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| Many dwell in County Carlow. | |||
| {{otheruses}} | |||
| {{Taxobox_begin | color=pink | name=Crabs}} | |||
| {{Taxobox_image | image = ] | caption = '']''}} | |||
| {{Taxobox_begin_placement | color = pink}} | |||
| {{Taxobox_regnum_entry | taxon=]ia }} | |||
| {{Taxobox_phylum_entry | taxon=]a }} | |||
| {{Taxobox_subphylum_entry | taxon=] }} | |||
| {{Taxobox_classis_entry | taxon=] }} | |||
| {{Taxobox_ordo_entry | taxon =] }} | |||
| {{Taxobox_subordo_entry | taxon=] }} | |||
| {{Taxobox_infraordo_entry | taxon='''Brachyura''' }} | |||
| {{Taxobox_authority | author = ] | date = 1802}} | |||
| {{Taxobox_end_placement}} | |||
| {{Taxobox_section_subdivision | color = pink | plural_taxon = ]}} | |||
| *] | |||
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| {{Taxobox_end}} | |||
| The term '''crab''' is often applied to several different groups of short (nose to tail) ] ]s with thick exoskeletons, but only members of the '''Brachyura''' are true crabs; other taxa, such as ]s, ]s, ]s, and ]s are, despite superficial similarities, not crabs at all. ]s, ]s and ]s belong to the ] and can be distinguished from true crabs by counting the legs - in Anomala, the last pair of ]s (walking legs) is hidden inside the ] and so only four pairs are visible (counting the claws), whereas uninjured true crabs always have five visible pairs. | |||
| ==True crabs== | |||
| True crabs are ]s in the ] Brachyura, in the ] ]. They have five pairs of walking legs (the first of which is modified into a pair of claws or ''chelae'') and typically a flattened shell. In all but a few crabs (for example, ]), the ] is folded under the ]. The form of the abdomen usually reveals the sex of the crab - males have a narrow abdomen, while females have a much wider abdomen, under which they carry their ]. Crabs are a very diverse group, mostly found in ], but with some groups living in ] or on land. Although famed for their tendency to walk sideways, crabs are in fact able to walk in any direction. | |||
| Classification within the crabs is traditionally based on the position of the ]s: whether they are found on the legs or on the ]. In the two "primitive" sections (sometimes called collectively the "Podotremata"), the gonopores are found on the legs (as in all other decapods); in the ], the male gonopores are on the legs, and the female gonopores are on the ]; in the ], the gonopores are on the sternum in both males and females. | |||
| ==Sexual dimorphism== | |||
| Most crabs show clear ] and so can be easily sexed. The abdomen, which is held recurved under the thorax, is narrow in males. In females, however, the abdomen is considerably wider, and retains a greater number of ]s. This relates to the carrying of the ] ] by the female crabs (as seen in all ]s). In those species in which no such dimorphism is found, the position of the gonopores must be used instead. In females, these are on the third ], or nearby on the sternum in higher crabs; in males, the gonopores are at the base of the fifth pereiopods or, in higher crabs, on the sternum nearby. | |||
| ==Other animals== | |||
| A number of animals with a similar shape are commonly called crabs, including the ] (an ]) and the ] (in the class ]). The crab louse is the origin of the slang term "crabs" for an infestation of ]. | |||
| ==]== | |||
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| ==External links== | |||
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| * ''Brachyura'' entry at | |||
| * ''Brachyura'' entry at | |||
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| *, Prof. Chuck Holliday, Dept. of Biology, Lafayette College. Contains links to articles on osmoregulation in crabs and other crustaceans. | |||
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Revision as of 03:23, 10 January 2006
Many dwell in County Carlow.