Fig. 179.—The slit in A, Bellerophon, B, Pleurotomaria, C, Schismope, D, Polytremaria, E, Haliotis (not drawn to scale).
Fig. 180.—Development of the tube in the tube operculates: A, Pterocycus rupestris Bens.; B, Opisthoporus birostris Pfr.; C, Spiraculum travancoricum Bedd.; D, Rhiostoma Housei Pfr.
It is singular that the tube does not appear to be of any use to the animal, since its internal extremity, in the complete form, is closed, and does not communicate with the interior of the whorl. It may be presumed, however, that in origin the tube served as a means of conveying air to the animal when the operculum was closed. The holes in the peristome of Pupina, Cataulus, and Anostoma (Fig. [154]) may be compared.
Fig. 181.—Eburna spirata Lam., E. Indies. F, foot; OP, operculum; P, penis; S, siphon; T, tentacles, with eyes at their base. (After Souleyet.)
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.