LECTURE XI. 189 



tinctly seen several days before the period of 

 hatching. All the shell-animals are of such a con- 

 stitution as perpetually to secrete or exsude from 

 their bodies a viscid moisture, and it is with this, 

 managed according to the exigences of the ani- 

 mal, that the shell is throughout life increased 

 in dimensions, and repaired when accidentally 

 broken in any particular part. The growth of shells 

 proceeds from the edges of the mouth or opening, 

 and thus the spires or turns of the Univalve shells 

 are gradually increased in number and size, till 

 the animal has arrived at the full limits of its 

 growth. The Bivalves are increased in a similar 

 manner, by the gradual enlargement of the out- 

 line of each valve. 



