A person who is familiar with the snail species in an area can tell most, if not all, of them apart by their shells. This is because a snail’s shell, having a vital function in the survival of its bearer, has been shaped by evolution to fit the specific needs of the snail that carries it. As a result, the shells of different species of snails tend to differ from each other.
Unlike the shells of snails, however, the internal shells of slugs have long lost their functions. They now reside, away from the direct effects of the environment, buried under the slugs’ skin. The shell sits atop some of the vital organs, such as the heart and the kidney. Hence, one could argue that it still has a protective function. But this argument is weakened much when one notices that many slugs don’t have shells at all, yet they get by just as well. Moreover, sometimes a slug’s shell can be so thin and flimsy that one wonders why the slug even bothered to make one.
If the snail shells are species specific, mostly because they are directly subject to natural selection, the reverse argument that follows is that the slug shells that are not selected anymore should not be species specific. This is indeed the case with the internal shells of the slugs in the widespread families Limacidae and Milacidae 1.
These are some slug shells that I found in soil samples collected during a field trip in Istanbul, Turkey in the summer of 2000. All they tell me is that some limacid slugs were present in the area. Otherwise, they can’t reveal their true identities, for they have been silenced by evolution.
1. Reuse, C., 1983. On the taxonomic significance of the internal shell in the identification of European slugs of the families Limacidae and Milacidae (Gastropoda, Pulmonata). Biologisch Jaarboek Dodonaea 51, 180–200.