FORMATION OF PEAML8. 231 



end of the tube or burrow. The mantle lobes of the ani- 

 mal ^re united, with a minute opening for the ^oot, whifh if 

 small, BUvjker-like. The heart is not pierced by the mtes- 



Fig. IW.—Mactra avails, natural size.— After Murse. 



tine, while the siphons are very long and fvirnished with 

 two shelly styles. 



Pearls are sometimes produced in bivalve shells by particles 

 of sand getting in between the mantle and the shell, which 



Fig. J70.~Patu)ptm ardlca, natural size.— After Morse. 



cause an irritation to the tissues of the mantle and the for- 

 mation of a nacreous shelly matter around the nucleus. 

 Excellent pearls are sometimes found in fresh-water mussels, 



