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 Avith 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. 
