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TEXTBOOK OF ZOOLOGY 



some shells are shaped like house roofs (Patella that lives in the sea 

 or Ferrissia, a fresh-water form). The worm shell (Vermetus 

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

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

 long peculiarly 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. 





Polyqyra ^ Polyqyra 

 texasiana dorfeuiHiana 



Helicina 



orb'iculata 



tropica 



f?etinella 

 indentata 

 paudiirata 



_ .... Euconulus 



Zonitoides chersinus 



orboreus trochulus 



Strobilops 



lobyrinthica 



texasiana 



(qostrocopta 

 armifera 



Succinea Zuqfandina Pupo'ides Phiiornycu^ 

 Qvam sincjleyana marq'inatus carolinensis 



Fig. 132. — Common terrestrial snails. 



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. 



The shell which is largely composed of carbonate of lime is 

 secreted by the mantle and usually consists of three layers. Em- 

 bedded within the latter may be pigments that give the occasional 

 brilliant colors to certain species. The thickness of the layers is 

 dependent on the richness of lime salts in the environment; thus, 

 snails living in an acid bog have thin transparent shells, whereas 

 the same species inhabiting an area rich in lime salts have thicker, 



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