Thecostraca are a subclass

Class (biology)

In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...

 of marine invertebrate

Invertebrate

An invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...

s containing about 1,320 described species. Many species have plankton

Plankton

Plankton are any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. That is, plankton are defined by their ecological niche rather than phylogenetic or taxonomic classification...

ic larvae which become sessile

Sessility (zoology)

In zoology, sessility is a characteristic of animals which are not able to move about. They are usually permanently attached to a solid substrate of some kind, such as a part of a plant or dead tree trunk, a rock, or the hull of a ship in the case of barnacles. Corals lay down their own...

 or parasitic as adults.

The most important subgroup are the barnacle

Barnacle

A barnacle is a type of arthropod belonging to infraclass Cirripedia in the subphylum Crustacea, and is hence related to crabs and lobsters. Barnacles are exclusively marine, and tend to live in shallow and tidal waters, typically in erosive settings. They are sessile suspension feeders, and have...

s (infraclass Cirripedia), constituting about 1,220 known species.

The subgroup Facetotecta

Facetotecta

Facetotecta is a poorly known infraclass of thecostracan crustaceans. The adult forms have never been recognised, and the group is known only from its larvae, the "y-nauplius" and "y-cyprid" larvae...

 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 tantulocarida

Tantulocarida

Tantulocarida is a highly specialised group of parasitic crustaceans that consists of about 33 species, treated as a subclass of the class Maxillopoda . They are typically ectoparasites that infest copepods, isopods, tanaids, amphipods and ostracods...

ns (no larval tantulocaridans are known, so this would solve two puzzles at once).

The group Ascothoracida

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....

 contains about 100 species, all parasites of coelenterates and echinoderm

Echinoderm

Echinoderms are a phylum of marine animals. Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone....

s.

Classification

This article follows Martin and Davis in placing Thecostraca as a subclass of Maxillopoda

Maxillopoda

Maxillopoda is a diverse class of crustaceans including barnacles, copepods and a number of related animals. It does not appear to be a monophyletic group, and no single character unites all the members.-Description:...

 and in the following classification of thecostracans down to the level of orders:

Subclass Thecostraca Gruvel, 1905

• Infraclass Facetotecta
  Facetotecta
  Facetotecta is a poorly known infraclass of thecostracan crustaceans. The adult forms have never been recognised, and the group is known only from its larvae, the "y-nauplius" and "y-cyprid" larvae...
  
  Grygier, 1985
• Infraclass Ascothoracida
  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....
  
  Lacaze-Duthiers, 1880
  • • Order Laurida Grygier, 1987
    • Order Dendrogastrida Grygier, 1987
• Infraclass Cirripedia Burmeister, 1834
  • Superorder Acrothoracica
    Acrothoracica
    The Acrothoracica are a superorder of barnacles.Acrothoracicans bore into calcareous material such as mollusc shells, coral, crinoids or hardgrounds, producing a slit-like hole in the surface known by the trace fossil name Rogerella. They are typically smaller than other types of barnacle, being...
    
     Gruvel, 1905
    • Order Pygophora Berndt, 1907
    • Order Apygophora Berndt, 1907
  • Superorder Rhizocephala
    Rhizocephala
    Rhizocephala are derived barnacles that parasitise decapod crustaceans. Their bauplan is uniquely reduced in an extreme adaptation to their parasitic lifestyle, and makes their relationship to other barnacles unrecognisable in the adult form...
    
     Müller, 1862
    • Order Kentrogonida Delage, 1884
    • Order Akentrogonida Häfele, 1911
  • Superorder Thoracica
    Thoracica
    Thoracica is a superorder of crustaceans which contains the most familiar species of barnacles found on rocky coasts, such as Semibalanus balanoides and Chthamalus stellatus. They have six well-developed limbs, and may be either stalked or sessile. The carapace is heavily calcified...
    
     Darwin
    Charles Darwin
    Charles Robert Darwin FRS was an English naturalist. He established that all species of life have descended over time from common ancestry, and proposed the scientific theory that this branching pattern of evolution resulted from a process that he called natural selection.He published his theory...
    
    , 1854
    • Order Pedunculata Lamarck
      Jean-Baptiste Lamarck
      Jean-Baptiste Pierre Antoine de Monet, Chevalier de la Marck , often known simply as Lamarck, was a French naturalist...
      
      , 1818
    • Order Sessilia
      Sessilia
      Sessilia is an order of barnacles, comprising the barnacles without stalks, or acorn barnacles. They form a monophyletic group and are probably derived from stalked barnacles. The order is divided into three suborders. Brachylepadomorpha contains a single family, Neobrachylepadidae, while...
      
      Lamarck, 1818