Acoelomorpha is a subphylum of very simple and small soft-bodied animals with planula-like features which live in marine or brackish waters. They usually live between grains of sediment, swimming as plankton, or crawling on other organisms, such as algae and corals. With the exception of two acoel freshwater species, all known Acoelomorphs are marine. Systematics 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 comprise millimetre-sized, mostly interstitial worms, distributed into two families. Phylogeny The soft bodies of acoelomorphs and the lack of some of the key bilaterian traits make them 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. Actually, Acoelomorpha appeared to constitute a separate, deep-branching phylum, kingpin of bilaterian evolution. Yet their evolutionary affinities remain enigmatic as they might be the sister-group either to all other bilateral animals or to all deuterostomes. 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. 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