144 



INTRODUCTION TO ZOOLOGY. 



is always notched or pi-olou^^cd into a spout, and the 

 odontophore in sucli a cnse is long and narrow, and 

 is called a radula. It is capable of drilling or sawing 

 a hole through the shell of nnother mollusk. 



Tlie odontophore con- 

 ists of a iiienilirane of 

 horny material, thickly set 

 with numerous curved 

 teeth. It i)asses over the 

 surface on which the snail 

 is crawling and rasps off 

 auytliing that the snail can 

 eat. 



>Dr>NTi IPHI )ltl;; 



Most sea snails l)reathe by means of a gill, but the 

 greater number of land and fresh-water snails breathe 

 by a lung. Usually a snail tliat has an operculum, 

 or plate to close the aperture, is a gill Ijreather. One 

 without an opereuhun is a lung breather. 



Most snails have eyes, and in the case of land 

 snails the eyes are placed at tlie ends of eye stalks. 

 In the greater number of our p(.)nd snails, however, 

 the eyes are not ()n stalks, Init are situated at the base 

 of t(;ntacles. 



Some snails, called slugs, are without any shell, 

 or have only a, rudimentar)' shell which is concealed 

 beneatli the epidermis. 



Snails belong to a t'lass called tiasteroi)ods. The 

 name is given bccatise the snail lo(.)ks as if it were 

 crawling on its l)elly. 'Idicy are also called univalves, 

 because the shell is in one piece. 



