Arthropoda. In this great division of the animal kingdom, the body consists of a usually definite number of segments, each bearing a pair of hollow and almost always jointed limbs, into which the body muscles proceed.
In all cases, the epidermis gives rise to an external horny layer of Chitin (q.v.), which usually attains considerable strength and thickness, and in crustaceans is further strengthened by impregnation with salts of lime. The segments of the body and their corresponding appendages exhibit a considerable degree of differentiation, especially in the anterior region of the body, where also some or many segments may completely coalesce, their appendages also becoming extraordinarily modified for various functions; so that it requires the combined research of both the embryologist and the comparative anatomist to analyse the organism into its constituent parts. The nervous system consists of a brain above the mouth, and of a ventral chain of ganglia united by longitudinal and transverse commissures; a nerve-ring round the oesophagus connects the ventral chain with the brain above. One pair of ganglia is developed for each segment, although some of these also coalesce more or less completely in the adult.
The Arthropoda divide naturally into two great alliances—the water-breathers or Branchiata (see GILLS), and the air-breathers or Tracheata (see RESPIRATION); the former including the Crustacea, and the latter the Prototracheata or Peripatidea, the Myriapoda, the Arachnida, and the Insecta. The relation of the Arthropoda to other groups will best be understood from ZOOLOGY. Separate articles will be found on the CRUSTACEA, MYRIAPODA, ARACHNIDA, and INSECTS. A distinct class has been recently established for the genus Peripatus (q.v.), which persists as a survivor of the ancestral insects. In several of its characters, it links together the worm and arthropod types, and gives a new basis to Cuvier's union of the two under the title Articulata.