MOLLUSCA PROPEE. 



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

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



Fig. 84. — Grasteropoda. a Shell of the Turritella eommwnia, showiij^ a round mouth; 

 & Shell of the common whelk ^BncciTium undaiwrn), 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, b), the mouth of the shell 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 



