Thecostraca
Thecostraca is a class of marine invertebrates containing over 2,200 described species. Many species have planktonic larvae which become sessile or parasitic as adults. The most important subgroup are the barnacles (subclass Cirripedia), constituting a little over 2,100 known species. The subgroup Facetotecta contains a single genus, "Hansenocaris", known only from the tiny planktonic nauplii called "y-larvae". These larvae have no known adult form, though it is suspected that they are parasites, and their affinity is uncertain. Some researchers believe that they may be larval tantulocaridans. No larval tantulocaridans are known, so this would solve two puzzles at  once. The group Ascothoracida contains about 110 species, all parasites of coelenterates and echinoderms. Classification. This article follows Chan et al. (2021) and the World Register of Marine Species in placing Thecostraca as a class of Crustacea and in the following classification of thecostracans down to the level of orders. Previously, Thecostraca was considered a subclass of Maxillopoda. Significant changes in the organization of Cirripedia's orders, families, and genera were introduced in 2021 by Chan et al. and accepted by the World Register of Marine Species. Class Thecostraca