Etymology The term "acoelomorph" derives from the
Ancient Greek words (), the
alpha privative, expressing negation or absence, (), meaning "cavity", and (), meaning "form". This refers to the fact that acoelomorphs have a structure lacking a fluid-filled body cavity.
Classification 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. •
Nemertodermatida comprises millimetre-sized, mostly interstitial worms, distributed into two families.
Phylogeny The soft bodies and the lack of some of the key bilaterian traits make acoelomorphs difficult to classify. 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. 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, therefore involving the
polyphyly of flatworms. Acoelomorpha appeared to constitute a separate,
deep-branching phylum, the kingpin of bilaterian evolution. Yet their evolutionary affinities remain enigmatic, as they might be the sister-group either to all other bilateral animals '', 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. The close evolutionary relationship between Acoelomorpha and
Xenoturbella is supported by the morphology (structure of epidermal
cilia), the embryology (direct development without a feeding larval stage), and the concatenation of hundreds of proteins. • 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, and Xenacoelomorpha as sister group to all other bilaterians (i.e., the
Nephrozoa hypothesis) on the other. However, the Nephrozoa hypothesis might reflect methodological errors resulting from model violations in the phylogenomic inference. == Anatomy ==