Some snails have also been the source of purple dyes at least since 1600BC in Crete. When exposed to sunlight, it becomes red, then purple. Use of it and purple-dyed cloth was controlled in the Roman Empire. So purple cloth was used only for temple hangings and robes and clothing of nobles. The earliest gastropod shells looked like a hose coiled on the ground.
Most gastropods have an asymmetrical, spiral shell with a single aperture.
Some have suggested that this led to reduced number of gills, retractor muscle and nephridia to one pair. Others have suggested that as the shell later evolved into a taller spiral, the visceral mass had to be carried obliquely for balance and this causes the viscera to bulge on one side and reduce the space on one side of the shell for gill, nephridia and right auricle. Therefore the mantle cavity, gills, anus, nephridiopores now are behind the head. The mantle cavity moved from the rear to the front so the head now can be retracted more readily. Slowly Gastropods typically move forward by raising a portion of the sole of the foot, extruding it forward, and returning that portion to the surface.