GASTEROPODA. 3 



shape (as in Dcntalium), but is usually coiled up into a 

 spiral. The "spiral univalve" (fig. 384) may, in fact, be 

 looked upon as the typical form of the shell in the Gastero- 

 poda. In some cases the coils of the shell — termed techni- 

 cally 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 convolu- 

 tion 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 Flanorhis). In most cases, how- 

 ever, 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. 

 383) is the one which may be looked upon as most char- 

 acteristic of the Gasteropods, the shell being composed of a 



--d 



Fig. 383. — Cassis cancellata, a spiral Gasteropod, a, The "spire," placed at the posterior 

 end of the shell ; &, The "mouth," placed at the anterior end of the shell ; c. Inner or colum- 

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



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 junc- 

 tion 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 (columella) round which the whorls are 

 coiled is usually solid, when the shell is said to be " imper- 

 forate ; " but it is sometunes hollow, when the shell is said 



