Acoela
| Acoela Temporal range: | |
|---|---|
| Many flatworm-like, orange individuals of the Waminoa acoel on a Plerogyra coral (whitish bubbles) | |
| Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Xenacoelomorpha |
| Subphylum: | Acoelomorpha |
| Order: | Acoela Uljanin, 1870 |
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. About 400 species are known, but probably many more not yet described.
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.