Earth and Environmental Sciences Invertebrate Paleontology Fall 1999 November 5. Extinct forms are classified with the hope that certain shell shapes are consistently associated with gill structure. However, homeomorphy makes this an unlikely proposition.
Torsion crosses the nerve tracts and the mantle cavity is brought forward over the head.
Most have an operculum. The shell is cup-shaped, planispiral or helical, most commonly the last. Two nephridia and two auricles. Usually have two bipectinate gills, but some have only one. Radula has a large number of unspecialized teeth.