=The Operculum.=--The operculum is a cuticular development of a group of cells situated on the dorsal side of the foot, exactly over the terminal point of the fibres of the columellar muscle. It is so situated that in crawling it is generally carried free of the shell, sometimes at the extreme upper end of the foot, more usually somewhat nearer to the shell (Fig. 181). In _Pterocyclus_ it is pushed back into the umbilicus when the animal is in motion. The operculum is present in nearly all land, fresh-water, and marine Prosobranchiata, absent in all Opisthobranchiata in the adult state, except _Actaeon_, and in all Pulmonata, except _Amphibola_. It has been lost in the following marine Prosobranchiata: many Cancellariidae and Conidae, _Oliva_ (though present in _Olivella_ and _Ancilla_), Harpidae, Marginellidae, _Voluta_ proper (though present in _V. musica_), nearly all Mitridae, Cypraeidae, Doliidae, Ianthinidae; and, of land genera, in Proserpinidae. It is evident, therefore, that its presence or absence is of limited value in classification. In some species of _Ampullaria_ and _Natica_ it is horny, in others shelly. Dall found that in a number of specimens of _Volutharpa ampullacea_, 15 p.c. had opercula, 10 p.c. traces of the operculigenous area, but no operculum, the rest no trace of either. Monstrosities of _Buccinum undatum_ sometimes occur, which have two, or in rare case three opercula. As a rule, the operculum exactly fits the mouth of the shell. But in cases where the mouth is very large (_e.g._ _Conus_, _Strombus_, _Concholepas_, some _Bullia_), it only covers a very small portion and is quite inadequate as a protection (Fig. 62, p. 155). Again, when the shell has assumed a more or less limpet-shaped form, and habitually adheres to flat surfaces without much occasion for locomotion, the operculum becomes degraded and is probably on the way to being lost altogether. This is the case with _Navicella_ (a modified _Nerita_, see Fig. 13, p. 17), _Concholepas_ (a modified _Purpura_), _Sigaretus_ (a modified _Natica_). Probably the more completely patelliform shells of _Crepidula_, _Haliotis_, _Fissurella_, and _Patella_ have reached the stage at which the operculum has been lost entirely. In _Navicella_, besides becoming degraded, the operculum has actually become partly internal, and apparently serves the purpose of separating the viscera from the upper part of the foot, something like the shelly plate in _Crepidula_. This explains why the operculum in this genus is polished on both sides.[349] Some authors have imagined that the operculum is homologous (_a_) to the second valve in Pelecypoda, (_b_) to the byssus. It differs, however, morphologically from the former in the essential point of not being produced by the mantle, and from the latter in not being produced by a special gland. [Illustration: Turbo Turbo Livona Ampullaria Natica Sarmaticus) (Callopoma)