Acoela

Acoela, or the acoels, is an order of small and simple invertebrates in the subphylum Acoelomorpha of phylum Xenacoelomorpha, a deep-branching bilaterian group of animals, which resemble flatworms. Historically, they were treated as an order of turbellarianflatworms. About 400 species are known, but probably many more are not yet described.

The etymology of "acoel" is from the Ancient Greek words (a), the alpha privative, expressing negation or absence, and (koilía), meaning "cavity". This refers to the fact that acoels have a structure lacking a fluid-filled body cavity.