The remaining members of the sub-kingdom Annulosa are distinguished by the possession of jointed appendages, articulated to the body; and they form the second primary division - often called by the name Articulata. As this name, however, has been employed in a wider sense than is understood by it here, it is perhaps best to adopt the more modern term Arthropoda.
The body (fig. 112) is composed of a series of segments, arranged along a longitudinal axis; each segment or "somite," occasionally, and some almost always, being provided with articulated appendages. Both the segmented body and the articulated limbs are more or less completely protected by a chitinous exoskeleton, formed by a hardening of the cuticle. The appendages are hollow, and the muscles are prolonged into their interior. The nervous system in all, at any rate in the embryonic condition, consists of a double chain of ganglia, placed along the ventral surface of the body, united by longitudinal commissures, and traversed anteriorly by the oesophagus. The haemal system, when differentiated, is placed dorsally, and consists of a contractile cavity, or heart, provided with valvular apertures, and communicating with a perivisceral cavity, containing corpusculated blood. Respiration is effected by the general surface of the body, by gills, by pulmonary sacs, or by tubular involutions of the integument, termed "tracheae." In no member of the division are vibratile cilia known to be developed. According to Professor Huxley, an additional constant character of the Arthropoda is to be found in the structure of the head, which is typically composed of six segments, and never contains less than four.
The Arthropoda are divided into four great classes - viz., the Crustacea, the Arachnida, the Myriapoda, and the Insecta; which are roughly distinguished as follows:
Crustacea. Respiration by means of gills, or by the general surface of the body. Two pairs of antennae. Locomotive appendages, more than eight in number, borne by the segments of the thorax and, usually, of the abdomen also.
Arachnida. Respiration by pulmonary vesicles, by tracheae, or by the general surface of the body. Head and thorax united into a cephalothorax. Antenna (as such) absent. Legs eight. Abdomen without articulated appendages.
Myriapoda. Respiration by tracheae. Head distinct; remainder of the body composed of nearly similar somites. One pair of antenna. Legs numerous.
Insecta. Respiration by tracheae. Head, thorax, and abdomm distinct. One pair of antenna. Three pairs of legs borne on the thorax. Abdomen destitute of limbs. Generally two pairs of wings on the thorax.