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