Most snails, of course, have a coiled anatomy to fit the shape of the shell. Also since there’s only one opening from the shell (with exceptions like keyhole limpets), their intestinal tract has to loop back on itself so the mouth, anus, respiratory organs, and reproductive orifices are all situated pretty close together around that one opening. They’re not immediately adjacent to each other, as you can see, but they’re a lot closer than in most animals.
This looping back of the GI tract is called torsion. It occurs in the embryo or, in marine gastropods with free-swimming larvae, in the larval stage (veliger). It can take as little as a few minutes and results from a muscle on one side of the body being stronger than the corresponding muscle on the other side. In two quick 90-degree bends, the embryo or larva basically folds in half before secreting the shell. It’s followed by coiling, assuming the spiral form within the shell. This allows the shell to enlarge with age without becoming so long it makes locomotion difficult.
Snails and slugs both evolved from ancestors with coiled shells, so slugs show many of the same features. Both of them have a mantle, for example. Like a person’s mantle—a cloak hung over the shoulders to keep warm—a snail’s mantle enfolds its body and secretes the shell. A slug’s mantle, however, looks more like a saddle on its back, since it has no shell to secrete. After undergoing torsion (the more primitive condition), the slug embryo undergoes a degree of detorsion to straighten out again, moving the anus farther posteriorly.
Detorsion is most complete in the nudibranchs or sea slugs, such as Aplysia. Here, the anus is all the way back at the posterior end.
Picture Credit : Google
