SNAILS AND MUSSELS 



there their twisted shells attract attention and they are easily 

 remembered. A snail's shell is a spiral cone and beneath its 

 apex is the back or dorsal side of the animal. Most snail shells 

 are right-handed, some are left-handed. When a right-handed 

 shell is held with its opening toward the observ^er and its apex 

 up, the opening will be at the observer's right. Held in the 

 same position the shell of a left-handed snail will have its 

 opening at the left (Fig. 240). 



^— 1 



Fig. 240. — Diagrams of snail shells, showing 

 regions and types of twisting. A. — Right-handed 

 shell: I, apex; 2, spire; 3, body whorl; 4, columella; 

 5, opening or aperture; 6, lines of growth. B. — 

 Left-handed shell. 



Shell is the hardened secretion of surface and border cells 

 of the fleshy mantle beneath it, and lines of growi;h show suc- 

 cessive additions of new shell. The shell is twisted on an axis 

 known as the columella (Fig. 240). 



A' snail's body is coiled and twisted like the shell and ex- 

 tends into its apex. A fresh water snail has a distinct head 

 bearing two conspicuous tentacles with a small black eye at 

 the base of each (Fig. 241). Land snails which occasionally 

 wander down to the water margin can be distinguished by 

 their four tentacles. Snails have one to three chitinous jaws 

 in the upper part of the mouth and on the floor of it a ribbon- 

 like tongue or radula whose outer surface is covered with 

 rows of homy teeth (Fig. 241). This radula is their all- 



305 



