Etymology

The term "acoelomorph" derives from the Ancient Greek words ἀ (a), the alpha privative, expressing negation or absence, κοιλία (koilía), meaning "cavity", and μορφή (morphḗ), meaning "form".^[3][4] This refers to the fact that acoelomorphs have a structure lacking a fluid-filled body cavity.

Classification

Various members of the Acoela class.^[5]

Various members of the Nemertodermatida class.

The subphylum Acoelomorpha is divided into two classes. There are at least 408 described species, with a majority of these falling within the Crucimusculata infraorder in Acoela.

• Acoela comprise small flattened worms, classified into a dozen families.^[6]
• Nemertodermatida comprise millimetre-sized, mostly interstitial worms, distributed into two families.^[7]

Phylogeny

The soft bodies of acoelomorphs and the lack of some of the key bilaterian traits make them difficult to classify.^[8][9] Traditionally, based on phenotypic features, acoelomorphs were considered to belong to the phylum Platyhelminthes, which was long seen as the sister group to all other bilaterian phyla.^[10] However, a series of molecular phylogenetics studies at the hinge between the 20th and 21st centuries demonstrated that they are fast-evolving organisms not closely related to platyhelminthes,^{[11][12][13][14]} therefore involving the polyphyly of flatworms.^{[15][16][17][18]}

Actually, Acoelomorpha appeared to constitute a separate, deep-branching phylum, kingpin of bilaterian evolution.^[19] Yet their evolutionary affinities remain enigmatic as they might be the sister-group either to all other bilateral animals^[15][16][17] or to all deuterostomes.^[18] Resolving this debate would indicate whether acoelomorphs are simple or simplified. If they are the sister group to Bilateria, it would point to a simple body plan for the first bilaterian. Alternatively, if acoelomorphs are related to deuterostomes, this would imply that their organisation is the result of secondary simplification.^[20]

Xenoturbella, the sister group to acoelomorphs

In addition, comparative analyses of morphological, developmental, and molecular characters raised two points.

• Xenoturbellida is the sister group to acoelomorphs, constituting the so-called Xenacoelomorpha clade.^[21] The close evolutionary relationship between Acoelomorpha and Xenoturbella is supported by the morphology (structure of epidermal cilia^[22]), the embryology (direct development without a feeding larval stage^[23]), and the concatenation of hundreds of proteins.^[24][21][25]
• The phylogenetic placement of Xenacoelomorpha among bilaterian animals is not yet well defined, despite increased taxon and gene sampling, (re)-analyses of published data sets, and use of more sophisticated models of sequence evolution in phylogenomic studies. There is a conflict between two evolutionary hypotheses, with Xenacoelomorpha being the sister group to Ambulacraria within deuterostomes (i.e., the Xenambulacraria hypothesis) on the one hand,^[26] and Xenacoelomorpha as sister group to all other bilaterians (i.e., the Nephrozoa hypothesis) on the other.^[25][27][28] However, the Nephrozoa hypothesis might reflect methodological errors resulting from model violations in the phylogenomic inference.^[26]