402 



MOLLUSCA : GASTEKOPODA. 



Univalves. Some, however, have no shell in the adult 

 < state, though all have a shell when first hatched. 



Many kinds of gasteropods have a lid or door, called the 

 opercuLum, with which they close the opening to the 



FIG. 643. 



Apex 



buturc 



' Spire. W, whorls. 



Lines of Growth .--_.--- 



Umbilicus 



Columella. 

 Outer lip. 

 . Aperture. 



Canal. 



Names of the parts of a Gasteropod Shell. 

 FIG. 544. 



,, , lei 



Lingual teeth of Achatina, with central and lateral teeth enlarged. 



FIG. 545. 



Tongue of the Whelk, Buccinum undatum. Woodward. 



shell when they withdraw within. It is a horny plate, 

 sometimes strengthened by shelly matter. 



