The phylum Arthropoda (Gr. arthros joint + podos foot) contains segmented animals whose epidermis secretes an exoskeleton of rings. The rings are of varying widths corresponding with the segments and are connected by more or less flexible membranes that act as joints. Many segments bear paired lateral appendages, each with a similar chitinous exoskeleton of jointed rings. The exoskeleton is an organic chemical complex that includes a nitrogenous polysaccharide -chitin-as its chief element.
The phylum Arthropoda includes: the crabs, shrimps, barnacles, lobsters, most members of the zooplankton, and other crustaceans (Class Crustacea [L. crusta, a hard shell]), flies, fleas, cockroaches, springtails, bees, grasshoppers, moths, butterflies, lice, beetles, and other insects (Class Insecta [L. inciso, into distinct parts]); spiders, scorpions, mites, ticks, harvestmen, and their allies (Class Arachnida [Gr. arachne, spider]); centipedes (Class Chilopoda [L. Chilos + poda, jaw