MUSSELS AND SNAILS. 161 
and passes from these organs to the auricles. Part of 
the blood also flows through the renal organs, which 
take from the blood (excrete) the nitrogenous waste 
matters and empty them into the outgoing current of 
water. The ventricle forces the blood, constantly 
coming to the auricles, away from the heart all over 
' the body. 
As in other animals, the separate cells take from the 
circulating liquid the substances necessary to produce 
eg 
PY: 
0.9. 
Fic. 126.—Cross-section of Fic. 127.—Nervous System of Ano- 
Anodon. n, intestine; v.e., don. ¢.g.,ganglia near the mouth; 
heart; z.g. and o.g., gills; P8&., ganglia of the foot; o.¢., 
J, foot; .é., mantle. ganglia of the posterior adductor. 
more cells like themselves or to manufacture intercel- 
lular structures. The-cells of the mantle secrete 
carbonate of lime, which forms in layers on the inside 
of the shell and along its edges, thus increasing both 
its thickness and its size. If the mantle be irritated by 
the introduction of minute particles of foreign matter or 
by disease the secretion of carbonate of lime at a defi- 
nite point is hastened and a pearl is formed. 
Respiration. As the water passes through the gills 
it flows over and around many blood-vessels, through 
