Arthropoda  (är-thrŏp ´o-dȧ).—Articulated animals with jointed feet, as crabs, insects, etc.

The phylum Arthropoda embraces an immense assemblage of small animals, inhabiting salt and fresh waters, the land, and the air above it. The typical members of this group have a body divided into segments, jointed limbs, some of which are modified into jaws, and a more or less firm external skeleton. The general organization is complex, with the nervous system and senses well developed, in some divisions showing powers of perception and brainwork of a very high order. The chief divisions, or classes, of the Arthropoda are given below in the order of rank, from those simplest in organization to the most complex. Members of the first three classes breathe by gills, and are termed Branchiata, the remainder are air breathers or Tracheata.

Crustacea —Crabs, lobsters, shrimps, barnacles, beach fleas.

Trilobita —Trilobites; eurypterids (fossil only).

Xiphosura —Horseshoe crabs.

Onychophora —Peripatus.

Myriapoda —Centipedes; millipedes.

Arachnoidea —Spiders, mites, ticks, scorpions.

Insecta —Insects.