206 SNAILS AND SLUGS [CH. VII 



Fig. 39. Helix pomatia. Side view of shell and animal expanded. 

 From Hatschek and Cori. 



1. Mouth. 2. Anterior tentacles. 3. Eye tentacles. 4. Edge 

 of mantle. 5. Kespiratory pore. 6. Anus. 7. Apex of 



shell. 8. Foot. 9. Eeproductive aperture. 



position, and about to walk down along the suture; he 

 will then keep constantly turning towards his right hand. 

 The majority of gastropod shells are dextral ; occasion- 

 ally sinistral examples are found of species normally 

 dextral, e.g. the common whelk, while in some few the 

 sinistral condition is the rule. 



On the under surface of the shell is a small opening, the 

 " umbilicus," leading into a hollow pillar, the " coiumella," 

 which is formed by the contact of successive whorls of the 

 shell. The umbilicus is completely hidden by a reflected 

 portion of the shell-lip in H. aspersa, but in H. pomatia it 

 is partly exposed. 



The " apex " is the oldest part of the shell and was 

 at the first sufficient to contain the entire young snail. 

 As the animal grew a larger shell became necessary, and 

 accordingly additions were made to the free lip of the 



