囊胸亚纲总纲

囊胸亚纲原先为蔓足下纲（藤壶）底下的囊胸目，现在两者被视为是独立的两个亚纲，与带甲亚纲共同组成鞘甲纲。

囊胸亚纲物种的胸部具有六对二分枝的附肢，腹部分为四节，尾节上具有尾叉。比起藤壶，囊胸亚纲物种的这些特征反而与桡足亚纲更为相似。此外，它们还具有分作两瓣的甲壳，且雌性的甲壳会较雄性宽大。

Ascothoracida is a small group of parasitic marine crustaceans, comprising around 100 species and divided into Dendrogastrida and Laurida. They are found throughout the world on cnidarians and echinoderms. Dendrogastrida are parasites on echinoderms, and Laurida are parasites on cnidarians, except from the species Waginella Grygier, which is also a parasite on echinoderms (crinoids). Piercing and sucking mouthparts are used for feeding, and more advanced forms also absorb nutrients through a modified integument of the carapace. More basal forms are ectoparasitic, but most genera are meso- and endoparasitic. The sexes are separate, except from secondary hermaphroditic species of the Petrarcidae. In many species the larger female often have smaller males living inside her mantle cavity.

Ascothoracida was previously ranked as an order within the infraclass Cirripedia (barnacles), but now both Ascothoracida and Cirripedia are considered separate subclasses. Those two subclasses, along with Facetotecta, make up the class Thecostraca.

The thorax of Ascothoracida species 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.