ARTHROPODA, a phylum of the animal kingdom, corn prising animals with a segmented body enclosed in a firm in tegument or exoskeleton, provided with jointed limbs some of which are modified to serve as jaws. The group includes the classes Crustacea, Myriopoda, Insecta and Arachnida, together with several less extensive groups. It corresponds to the class Insecta of Linnaeus, but that name has, by more than a century of usage, been restricted to one of its subdivisions.
The Arthropoda have many features in common with the Annelid worms (see ANNELIDA), from some primitive form of which, it can hardly be doubted, they have been derived. They agree with the Annelida in the segmented body, the segments or "somites" of which arise during development in regular order from front to rear, new somites being added in front of an unsegmented terminal region or "telson"; in the structure of the nervous system, consisting of a ventral double chain of segment ally arranged ganglia connected by a pair of cords which en circle the gullet with a pair of ganglia (the "brain") in front of the mouth; and in having segmental hollow appendages (para podia, limbs) moved by muscles. They differ from the Annelida in having the external cuticle—which i. the Annelida is a delicate membrane composed chiefly of a substance known as chitin— thickened and stiffened so as to form an exoskeleton, remaining thin and flexible only at the joints between the segments of the body and limbs ; in having one or more pairs of appendages in the neighbourhood of the mouth converted into paired, laterally moving, jaws; and in having the apparent body-cavity (haemo coel) forming part of the blood-system communicating with the contractile dorsal vessel or heart by segmentally arranged valved openings or Ostia, the true body-cavity (coelom) being almost obliterated.
The possession of a rigid exoskeleton, of which the segments form jointed levers moved by muscles, is associated with a much more complicated muscular system than is found in the Annelida and has rendered possible the development of elaborate mecha nisms for carrying on the varied activities of life. The appendages are modified in an endless variety of ways for creeping, swimming or flying, for catching and killing prey, for biting or grinding food, for the support of sense-organs, for aiding the functions of reproduction or for protecting the eggs and young. Along with these developments, the nervous system and the organs of spe cial sense, in particular the eyes, become correspondingly complex and efficient. The protection which the continuous cuticle affords to the underlying tissues has facilitated the transition, many times repeated in the evolution of the group, from aquatic to terrestrial life, in which the insects above all have been con spicuously successful.
With the body and limbs enclosed in a continuous and inex tensible envelope, growth is only possible if the envelope is peri odically shed and renewed, and this process of "moulting" or "ecdysis" is very characteristic of Arthropoda. In some, growth appears to continue at a diminishing rate throughout life, and ecdysis occurs at successively longer intervals. This is probably the case with most of the higher Crustacea. In the winged in sects, however, growth ceases at the end of the larval stage and the adult insect does not undergo ecdysis. Although the general association of ecdysis with growth is obvious there are exceptions which forbid the assumption that the two phenomena are causally connected. Thus in certain Decapod Crustacea (prawns) re peated ecdysis may occur without any perceptible increase in size, and in individuals subjected to starvation there may even be a shrinkage in bulk of the animal. On the other hand, certain parasitic Crustacea increase enormously in size without ecdysis, the cuticle, which remains thin and membranous, extending apparently by some process of interstitial growth.
A remarkable peculiarity, which is all but universal throughout the Arthropoda, is the absence of vibratile cilia from all parts of the body both in the larval and in the adult stages. In this char acter the Arthropoda differ from all other animals except the Nematode worms. The only Arthropoda known to possess active ly moving cilia are certain species of Peripatus (Onychophora) in which the epithelium of the receptaculum seminis is ciliated. The absence of cilia from the external surface of the body is doubtless correlated with the development of a strong cuticle; but it is not clear how this can explain their absence from internal passages such as nephridial and genital ducts.