IX 



THE OPERCULUM 



267 



"OP 



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, some- 

 times at the extreme upper end of the foot, 

 more usually somewhat nearer to the shell 

 (Fig. 181). IwPterocyclus 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 Prosobran- 

 chiata, absent in all Opisthobranchiata in 

 the adult state, except Actaeon^ and in all 

 Pulmonata, except Amj^hihola. It has been 

 lost in the following marine Prosobran- 

 chiata : many Cancellariidae and Conidae, 

 Oliva (though present in Olivella and 

 Ancilla)^ Harpidae, Marginellidae, Voliita proper (though pres- 

 ent in V. musicci)^ nearly all Mitridae, Cypraeidae, Doliidae, 

 lanthinidae; and, of land genera, in Proserpinidae. It is evi- 

 dent, therefore, that its presence or absence is of limited value 

 in classification. In some species of Ampullar ia and Natiea 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 some- 

 times 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, 

 Concliolepas^ 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 Navi- 

 cella (a modified Neynta^ see Fig. 13, p. 17), ConcJiolepas (a modi- 

 fied Purpura)^ Sigaretus (a modified Natiea). Probably the more 

 completely patelliform shells of Crepidula^ Haliotls, Fissurellay 



