= SS 

Figure 2. - The prosobranch operculum. A, The operculum in the 
shell aperture, sealing off the interior of the shell; B, position of 
operculum while the snail is active. 
tentacle tentacle 
VA ¢ 
~ 
=sS 

A 

Figure 3, - Position of the eyes and method withdrawal of the 
tentacles in the three pulmonate snail orders. A, BASOM- 
MATOPHORA, eyes at bases of contractile tenacles; 
B, SYSTELLOMMATOPHORA, eyes at the tips of contractile 
tentacles; C, STYLOMMATOPHORA, eyes at the tips of 
retractile tentacles. 
