What do all types of worms have in common?

Worms that inhabit planet Earth are of three different types: Flat, Round, and Ringed. Despite the difference in the level of their organization, they have many common features of the external and internal structure.
Their body is elongated. The musculoskeletal system is a skin-muscular sac. They have neither external nor internal skeleton. Oxygen enters their body through the skin, which must be moist or the worm must live in water. The excretory system is in the form of tubes. They have poorly developed sense organs.