What is the bottom side of the worm called?

The back side is called the Anus, The front side is called the prostomium

Find the anterior (front) end of the earthworm by locating the fleshy bump over its mouth, called the prostomium. The posterior (back) end has a small hole where solid waste is expelled, called the anus ... Notice that the earthworm has a rounded dorsal (back) surface and a flatter ventral (belly) surface