MOLLUSC A : GASTEROPODA. 333 



entirely retract itself; and it is enabled to swim freely by means 

 of two ciliated lobes arising from the sides of the head ; thus, 

 in many respects, resembling the permanent adult condition of 

 the Pteropoda. In the branchiate Gasteropoda, however, of fresh 

 waters, the young do not possess these ciliated buccal lobes. 



Shell of the Gasteropoda. The shell of the Gasteropods 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 consider- 

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

 shape (as in Dentalium}, but is usually coiled up into a spiral. 

 The "spiral univalve" (figs. 136, 137) may, in fact, be looked 

 upon as the typical form of the shell in the Gasteropoda. In 



Fig. 135. A, Young of Eolis, a water-breathing Gasteropod, showing the provi- 

 sional buccal lobes. B, Adult Pteropod (Limacina Antarctica). (After Wood- 

 ward.) 



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 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 



