acoelenterate

Acoelenterata is an obsolete taxonomic term once used to describe a group of animals deemed to lack a coelenteron, the true gastrovascular cavity found in coelenterates. The term was typically set in opposition to Coelenterata, which encompassed the cnidarians (jellyfish, corals, sea anemones) and ctenophores (comb jellies). In early classifications, Acoelenterata referred to organisms thought to lack a true digestive cavity, such as sponges (Porifera) and, in some accounts, Placozoa and other very simple metazoans. The exact composition varied among authors, but the unifying idea was the absence of a coelenteron rather than a shared tissue organization.

Historically, Acoelenterata was not considered a monophyletic or natural group. With advances in comparative anatomy, embryology, and especially molecular phylogenetics, it became clear that this grouping mixed distantly related lineages that did not form a single evolutionary lineage. Modern taxonomy generally treats sponges (Porifera) and placozoans as separate from eumetazoans, which include the true tissues and, in many cases, a gastrovascular cavity. Consequently, the concept of Acoelenterata has fallen out of use and is retained mainly as a historical reference in discussions of the evolution of animal body plans.

See also: Coelenterata; Porifera; Placozoa; Eumetazoa; Gastrovascular cavity; Coelenteron.