Ascothoracida
Ascothoracida is a small group of crustaceans, comprising around 100 species. They are found throughout the world, and are parasites on cnidarians and echinoderms.
Ascothoracida was previously ranked as an order within the infraclass Cirripedia, but now considered a separate infraclass. Those two infraclasses, along with Facetotecta, together make up the subclass Thecostraca.
The thorax has six pair of biramous appendages, while the abdomen has four segments and a terminal telson with a caudal furca. This arrangement is similar to that seen in copepods. In addition, there is a bivalved carapace, which is expanded in females.
