FORMATION OF PEARLS. 



231 



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

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

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





Pig. iSQ.^Mactra ovalis, natural size. — ^After Iloise. 



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. 110.— Panoposa arctica, natural size.— iftfr Morse. 



cause an irritation to the tissues of the -mantle; and the f ar> 

 mation of a nacreous shelly matter around the nucleus. 

 Excellent pearls are sometimes found in fresh-water mussels, 



