MOLLUSCA PROPER. 



183 



retained throughout life without any alteration. In the great 

 majority of cases, however, the cone is considerably elongated 

 so as to form a tube, which may retain this shape (as in the 

 " tooth-shell "), but which is usually coiled up into a spiral. 

 The " spiral univalve " may, in fact, be regarded as the typical 



FIG. 84. Gasteropoda, a Shell of the Turritella communis, showh> & round mouth; 

 & Shell of the common whelk (Buccinum undatum\ showing the mouth notched for 

 a respiratory siphon. 



form of the shell in the Gasteropoda (Fig. 84). The coils of 

 the spiral are termed the " whorls," and are usually more or 

 less amalgamated on one side. In most cases, too, the whorls 

 are wound obliquely round a central axis or pillar, increasing 

 gradually in size to the mouth. The last whorl is the largest, 

 and is termed the " body-whorl." The mouth of the shell in 

 many forms is unbrokenly round or " entire'" (Fig. 84, a\ and 

 it is found that most of these shells subsist upon vegetable 

 food, as, for instance, the common periwinkles. In others, 

 again (Fig. 84, #), the mouth of the snell is notched or is pro- 

 duced into a canal, as in the common whelk, and it is found 

 that these live upon animal food, or are " carnivorous." There 

 may be more than one of these canals or tubes, but they do 

 not necessarily indicate the nature of the food, as their func- 

 tion is to protect the respiratory siphons. 



The Gasteropoda are divided into a good many groups, 

 of which the more important may be briefly noticed, the fore- 

 going applying chiefly to the ordinary forms, which, therefore, 

 need no further description. The remaining members of the 



