There are at least seven orders: the stalk-eyed Brachyura, Macrura, Schizopoda, Stomatopoda, and the sessile-eyed Sympoda, Isopoda, Amphipoda.
Between the Brachyura and Macrura some authors uphold an order Anomura, though in a much restricted sense, the labours of Huxley, Boas, Alcock and conjointly Alphonse Milne-Edwards and Bouvier, having resulted in restoring the Dromiidea and Raninidae to the Brachyura, among which de Haan long ago placed them.
The French authors argue that from the macruran lobsters (Nephropsidae) anciently diverged two lines: one leading through the Dromiidea to the genuine Brachyura; or crabs, the other independently to the Anomura proper, which may conveniently be named and classed as Macrura anomala.
- For the present, as of old, the true Brachyura are divided into four tribes: Cyclometopa, with arched front as in the common eatable crab; Catometopa, with front bent down as in the land-crabs and the little oyster-crab; Oxyrhyncha, with sharpened beak-like front as in the various spider-crabs; Oxystomata, including the Raninida, and named not from the character of the front but from that of the buccal frame which is usually narrowed forwards.
The Brachyura anomala, or Dromiidea, "have preserved the external characters and probably also the organization of the Brachyura of the Secondary epoch" (Milne-Edwards and Bouvier, 1901).
This is seen, for example, in the Brachyura among the Decapoda.
It is most typically developed in the most specialized Decapoda, the Brachyura, while the more primitive groups of Malacostraca, the Euphausiidae, Penaeidea and Stomatopoda, retain the primitive order of appearance C FIG.
The earliest forms, from the Lower Oolite and later, belonging chiefly to the extinct family Prosoponidae, have been shown to have close relations with the most generalized of existing Brachyura, the deep-sea Homolodromiidae, and to link the Brachyura to the Homarine (lobster-like) Macrura.