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 coelum, 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 oesophagus. 



All sections of the group, except the Bivalves, 

 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 



