\r(mi& 



HE mollusks considered in 

 the previous chapter were 

 marine, many of them lit- 

 toral species, living on the 

 rocks that are uncovered for hours each day 

 when the tide has receded. To protect some from 

 the evaporation of their fluids in such a situation, 

 and to preserve others from enemies when swept off 

 the rocks by rough seas, all those that had distinct 

 whorled shells had the orifice closed by a door or 

 operculum. It can be clearly seen how such an 

 adjunct to the open shell made a transition from 

 marine to terrestrial life easy. Such a form as 

 Littorina rudis, that spends weeks above high water, 

 and is believed to be in the process of changing to a 

 land-snail, was probably the ancestor of the Round- 

 mouthed Snail (Cyclostoma elegans), which is clearly 

 a marine snail that has been so modified tlmt it lives 



228 



