They are covered by the rigid exoskeleton and usually have flexible joints. These jointed appendages provide arthropods with such strength and flexibility that the variation in structure and function of appendages is enormous. It is so great that a survey of the phylum and its systematics is primarily a study of variation in appendages. Only a few species are terrestrial. Crustaceans differ from the other subphyla because they have fundamentally biramous or double-branched appendages.
Crustaceans have two pairs of antennae and usually have compound eyes with multiple lenses.
Crustaceans along with insects are commonly called mandibulates because they have opposing mandibles derived form an anterior pair of appendages. Pictured here is the crayfish. This subphylum of mandibulares includes centipedes, millipedes, and insects. They have uniramous, or single-branched appendages. Class Insecta Insects are by far the largest group of organisms on earth, and probably include 1 to 10 million species.