CHAPTER VII. 



SNAILS AND SLUGS. 



THE fauna of the British Isles is fairly rich in the 

 terrestrial and fresh-water Gastropod Molluscs popularly 

 known as Snails and Slugs. The shell of the common 

 garden snail, Helix aspersa, or of the larger Helix pomatia 

 may be taken as typical. It is spirally coiled, and marked 

 with numerous fine lines of growth running parallel with 

 the margin of the matter of the shell and indicating 

 successive positions of the free edge during the growth. 

 The surface is decorated with several bands of colour, 

 whose arrangement and pattern are liable to great varia- 

 tions. A spiral "suture" runs from the tip or "apex," 

 round and round the shell to the upper edge of the 

 mouth: this suture marks the line along which the 

 growing margin of the shell has become fused with the 

 already existing whorls. It will be seen that the direction 

 of growth has always been towards the animal's right 

 side : the shell exhibits a " dextral " or right-handed spiral. 

 This will be more easily grasped if the reader imagine 

 himself to be on the tip of the shell, in its natural 



