CHAPTER XVIII 

 A LAND SNAIL AND OTHER MOLLUSKS 



INCLUDED with the bivalves in the phylum Mollusca are four 

 other classes. Two of these, the Amphineura and Scaphopoda, 

 are comparatively rare; the third, the Cephalopoda, is repre- 

 sented by some very interesting marine species like the octopus 

 and chambered nautilus; and the fourth, the Gastropoda, is 

 abundantly represented almost everywhere by the snails and 

 slugs. The activities and structure of snails may best be illus- 

 trated by a consideration of a common land snail. 



Life on Land. Mollusks are naturally aquatic animals, and 

 when certain of them forsook the water for a life on land, their 

 habits and structures changed in order to meet the terrestrial 

 conditions. In the first place the evaporation of water from 

 the body had to be retarded. This is accomplished partly by the 

 shell and partly by the layer of viscid substance, the mucus, 

 which covers the skin. However, in spite of these protections 

 from evaporation, land snails can exist only in a moisture-laden 

 atmosphere, becoming active only during damp weather and on 

 dewy nights, when there is no sun to dry up their bodies. When 

 placed in a dry vessel, snails withdraw into then* shells and re- 

 main inactive until they are moistened; such an experiment 

 may be carried out in any laboratory. In prolonged dry weather 

 the mucus secreted by the edge of the mantle forms a thin mem- 

 brane, the epiphragm, across the opening of the shell, which 

 prevents desiccation. 



Protection. THE SHELL. Snails are protected by their 

 shells not only from evaporation but also from mechanical in- 

 jury and from many enemies. The shell is coiled about a cen- 



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