Terrestrial tracheate Arthropoda, zvith a head, followed by a series, usually numerous, of similar somites: with a pair of antennae, mandibles, and maxillae. Limbs 6-7 jointed, ending with a claw.

The body is elongate, and of nearly the same diameter throughout, and either flattened dorso-ventrally or cylindrical. In the Archipolypoda it appears to have been fusiform. The head is generally somewhat flattened. The numbers of somites in the body varies from five (Eupauropus) to some hundreds in the Geophilidae (Chilopoda). In Chilopoda the head is followed by a large 'basilar segment/ usually stated to be composed of three or four fused somites1. The remaining somites are either all similar, or some are smaller than others and partially concealed by them, and each somite consists of a single tergal and sternal plate united at the sides by a soft pleural membrane. In Diplopoda the tergal region makes frequently (e. g. Iulidae) at least three-fourths of a circle, or when flattened dorso-ventrally it has lateral expansions (e. g. Polydesmidae). To each tergum corresponds a pair of sterna, one in front of the other, and the first somite has the tergum expanded anteriorly and it may then conceal the head completely (Eupauropus) or partially as in many others.

The antennae are 7-jointed in Diplopoda, and sometimes lodged in grooves of the head. They are long and many jointed in Chilopoda. In Pauropoda (Diplopoda) the last joint of the antennae is bifid and the organ is terminated by three long jointed appendages. The mandibles have a broad masticatory surface in the vegetable-feeding Diplopoda, a toothed margin in the carnivorous Chilopoda, in some of which a rudimentary palp is said to exist. The maxillae in the former group are represented by a four-lobed plate derived from a single pair of limbs in the embryo; in the latter each maxilla consists of a palp and bilobed median process. The Polyzonidae (Diplopoda) have the jaws united to form a sucking tube. The first pair of post-maxillary appendages in Chilopoda is limb-like, the bases of the limbs in contact, as is the case in the second pair, the limbs of the 'basilar segment/ which form the stout curved poison claws. The poison gland is lodged in the two last joints and opens on the convex side of the apex. The third pair of limbs (which have no corresponding tergum) is leg-like and sometimes wanting (Lithobius, Scutigera). The remaining somites bear a single pair of jointed limbs, the last pair being usually long and turned backwards.

The first two or three somites in Diplopoda may carry a single pair of limbs, of which the first pair are shorter and turned forwards and sometimes modified in the male; or there are two pairs to every somite in Eupauropus, as there are in the middle and posterior somites of other members of the class. The third somite is apodous in hdidae, and the seventh in the male carries one pair of limbs and the copu-latory organ. The limb is usually 7-jointed, the terminal joint being a claw.

The chitinoid cuticle is generally thick and firm in the terga and sterna. Hairs are sometimes well developed in Diplopoda, e. g. Polyxejiidae, in which they are aggregated in bundles. Bundles of spines occur in the extinct Protosyngnatha, and in the extinct Archipolypoda six longitudinal rows of spines or tubercles. The majority of Diplopoda possess a longi-. tudinal lateral series of foramina repngnatoria, the apertures of glands secreting a mal-odorous fluid. In Fontaria sp.? (Diplopoda) the secretion contains a ferment and a substance which breaks up into hydrogen cyanide and oil of bitter almonds. In Glomeris the secretion is sticky and probably serves to retain the animal in a rolled up condition when it falls. Geophilus Gabrielis, a Chilopod, has a ventral series of glands, in which a large number of tubules open on a perforated plate. The use of the red secretion is unknown1.

1 Cf. Balfour, Comp. Embryology, p. 325. L 1 2

The nervous system consists of a supra- and a sub-oesophageal ganglion with a ventral chain. The number of ventral ganglia corresponds as a rule to the number of somites, and the Diplopoda have two to each tergal ring. The ganglia supplying the appendages of the 'basilar segment' in chilcpoda are fused together. Siphonophora (Diplopoda), the Geophilidae, and Crypteps (Chilopoda), are blind. Scutigera among Chilopoda has a poly-meniscous eye on each side of the head, other Myriapoda have a number of monomeniscous eyes grouped together in the same position. lulus and Glomeris among Diplopoda are stated to have a monostichous ommateum, and the Chilopoda a diplostichous, though the vitreous layer is evanescent. The eye is apostatic or cup-shaped, but the layer of vitreous cells does not share in the invagination. The visual rods at the sides of the cup are directed inwards more or less horizontally, and there are no pigment cells between the visual rods whether at the sides or base of the cup.

Each visual cell is said to support many visual rods in the Diplopoda. Scutigera has a retinulate ommateum, and the visual cells secrete rhabdomeres surrounding a conical vitreous body 2. Sensory hairs are found upon the antennae, and the apical joint of those organs in some Diplopoda is furnished with sensory structures very similar to the structures found in Insecta. A cavity opens externally in Scutigera between the mandibles and maxillae, and one or more cavities beneath or near the eye in some Diplopoda. Their function is unknown, but the presence of hairs within the cavity of Scutigera, and of a single moveable hair in the cavities of Polyxenus lagurus, point perhaps to that of hearing.

The digestive tract has a short stomodaeum with salivary glands 3, a long straight mesenteron, disposed however in a single longitudinal fold in Glomeris and Sphaerotheriurn (Diplopoda), beset with short glandular tubules, and a short proctodaeum. The anus is terminal, and in Diplopoda is inclosed between two valves borne by the last somite. There is a dorsal heart composed of a series of chambers (two to each somite in Diplopoda), with valved lateral ostia and inclosed in a pericardial sinus formed by a horizontal membrane, and furnished with alary muscles as in Insecta. Each chamber gives off one pair, or in Diplopoda two pairs, of lateral arteries. There is an anterior aorta which sends forward a cephalic vessel, and two lateral vessels which form an oesophageal ring and unite ventrally into a vessel which lies dorsally to the nerve cord and runs backwards. A single pair of stigmata opens in the pleural membrane of all or certain somites in Chilopoda, with the exception of the genus Scutigera, which has a single median dorsal stigma to each somite. In the Diplopoda, on the contrary, each somite carries two pairs of stigmata, opening ventrally, one at the base of each pair of limbs; but in Glomeris the three first pairs of stigmata belong each to a separate somite.