The Ulvophyceae or Ulvophytes are class of green algae, distinguished mainly on the basis of ultrastructural morphology. The sea lettuce, Ulva, belongs here. Other well-known members include Ulothrix and Acetabularia. The Ulvophytes are diverse in their morphology and their habitat. Most are seaweeds such as those listed above. Others, such as Cladophora, Rhizoclonium and Pithophora live in fresh water and in some areas are considered weeds.