THE SNAIL 



383 



The former feeds on secretions produced by the latter. A few species 

 of sponges, echinoderms, annelids, and mollusks are parasitized by 

 various species of mollusks. (Detailed Description, based on Helix.) 



External Anatomy 



Shell. — The shell of the snail may be in the form of a low, broad, 

 or flattened spiral (Humholdtiana chisosensis and Polygyra roemeri)^ 

 or a long, tapering spire {Lymnaea stagnalis) ; on the other hand, 

 some shells are shaped like house rooms {Patella that lives in the sea 



Po\yqyra , Polyqyra 

 texasiana dorfeuilUana 



Helicina 



orhiculata 



tropica 





Retinella 

 indentata 

 pauc/llrata 



_ .... Euconulas 



Zonjtoides chersinus 



arboreus trochulus 



Strob]]ops (^asbrocopta 

 labyrinthica armifera 

 texQsiono 



Succinea Zuqiandina Pupoides Philqmycu^ 

 Q\/ara sincjleyana. marqinatus carolinensis 



Fig. 150. — Common terrestrial snails. 



or Ferrissia, a fresh-water form). The worm shell {VermeUis 

 spiratus) is so loosely coiled that it superficially resembles a worm. 

 Some shells, such as those belonging" to the genus Miirex, may have 

 long peculiarl}^ curved spines extending out from the main shell 

 body that give to the shell a grotesque appearance. In the sea and 

 land slugs the shell is either rudimentary^ internal, or absent. 



If the shell is held with the aperture toward the observer and the 

 aperture is on the left, the shell is said to be sinistral; if on the right, 

 the shell is dextral. Most species are normally dextral, but occa- 

 sionally a reversal occurs which has been found to be inherited. 



