The name Anomura derives from an old classification in which
reptant decapods were divided into Macrura (long-tailed), Brachyura (short-tailed) and Anomura (differently-tailed). The alternative name Anomala reflects the unusual variety of forms in this group; whereas all crabs share some obvious similarities, the various groups of anomurans are quite dissimilar.[3]
The group has been moulded by several instances of
carcinisation – the development of a crab-like body form.[4] Thus, the
king crabs (Lithodidae),
porcelain crabs (Porcellanidae) and
hairy stone crab (Lomisidae) are all separate instances of carcinisation.[4]
As decapods (meaning ten-legged), anomurans have ten
pereiopods, but the last pair of these is reduced in size, and often hidden inside the gill chamber (under the
carapace) to be used for cleaning the gills.[5][3] Since this arrangement is very rare in
true crabs (for example, the small family
Hexapodidae),[6] a "crab" with only eight visible pereiopods is generally an anomuran.[3]
Some of the internal relationships within Anomura can be shown in the cladogram below, which shows Hippidae as
sister to Paguroidea, and resolves Parapaguridae outside of Paguroidea:[7]
^Jonas Keiler; Stefan Richter (2011). "Morphological diversity of setae on the grooming legs in Anomala (Decapoda: Reptantia) revealed by scanning electron microscopy". Zoologischer Anzeiger. 250 (4): 343–366.
doi:
10.1016/j.jcz.2011.04.004.