'The Shell and its Formation 39 



the case with the pond-snails, who probably find 

 carbonate of lime not very plentiful in their little 

 world, neither is it greatly needed as a protection. 

 A strong shell is required by those land-snails 

 whose habitat exposes them to the attacks of birds 

 like the thrush, but it is essential that the shell 

 should not be heavy. The carbonate of lime is in 

 this case obtained from the plants upon which the 

 snails feed, and it is therefore only to be expected 

 that on chalky soils the snails should be exceedingly 

 abundant, whilst in sandy districts they are rare 

 or entirely absent. But it does not follow that on 

 chalky soil the shells will be necessarily thick, 

 Clausilia lariiinata, for example, plentiful in beech 

 woods on the chalk, having a thin semi-transparent 

 shell, apparently with little chalk in it. 



In the slugs of our gardens and hedgerows the 

 shell has been reduced to a little shield covering 

 the breathing organ, or to a few granules of lime 

 beneath the mantle ; whilst many of the Sea-slugs 

 have found it an advantage not to develop the 

 shell at all, though they are born with the nucleus 

 of a shell like all other moUusks. A somewhat 

 similar case to that of the terrestrial slugs, so far 

 that is as the shell is concerned, will be found 

 among the Cephalopods : the Sepia produces beneath 

 its mantle the familiar " cuttle-bone," in the Squid 

 this is reduced to the long transparent " pen " of 

 animal matter only, whilst in the Octopus the shell 

 is represented by two little stylets in the substance 

 of the mantle. 



