Coelomate Metazoa, with a bilaterally symmetrical body, composed of a series of somites, usually disposed in dissimilar groups; with a pair of hollow-jointed limbs attached to more or fewer of the somites; with a chitinoid cuticle, and body-muscles not arranged in continuous layers, and a nervous system composed of a supra-oesophageal ganglion and a ventral chain of ganglia.

The segmentation of the body is rarely lost. Successive somites are connected to one another, either by a soft intersegmental membrane or by fusion. The somite itself may be ring-like, or consist of a dorsal plate, the tergum, and a ventral plate, the sternum, connected laterally by a soft pleural membrane, or more rarely by distinct pieces, a dorso-lateral epi-meron and ventro-lateral episternum. These last named parts may sometimes be distinguished as regions when the parts of a somite are continuous, e.g. higher Crustacea. The tergum may occasionally be broken up secondarily, as in the thoracic somites of Insecta.

A head region is nearly always distinguishable. It consists of a prae-oral or pro-cephalic region, to which are fused a variable number of post-oral somites, and it then either remains distinct (Insecta, Myriapoda) or becomes continuous with a part or the whole of the thorax, forming a cephalo-thorax (Arachnida, many Crustacea). A thorax is not marked off in the Myriapoda. In Insecta it consists of three somites; in Arachnida it may be considered as consisting of four; in the higher Crustacea of eight, and of a variable number in the lower. The somites behind the thorax constitute the abdomen. This region is not specially differentiated in Myriapoda, but is clearly distinguishable in the other classes. The number of its somites varies much from class to class, and within the limits of the same class.

The Insecta, Myriapoda, and Protracheata carry upon the head a pair of sensory antennae, which are probably to be regarded as processes of the procephalic lobes, and not as homologous with the remaining appendages. The Arachnida possess a pair of prae-oral appendages in the adult, which are post-oral in the embryo. The first antenna of the Crustacea is probably post-oral, like the second antenna and remaining appendages. The appendage or limb is composed of a series of articulated joints, and in the Crustacea possesses typically a basal portion bearing an external and internal branch (exo- and endo-podite). The muscles moving the joints are contained partly within the limb itself, partly within the somite that carries it. One pair of limbs at least, and usually more, are modified to act as jaws. The external shape of the limb varies much, according to its function.

The body wall is composed of a chitinous cuticle, sometimes thin, sometimes thick and then laminated, and in Crustacea hardened by calcareous deposit. It is formed by an underlying and single layer of ectoderm or hypodermis cells. When the animal grows, this cuticle is shed or undergoes ecdysis at stated periods, and at the same time its internal extensions into the stomodaeum, proctodaeum, gland ducts, tracheae, and sometimes the tendons, are shed also. Beneath the hypodermis a thin basement membrane is nearly always to be detected. Peripatus (= Protracheata) alone has a continuous layer of circular muscles and bands of longitudinal muscles, all of which are non-striated. In other Arthropoda the muscles are striated, and are disposed in separate fascicles. Ciliated epithelium is universally absent, except, perhaps, in the funnels of the nephridia, etc. of Peripatus. The integument is remarkably poor in glands. Connective tissue is variably developed, to the greatest degree, perhaps, in the higher Crustacea, in Scorpio and Limulus among Arachnida, and as a fatty tissue in Insecta.

The supra-oesophageal ganglion in its simplest form or archi-cerebrum innervates the eyes and special sense-organs of the head, as in the Arachnid Limulus. In other Arthropoda (Scorpions ?) it is probably always fused with other ganglia, and is a syn-cerebrum. In Insecta it gives origin in addition to the nerves of special sense, to the stomato-gastric sympathetic system, and in most Crustacea to the nerves of both pairs of antennae (see p. 187) and to the stomato-gastric in part. The ganglia supplying the jaws are fused together into an infra-oesophageal ganglion, and the succeeding ganglia, typically one to each somite, frequently undergo greater or less concentration. Each ganglion consists of a right and left half, which are sometimes distinct, and only connected by transverse commissures (a few Crustacea). Successive pairs of ganglia are united by two longitudinal commissures, which may retain their primitive distinctness or become united in one sheath. The structure of the nervous system of Peripatus is unique among Arthropoda.

Eyes are confined, with two exceptions, to the head. The cuticle corresponding to the eye is thickened to form one or many lenses; hence mono- and poly-meniscous. The hypodermic cells beneath the thickened cuticle constitute the ommateum, and remain either in a single or form a double layer; hence mono- or diplo-stichous. The monostichous ommateum is said to be apostatic when cup-shaped, and epistatic when in close contact with the cuticle and following its curvature. In the diplostichous eye the anterior layer forms the vitreous layer. Its cells either retain their regular arrangement or become disposed in groups or vitrellae, which develope a transparent vitreous body. The posterior layer forms the retinal layer; when its cells are collected into groups or retinulae it is said to be retinulate. Each cell in a group generally forms a clear visual rod or rhabdomere. The rods belonging to a group of cells commonly fuse into a rhabdome. The retinal cells are often more or less pigmented. Pigment cells are also present between them, and between the vitrellae. They are derived from the hypodermis, or perhaps in some instances from intrusive mesoblast, hence the eye is auto- or exo-chromic. A mono-meniscous eye may be retinulate.