144 MANUAL OF NATURAL HISTORY. 



tentacles cylindrical, shorter than and under eye- 

 peduncles. Living on the land. 



1. Family. — W ater-lo ving Slugs (Onchidiidse). Eye- 



peduncles contractile (not retractile), eyes 

 at their ends ; body covered with a large 

 coriaceous mantle ; shell none. 



2. Family. — Burrowing-Slugs (Testacellidse). Eye- 



peduncles retractile; mantle usually con- 

 cealed under shell ; breathing orifice on hind 

 part of mantle ; shell external, small, on hind 

 part of body. 



3. Family. — True-Slugs (Limacidse). Eye-peduncles 



retractile ; mantle shield-shaped, covering the 

 shell ; breathing orifice on right side ; caudal 

 gland none ; shell rudimentary, internal. 



4. Family. — Arions (Arionidse). Eye-peduncles 



retractile ; mantle shield-shaped, covering 

 the shell ; breathing orifice on right side ; 

 foot with a distinct caudal gland near the 

 end ; shell internal, rudimentary, or external 

 and well developed. 



5. Family. — Snails (Helicidse). Eye-peduncles re- 



tractile ; mantle lining the shell ; caudal 

 gland none ; shell external, spiral, well-de- 

 veloped 



1. Buliminw. Aperture of shell longer than 



wide, columella not truncate ; spire ele- 

 vated. 



2. Helicince. Aperture of shell wider than 



long ; columella simple in front ; spire 

 moderate. 



