 

 stance which, on hardening, 

 becomes a pearl (Fig. 90, 5). 

 Before the hardening takes 

 place the larva may move to 

 some other part of the man- 

 tle-fold. Again it may be 

 the stimulus for producing a 

 pearl. 



Examination of pearls 

 often reveals a small particle 

 of matter about which the 

 pearl was formed in layers. 

 Professor Jameson thinks the 

 substance at the center of 

 the pearls he has examined 



is the body-waste, or excrement, of the parasite (Fig. 90, 0), 

 except in those pearls in which he has found the dead body 

 of the parasite itself. Even the temporary presence of the 

 parasite is a sufficient stimulus for the production of a pearl. 

 The pearl may get so large that it breaks through the outer 

 epithelium of the mantle-fold and becomes attached to the 

 valve itself. If the pearl breaks through the inner (ciliated) 

 epithelium (Fig. 90, 3) of the mantle-fold, it falls into the 

 mantle-cavity, and finally to the outside, which accounts for 

 the fact that pearls are often found in the sand. 



FIG. 90. Section of Mantle-Fold of 

 Mussel showing how a Pearl is 

 formed. (After Jameson) 



1, cells of mantle ; 2, external epithelium 

 of mantle ; .'$, internal (ciliated) epi- 

 thelium ; 4, position of shell ; 5, pearl ; 

 6, remains of parasite 



