CHAPTER VIII. 



Sutures 



BRANCH MOLLUSCA. 



CLASS GASTROPODA. 



THE gastropods include the snails and slugs. They are 

 of many kinds, terrestrial and aquatic, in fresh water and in 

 salt water, shelled and shell-less, symmetrical and unsym- 

 metrical, herbivorous and carnivorous. 



The Shell. The shells of gastropods are usually of one 

 piece, therefore they are often called univalves in distinc- 

 tion from the bivalves. Ape x 

 This one-pieced shell is 

 almost always in the 

 form of a cone. Some- 

 times the cone is nearly 

 straight, as in the tooth- 

 shells ; again it is in the 

 form of a very low, wide 

 cone, as in the limpets ; 

 but in the great majority 

 the cone is twisted into a 

 spiral, making a thick, 

 short cone out of a long 

 and slender one. Some- 

 times this primary cone 

 is wound upon itself to form a plane spiral like a watch 

 spring, as in Planorbis. But ordinarily the spiral is an 

 ascending spiral, which may be illustrated by holding the 

 outer end of a watch spring and pushing the inner end out 



127 



... Lip 



Aperture 



FIG. 79. PARTS OF A SNAIL SHELL. 



