In the tenth edition of Systema Naturae, Linnaeus (1758) included 87 species of crustaceans that he placed in only six genera distributed through two of the classes that he recognised at the time (Table 1). The Linnaean genus Cancer was by far the largest of the Crustacea-containing genera, comprising 59 species, and, although dominated by brachyuran crabs, this genus was heterogeneous by modern standards as it incorporated stomatopods, amphipods and anostracan Branchiopoda, as well as representatives of five different infraorders of decapods. The Linnaean genus Monoculus was even more heterogeneous, containing a xiphosuran chelicerate and a pteropod mollusc in addition to a variety of crustaceans which included a branchiuran fish louse, four branchiopods (one each from the Notostraca, Conchostraca, Anomopoda and Onychopoda), an ostracod and a copepod (see Damkaer 2002). Monoculus Linnaeus was eventually suppressed (Fox 1951) and is the only one of Linnaeus’s original Crustacea-containing generic names no longer valid today. The third Linnaean genus classified in the class Insecta was Oniscus, which comprised eleven species, all but one of which are isopods. The exception is Oniscus ceti Linnaeus, an amphipod parasitic on cetaceans currently placed in the genus Cyamus. The three other Crustacea-containing genera were all classified by Linnaeus as members of the class Vermes but each was placed in a different order. The genus Lernaea contained just three species of parasitic copepods from fish hosts and was placed in the order Mollusca. Two other fish-parasitic copepods were included in the Linnaean genus Pennatula, belonging to the order Zoophyta. Pennatula is a valid genus of Cnidaria and the two copepod species included by Linnaeus are now classified within the siphonostomatoidan genus Pennella. Finally, the Linnaean genus Lepas comprised just five species of barnacles and was placed in the order Testacea. All of Linnaeus’s Crustacea-containing genera are heterogeneous. Monoculus and Pennatula, for exam-ple, both contain representatives of more than one phylum. Cancer contains representatives of two classes within the subphylum Crustacea but the remaining three genera each comprise representatives of just a single superorder: Oniscus contains only peracaridans, Lernaea, only neocopepodan copepods, and Lepas, only tho- racican cirripedes. The journey from these six genera to the modern classification of the Crustacea (cf. Martin & Davis 2001) is a fascinating one and can be seen as a search for natural order, set against a landscape of rap- idly increasing knowledge, theoretical advances and changing methodology. Numerous Crustacea have entered into symbiotic associations with host organisms and many have adopted a fully parasitic mode of life. The Linnaean genus Lernaea was based on parasitic copepods and was placed in a different class from the only free-living copepod included by Linnaeus, the unrecognisable Monoculus quad- ricornis. Such arrangements, with parasitic forms treated as representing a distinct higher taxon from their free-living relatives, have remained common practice since Linnaeus’s time. Classifications of this type were based primarily on differences and derived support from the morphological gaps that often exist between free- living taxa and their highly specialised parasitic relatives. The Epicaridea, for example, comprises parasites of crustacean hosts and has long been treated as a separate suborder within the Isopoda (see Bowman & Abele 1982; Martin & Davis 2001). New molecular evidence (Dreyer & Wägele 2002) now confirms Wägele’s (1989) proposal that the epicarideans are closely related to cymothoids. Epicarideans are no longer treated as a distinct suborder characterised in part by their adaptations to parasitism, instead they have been placed within a revised Flabellifera by Wilson (2003), and within the suborder Cymothoida by Dreyer & Wägele (2002) and by Brandt & Poore (2003). The latter authors also commented that the monophyly of the epicaride- ans has not yet been unequivocally established. On the basis of both molecular and morphological evidence it is now recognised that the epicarideans represent a specialised terminal branch of parasitic forms that has arisen within a primitively free-living clade.