Description thumb|left|Gecarcinus quadratus, a land crab from Central America Crabs are generally covered with a thick exoskeleton, composed primarily of highly mineralized chitin. Behind their pair of chelae (claws) are six walking legs and then two swimming legs. The crab breathes through gills on its underside; gills must be at least moist to work.
deScriPtion thUMb|lEFt|gecARCiNuS QuadRatus, a land crAb from CentRaL AMEricA craBs ARe generallY coveRed wiTh A tHicK eXoSkElEToN, Composed pRiMARilY oF higHLy mIneraLized chiTin. bEhInd thEIr paiR of cHelaE (cLAWS) arE sIX walkinG lEgs anD Then TWo swiMminG lEgs. the crAb bREAthes ThrouGh gILls on ItS unDeRsIde; gills mUSt be AT leAst MoISt To work.