Deroceras reticulatum is a European slug that has been introduced into the U.S. It is now common in gardens, abandoned farm fields and parks in the eastern states. This individual was 50 mm long. The hump with the breathing pore behind the head is the mantle.
Hidden inside the slug Deroceras reticulatum is a clue that betrays the slug's roots; a clue that exposes the identities of its ancestors. The clue is a shell buried under the slug's mantle. But unlike the familiar spiraling shells of snails, this one is only a thin, fragile membrane of calcium carbonate that is barely noticeable. It hardly deserves to be called a shell.
But what an evidence it is. It reveals wonderfully that slugs evolved from shell-bearing snails. The slug's shell is a vestige of a distant past. It is a perfect example for Darwin's1 exquisite logic: "Rudimentary organs may be compared with the letters in a word, still retained in the spelling, but become useless in the pronunciation, but which serve as a clue in seeking for its derivation. On the view of descent with modification, we may conclude that the existence of organs in a rudimentary, imperfect, and useless condition, or quite aborted, far from presenting a strange difficulty, as they assuredly do on the ordinary doctrine of creation, might even have been anticipated, and can be accounted for by the laws of inheritance."
The shell of Deroceras reticulatum revealed under its mantle. The concentric growth lines are vaguely visible on the shell.
1. Darwin, C. 1859. On the Origin of Species by Means of Natural Selection. Full text