MOLLUSCA 



247 



sea. The marine forms, moving in a relatively dense 

 medium, can afford to have thick shells, and in addition 



Drawing by R. Weber 



FIG. 64. Freshwater Mollusca (enlarged). A, a sinistral shell (Physa). B, a 

 dextral shell (Lymncea). 



they need to be protected from the buffeting of the 



waves if they live near the shore. Most snail shells Dextral and 



have what is called a dextral spiral ; that is, if the shell sheUs 



is held so that the aperture faces the observer, it is on 



the right-hand side. Sinistral shells have the aperture 



to the left, the whole spiral being reversed. Certain 



genera, as the fresh-water Physa, are regularly sinistral. 



Very rarely sinistral specimens of ordinarily dextral 



species are found ; these are much prized by collectors 



of shells. The reversal of the normal twist, as a rare 



abnormality, is not confined to mollusks ; even in man 



the heart is occasionally on the right instead of the left 



side. 



Some marine shells, such as the Murex, are protected Sea slugs 

 by great spinelike projections. Even the sea slugs, 

 naked and apparently without any resource against' 

 enemies, have special means of protection. Some are 



