Xenacoelomorpha

Xenacoelomorpha is a clade of small, bilaterally symmetrical marine animals comprising acoels, nemertodermatids, and xenoturbellids. Its members lack several organ systems found in many other bilaterians, including a through-gut.

Acoela: A group of mostly microscopic marine bilaterians lacking a through-gut and a coelom. Acoels are one of the three principal lineages included in Xenacoelomorpha.
Xenambulacraria: A proposed clade grouping Xenacoelomorpha with Ambulacraria. Some evolutionary analyses place xenacoelomorphs close to echinoderms and hemichordates in this grouping.
Digestive system: The organs and processes by which an animal takes in food, digests it, and absorbs nutrients. Most acoels have a digestive syncytium rather than a gut with an epithelial lining.
Xenoturbella bocki: A marine worm species in the genus Xenoturbella, first described from Scandinavian waters. This species is the type species of Xenoturbella, the clade's namesake genus.
Nemertodermatida: A small group of marine worms with a simple body plan and a digestive tract that varies among species. Nemertodermatids form a second principal lineage within the clade.
Nephrozoa: A proposed clade comprising most bilaterians other than Xenacoelomorpha. This hypothesis places xenacoelomorphs outside the major bilaterian group with complex excretory and digestive systems.
Gastrovascular cavity: A digestive cavity with a single opening that also distributes nutrients through the body. Some xenacoelomorphs have a simple gut cavity, unlike the gutless acoels.
Xenoturbella: A genus of marine worms with a simple body plan, belonging to Xenoturbellida. The genus supplies the first part of the name Xenacoelomorpha.
Xenoturbellida: A group of marine worms with a simple body plan, represented by the genus Xenoturbella. Xenoturbellids are the third principal lineage named in the clade.
Phylogenetics: The study of evolutionary relationships among organisms using inherited traits and molecular data. Different phylogenetic methods have produced competing placements for Xenacoelomorpha.

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