Class Gastropoda
The class Gastropoda contains about 70,000 living species and 15,000 fossil species. Gastropods are bilateral and most are herbivores. Gastropods go through a process during development called torsion. Torsion is a twisting where the mantle cavity (including the anus) will twist from originally being posterior (below) to the head and mouth to being anterior (above) to the head and mouth. This leads to a concern of waste getting in the head and mouth area. This concern is called fouling. There are several proposed explanations on different ways fouling is avoided (Hickman Jr. et al. 2009).
Check out the Multiple Organisms and Wisconsin land snails pages