An order of the crustacean class Maxillopoda. The cirripedes, particularly the organisms known as barnacles, are the most well-known and easily recognized members of the subclass Thecostraca. The thecostracans are characterized by two features. First, all exhibit a distinctive and unique larva, called the cypris (Fig. 1b). Second, when they have a carapace, it generally envelops the entire body, which typically is composed of a five-segment head, a seven-segment thorax, and a short or rudimentary limbless abdomen. In addition to the cirripedes, the thecostracans include the enigmatic Facetotecta, or “y-larvae,” as well as Ascothoracica, a group of parasites of echinoderms and anthozoan cnidarians.  See also: Ascothoracica; Crustacea; Maxillopoda