MOVEMENTS OF GASTEROPODS 



475 



can be made about the life of Gasteropods. We are safe in 

 saying, however, that though the majority are sluggish when 

 compared with Cephalopods, they are active when com- 

 pared with Lamellibranchs. 



The locomotion effected by the contractions of the 

 muscular foot is usually a leisurely creeping, but there are 

 many gradations between the activity of Heteropods in 



Fig. 271. — -A Pteropod [Cymbulia peronii), showing the wmg-Hke 

 expansions (pteropodial lobes) of the mid-foot - 



Open sea, the gliding of fresh-watef snails {Limncea) foot 

 upwards across the surface of the pool, the explorations of 

 the periwinkles on the sand of the shore, and the extreme 

 passivity of limpets {Patella), which move only for short 

 distances at a time from their resting-places on the rocks. 



The number of terrestrial snails and slugs, breathing the 

 air directly by means of a pulmonary chamber, is estimated 

 at over 6000 living species, while the aquatic Gasteropods 

 are reckoned at about 10,000, most of which are marine. 



