MOLLUSCA I GASTEROPODA. 261 



siderably elongated, so as to form, a tube, which may retain this 

 shape (as in D&ntaliwn), but is usually coiled up into a spiral. 

 The ' spiral univalve ' may, in fact, 

 be looked upon as the typical form, 

 of the shell in the Gasteropoda. In 

 some cases the coils of the shell 

 termed technically the 'whorls' 

 are hardly in contact with one an- 

 other (as in Vermetus.) More com- 

 monly the whorls are in contact, 

 and are so amalgamated that the 

 inner side of each convolution is 

 formed by the pre-existing whorl. 



In SOme Cases, the whorls of the Fig. 78. Helix aspersa, the Spotted 



shell are coiled round a central axis Snai1 ' a ' s *> iral> ^asteropod. 

 in the same plane, when the shell is said to be 'discoidal' (as 

 in the common fresh-water shell Planorbis) . In most cases, 

 however, the whorls are wound round an axis in an oblique 

 manner, a true spiral being formed, and the shell becoming 

 'turreted,' 'trochoid,' ' turbinated,' &c. This last form is the 

 one which may be looked upon as most characteristic of the 

 Gasteropods, the shell being composed of a number of whorls, 

 passing obliquely round a central axis, or ' columella,' having 

 the embryonic shell, or ' nucleus,' at its apex, and having the 

 mouth, or 'aperture,' of the shell placed at the extremity of 

 the last and largest of the whorls, termed the ' body- whorl.' 

 The lines or grooves formed by the junction of the whorls are 

 termed the ' sutures,' and the whorls above the body- whorl 

 constitute the ' spire ' of the shell. The axis of the shell (co- 

 lumella) round which the whorls are coiled is usually solid, 

 when the shell is said to be ' imperforate ; ' but it is sometimes 

 hollow, when the shell is said to be ' perforated,' and the aper- 

 ture of the axis near the mouth of the shell is called the 

 'umbilicus.' The margin of the 'aperture' of the shell is 

 termed the ' peristome,' and is composed of an outer and inner 

 lip, of which the former is often expanded or fringed with 

 spines. When these expansions or fringes are periodically 

 formed, the place of the mouth of the shell at different stages 

 of its growth is marked by ridges or rows of spines, which 

 cross the whorls, and are called 'varices.' In most of the 

 phytophagous Gasteropods (JELolostomata) the aperture of the 

 shell (fig. 79) is unbrokenly round, or ' entire,' but in the car- 

 nivorous forms (Siplionostomata) it is notched, or produced 

 into a canal. Often there are two of these canals, an anterior, 

 and a posterior, but they do not necessarily indicate the nature 

 of the food, as their function is to protect the respiratory 



