Body Shape

• Flatworms have flat, ribbon shaped bodies, while roundworms are round (but not segmented) and taper towards the end. Both are bilaterally symmetrical.
  
  

Reproduction

• Roundworms are sexed, with two different genders, while flatworms may either be sexed, hermaphroditic, or use asexual reproduction.
  
  

Coelom (Body Cavity)

• Flatworms are acoelomates; they have no body cavity between the digestive tract and the body wall. Roundworms are pseudocoelomates; while they have a body cavity, it is not composed of mesodermal epithelium and is thus not a true coelom.
  
  

Digestive System

• Flatworms have a one-way digestive tract. Waste is taken in and expelled through the same hole. Roundworms have a two-way tract, with a mouth and anus.
  
  

Cephalization (Headedness)

• Roundworms have nerve bundles on one end, but no optical receptors, making them less cephalized than flatworms. Flatworms have nerve bundles as well as a primitive brain and eyespots.