| Term | Definition |
| OPERCULUM (pl. OPERCULA) |
A generally oval, lid-like, CORNEUS or CALCAREOUS structure, situated on the posterior end of the FOOT of many gastropods, that closes off the APERTURE when the animal withdraws into its shell; see also: CONCENTRIC; EPIPHRAGM; MULTISPIRAL; OLIGOGYROUS; PAUCISPIRAL; POLYGYROUS; TRAP DOOR; UNGUICULATE, RIGICLAUDENT, FLEXICLAUDENT. See the website on opercula: The Humble Operculum Photo: Triplofusus giganteus (Kiener, 1840) |
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| OPERCULUM, ANNULATE | An OPERCULUM with a central nucleus, or nearly so, with other layers surrounding it in flattened rings. See also: MULTISPIRAL. | |
| OPERCULUM, CONCENTRIC | See: OPERCULUM, ANNULATE. | |
| OPERCULUM, MULTISPIRAL |
An OPERCULUM consisting of radiating WHORLS of uniform width; = OPERCULUM, ANNULATE; = POLYGYROUS SPIRAL. For illustrations, see The Humble Operculum website page: Operculum Structures. |
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| OPERCULUM, PAUCISPIRAL |
An OPERCULUM having only a few whorls or turns, of rapidly increasing width; = OLIGOGYROUS SPIRAL; MULTISPIRAL. For illustrations, see The Humble Operculum website page: Operculum Structures. |
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