section_path: Classification and phylogeny
section_name: Classification and phylogeny
paragraph_text: Classification and phylogeny
  
  The name "crustacean" dates from the earliest works to describe the animals, including those of Pierre Belon and Guillaume Rondelet, but the name was not used by some later authors, including Carl Linnaeus, who included crustaceans among the " Aptera " in his Systema Naturae. The earliest nomenclatural valid work to use the name "Crustacea" was Morten Thrane Brünnich 's Zoologiæ Fundamenta in 1772, although he also included chelicerates in the group.
  
  The subphylum Crustacea comprises almost 67,000 described species, which is thought to be just 1 ⁄ 10 to 1 ⁄ 100 of the total number as most species remain as yet undiscovered. Although most crustaceans are small, their morphology varies greatly and includes both the largest arthropod in the world – the Japanese spider crab with a leg span of 3.7 metres (12 ft) – and the smallest, the 100- micrometre -long (0.004 in) Stygotantulus stocki. Despite their diversity of form, crustaceans are united by the special larval form known as the nauplius.
  
  The exact relationships of the Crustacea to other taxa are not completely settled as of April 2012. Studies based on morphology led to the Pancrustacea hypothesis, in which Crustacea and Hexapoda (insects and allies) are sister groups. More recent studies using DNA sequences suggest that Crustacea is paraphyletic, with the hexapods nested within a larger Pancrustacea clade.
  
  The traditional classification of Crustacea based on morphology recognised four to six classes. Bowman and Abele (1982) recognised 652 extant families and 38 orders, organised into six classes: Branchiopoda, Remipedia, Cephalocarida,Maxillopoda, Ostracoda, and Malacostraca. Martin and Davis (2001) updated this classification, retaining the six classes but including 849 extant families in 42 orders. Despite outlining the evidence that Maxillopoda was non-monophyletic, they retained it as one of the six classes, although did suggest that Maxillipoda could be replaced by elevating its subclasses to classes. Since then phylogenetic studies have confirmed the polyphyly of Maxillipoda and the paraphyletic nature of Crustacea with respect to Hexapoda. Recent classifications recognise ten to twelve classes in Crustacea or Pancrustacea, with several former maxillopod subclasses now recognised as classes (e.g. Thecostraca, Tantulocarida, Mystacocarida, Copepoda, Branchiura and Pentastomida).
  
  The following cladogram shows the updated relationships between the different extant groups of the paraphyletic Crustacea in relation to the class Hexapoda.
  
  According to this diagram, the Hexapoda are deep in the Crustacea tree, and any of the Hexapoda is distinctly closer to e.g. a Multicrustacean than an Oligostracan is.