Their first appearance can be traced back to Precambrian time. There have been three major evolutionary changes in the gastropods. Development of the head Conversion of the shell from a shield to protective retreat Torsion These changes have lead to their success as a class. Gastropods have retained a flat foot. This foot allows them to creep along today as in ancient times.
Most gastropods are very mobile and active.
The head of the gastropod bears a pair of tentacles with an eye at each tentacle base. A retractor muscle withdraws the head and foot. The bottom of the foot bears a horny disc called the operculum. The operculum fills the aperture of the shell and acts as a protective door or lid. The shell is an asymmetrical spiral and functions as a portable retreat when danger approaches.