Collier's New Encyclopedia (1921)/Arthropoda
ARTHROPODA, a subdivision of the annulosa, or articulata, containing the classes belonging to that sub-kingdom which are of the highest organization. The body is very distinctly divided into rings or segments, sometimes, as in the myriapoda (centipedes and millepedes), mere repetitions of each other, but more frequently with some of them differentiated for special ends. In general, the head, thorax, and abdomen are distinct. Under the subdivision arthropoda are ranked in an ascending series the classes miriapoda, crustacea, arachnida, and insecta.