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.

[4] The etymology of "acoel" is from the Ancient Greek words ἀ (a), the alpha privative, expressing negation or absence, and κοιλία (koilía), meaning "cavity".

In addition to Convolutidae, there appears to be a potential new and yet unnamed family of acoels that also live in relationships with microalgal endosymbionts.

[11] Members of the class Acoela lack a conventional gut, so that the mouth opens directly into the mesenchyme, i.e., the layer of tissue that fills the body.

However, Acoela was separated from this phylum after molecular analyses showed that it had diverged before the three main bilaterian clades had formed, making flatworms as traditionally understood an evolutionary grade from which higher animals had evolved.