In gastropod shells, the aperture is the most vulnerable part. Various structures evolved to minimize this local vulnerability. A systematic account of these structures is presented and discussed in an evolutionary context.In a marine environment, early in the evolution of the gastropods, the operculum is supposed to have originated as a door-like accessory to shell aperture, protecting against predators. In amphibious species, it also functioned against desiccation. During the radiation of the pulmonate gastropods, the operculum got lost in most taxa. The pallial cavity, with narrow pneumostome, evolved as a superior adaptation to terrestrial life. On land, a variety of aperture-obstructing structures, like, e.g., the clausilium, also evolved among pulmonates. It is hypothesized that this was triggered later on in geological time, by the origin of small predatory animals that initially were lacking. The operculum could not fulfill a function against predators anymore, because it had become obsolete already in these early pulmonates. The terrestrial prosobranch snails did not achieve an enclosed pallial cavity. Consequently, when they radiated on land, the operculum kept a vital function against desiccation and, later on, against predatory animals as well. This hypothetical scenario might explain the wealth of apertural structures in pulmonate shells, without an operculum, as compared to the relatively simple roundish shell apertures of the always operculate terrestrial prosobranchs.
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