During development, gastropods undergo 180 degree twisting of their body that’s inside their shell, this is called torsion (Gillis, 2012). It is thought that torsion first came about as an adaptation for the planktonic larval stage, quite the beneficial mutation (Purchon, 1968). The coiling of the shell as it grows keeps it from growing too long and hampering the movement of the snail when (Gillis, 2012). As Cleveland P. Hickman, Jr. explains in his sixth edition of Animal Diversity, a result of torsion is that the anus is positioned right above the mouth of the snail. There are obvious draw backs to this kind of body plan, such as fouling which is when feces are deposited onto the head/gills of the (aquatic) snail. However, snails have methods to overcome such hindrances! The clever little snails extend their heads while eating and do not defecate when they are dining on their gourmet snail food (see Nutrition for more information on snail cuisine). They often move while defecating so that the water carries the excrement away from the snail. One may wonder why they bother with such a body plan, the answer is simple: protection. Torsion allows the snail to have but one exposed site in the shell, which reduces vulnerability. The only point of exposure is where the head is and so the snail can monitor the point of weakness, thus reducing the chances of sneak attacks from behind (Hickman, 2011).

Snails have other adaptations, specifically terrestrial snails, which allow them to live on land. As Hickman covers in Animal Diversity, to meet their respiration requirements, they have lost their gills and developed a lung from the mantle wall. An opening in the shell called a pneumostome is where wastes are expelled via the air from the lungs in a forceful burst, which also prevents fouling (Hickman, 2011).  




Move on to Reproduction for more interesting information!