GASTEROPODA. 201 



receptacle, but are more or less exposed at the back or 

 sides on the hinder part of the body ; they are herma- 

 phrodite ; and the shell is completely formed in the fry, 

 but often disappears in the adult or is incomplete. 

 According to Lacaze-Duthiers the Gasteropoda are 

 formed on an unsymmetrical plan ; the organs of diges- 

 tion are placed on one side, instead of in the middle as 

 in the Acephala ; and the organs of sense are more deve- 

 loped, and usually lodged in a head. 



Our knowledge of the plan of arrangement, so far as 

 regards the teeth on the lingual membrane of such 

 Gasteropoda as possess this curious apparatus, is too 

 imperfect to make it form part of any scheme of classi- 

 cation. Loven, Troschel, Gray, and Macdonald have to 

 a certain extent pursued the subject, and attach much 

 importance to it. Dr. Gray separated on this ground 

 his Ctenobranchiata into two suborders — Proboscidifera 

 and Rostrifera — treating the one as zoophagous, and 

 the other as phytophagous : but we find in the latter 

 division Conus, Cypraa, Aporrhais, Fusus, Vermetus, 

 Ccecanij Capulus, Calyptr&a, and many other genera 

 which are not vegetable-eaters, Pleurotomatida placed 

 among the Proboscidifera, and Conidce among the 

 Rostrifera (both of these families having precisely the 

 same kind and disposition of teeth) , besides many other 

 like incongruities. At the same time it is evident that 

 this spinous organ of deglutition affords a useful cha- 

 racter to distinguish certain genera and even higher 

 groups ; and I trust that a further examination of the 

 subject will enable us to make it available for that pur- 

 pose. 



The embryology, or history of the development of the 

 Gasteropoda, has been carefully investigated by a host of 

 able physiologists from the time of Stiebel (1815) to this 



K O 



