MOLLUSCA: GASTEROPODA. 409 



tegument, which is termed the " velum," and which is at first 

 merely a circlet of cilia round the head. The velurn has often 

 been compared with the wing-like cephalic fins of the Ptero- 

 poda, with which, however, it is only doubtfully homologous. 

 Amongst the Pulmonate Gasteropods, those which are strictly 

 terrestrial, undergo no metamorphosis, the velum being absent 

 altogether, whereas those that live in fresh water possess struc- 

 tures which correspond with the velum of the Branchiate 

 forms. 



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 with- 

 out 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. 216, 217) 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 Ver- 



Fig. 215. A, Young of sEolis, a water-breathing Gasteropod, showing the provisional 

 buccal lobes or "velum." B, Adult Pteropod (Limacina antarctica). (After 

 Woodward). 



metus). 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 



