Acoela is an order of animals treated either as a group of flatworm

Flatworm

The flatworms, known in scientific literature as Platyhelminthes or Plathelminthes are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrate animals...

s or as one of the two classes of the phylum Acoelomorpha

Acoelomorpha

The Acoelomorpha are a disputed phylum of animals with planula-like features that were considered to belong to the phylum Platyhelminthes. In 2004 molecular studies demonstrated that they are a separate phylum, although their position in the tree of life is contentious; most researchers believe...

, containing the majority of that phylum's species. It contains about 20 families.

As the most primitive bilateral animals

Bilateria

The bilateria are all animals having a bilateral symmetry, i.e. they have a front and a back end, as well as an upside and downside. Radially symmetrical animals like jellyfish have a topside and downside, but no front and back...

, Acoela provide interesting insights into early animal evolution and development. The best studied animal in this group is the European species Symsagittifera roscoffensis

Symsagittifera roscoffensis

Symsagittifera roscoffensis, formerly called Convoluta roscoffensis, is a free-living acoelomorph worm.-Appearance:Symsagittifera roscoffensis is a small flat worm. Due to the algae, Tetraselmis convolutae, which it assimilates into its body cavity, it has green colour...

.