FORMATION OF PEARLS. 



351 



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

 mal are united, with a minute opening for the foot, which is 

 small, sucker-like. The heart is not pierced by the intes- 



Fig. 174. Mactra walls, natural size. After Morse. 



tine, while the siphons are very long and furnished 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. 175. Panopcea arctica, 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 rnussels ; 



