xi PHYLUM MOLLUSCA 287 



peristome, which is sometimes entire and continuous, some- 

 times is broken by a deep notch or a spout-like prolonga- 

 tion or canal, formed in connection with the development 



Fig. 173. — Longitudinal median section of the shell of Triton nodiferus. 



of a spout-like prolongation of the mantle, the siphon, 

 which lies in it. The mouth of the shell in many Gastro- 

 poda is capable of being closed by means of an operculum 



