FORMATION OF PEARLS. 231 
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, suvher-like. The heart is not pierced by the intes- 
Fig. 169.—Mactra ovalis, 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. 170.—Panopea arctica, natural size.—Afier 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, 
