242 



MOLLUSCA. 



Shell of the Gasteropoda. The shell of the Gasteropoda is 

 composed either of a single piece (univalve), or of a number 

 of plates succeeding one another from before backwards (mul- 

 tivalve). The univalve shell is to be regarded as essentially a 

 cone, the apex of which is more or less oblique. In the 

 simplest form of the shell the conical shape is retained without 

 any alteration, as is seen in the common Limpet (Patella). 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 Dcntalium), but is usually coiled up into a spiral. The 

 " spiral univalve " (fig. 205) 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 another (as in Vermetus). More 

 commonly 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 shell are 

 coiled round a central axis 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 (fig. 204) is the one which may be looked 



Fig. 204. Cassis caucellata, a Spiral Gasteropod. \a " Spire," placed at the posterior 

 end of the shell ; b " Mouth," placed at the anterior end of the shell ; c Inner or colu- 

 mellar lip ; d Outer lip ; e Notch for the passage of a respiratory siphon. 



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," 



