6 THE LIFE OF THE MOLLUSCA 
from the exterior or from the interior, as occasion 
may require. 
The lime for the shell being probably obtained by 
the animal mainly from its food, it follows, especially 
in the case of the vegetable feeders, that where lime 
is abundant in the soil or water, and consequently in 
the plant tissues, the shell tends to become abnor- 
mally thick and heavy; while other members of the 
same species living where lime is scarce, will have 
exceedingly thin shells. The latter condition is very 
observable in the larger Land Snails of the Channel 
Islands, where, indeed, the living individuals will 
even resort to the shells of their dead comrades to 
obtain the requisite supply. Isolated cases of abnor- 
mally thick shells are probably due to physiological 
peculiarities of the individual, enabling it'to absorb 
lime more easily than its fellows. 
Internally the common trait in the Mollusca is the 
reduction of the ccelum, or body cavity, to a space 
around the heart, and the concentration of the prin- 
cipal nerve centres (“ganglia”) into a ring or collar 
surrounding the cesophagus. 
All sections of the group, except the_Biyalves, 
have a distinctive feeding organ called the “ radula,” 
and in their development almost always pass through 
what is known as the “‘veliger stage” (see infra, 
P- 93). 
The heart, which is well developed, is entirely 
arterial—that is to say, only receives the blood after 
