Summary.1The ability to perform a speedy backstroke by the legs, and some powers of reversible dorso‐ventral flattening and of shortening and elongation of the trunk is possessed by all Chilopoda. These proficiencies are shown to be correlated with characteristic habits of life and the basic organization of the body, including: the disposition of selerites and pleuron; the form of the coxa, coxa‐body and coxa‐trochanter joints; the general form of the legs; the endoskeletal tendon system; and the general lay out of the musculature.2Characteristic chilopodan morphology contributes to the parasagittal rock of the coxa on the body, which is additional to a promotor‐remotor swing. The resulting rotation along the leg axis promotes maximum strides and thrust against the ground at all phases of the backstroke.3The maintenance of body length and of alignment between trunk segments, in the absence of closely articulated intersegmental joints, is mediated by the elasticity of the sclerite cuticle and by the form of the joints. The structure of the cuticle, arthrodial membranes and joints in the four orders is described.4Lateral flexibility of the trunk is enhanced by the presence of intercalary sternites; dorso‐ventral flexibility is assisted by intercalary tergites; intercalary sternites and tergites in the Geophilomorpha facilitate the strong shortening of the body used in burrowing; simple longitudinal hinge lines in Scolopendromorpha facilitate flattening of a segment and the reverse; and the very simple condition of the intercalary joints in the larva ofAnisopus(Diptera) is considered both structurally and functionally in comparison with Chilopoda.5The endoskeletal tendon system of the head and trunk of the chilopodan orders is described, together with the very simple tendon system ofAnisopus.Paired segmentally arranged tendons support the dorsal, lateral and sternal longitudinal muscles. The segmental tendons in the head form transverse bars, as in many other Arthropoda. The lateral and sternal tendons of the trunk are fused on either side in each segment in the Anamorpha, and in Scutigeromorpha each pair is united ventrally by a large epineural sheet. The functional significance of the tendon system in relation to leg length and gaits is demonstrated.6The basic plan and mode of action of the trunk muscular system is described, and its relationship to gaits and other functions is considered.7The divergent habits of Chilopoda, superimposed upon the basic proficiencies are described, together with the facilitating morphology. These habits are mutually exclusive because the associated morphology is very different, (i) Perfection in burrowing has been achieved by the Geophilomorpha as a result of modifications in: the cuticle structure; the joints; the musculature of head and trunk; the limbs; the feeding mechanism and the respiratory system. These modifications are not considered to be primitive, but to be superimposed upon the basic morphology already described, (ii) Perfection in speedy running has been achieved by the Scolopendromorpha and Anamorpha as a result of modifications in: joint structure; the development of heteronomy in tergites, musculature and respiratory system; and other features, (iii) The habits and capabilities ofCraterostigmus tasmanianus,in so far as they have been ascertained, show that the anatomical peculiarities of the trunk of this animal are correlated with a secondary acquisition of extreme flexibility based upon a type of organization characteristic of the fleet Scolopendromorpha.8Pressures of at least 230 and probably 360–400 g./cm.2can be exerted by the body surface ofOrya barbaricaagainst the substrate. This figure greatly exceeds those obtained for burrowing Annelida in comparable experiments, and the latter are much greater than have previously been recorded.9Particular specializations for burrowing exist in: the form and mode of action of the head, poison claws, tentorium and mandible; the modifications on the anterior third of the body, including the carpophagous structures in those species which possess them; and in the elaboration of pericardial tracheae.10The presence in the Geophilomorpha of dorsal spiracle‐less atria possessing typical atrial cuticle, and of dorsal atria with spiracles in the Scutigeromorpha, suggests that the ancestral Chilopoda may have possessed both lateral and dorsal tracheal systems. The divergencies in tracheal systems from such a type suit particular needs in the several orders: extreme shape changes and temporary spiracular occlusion in Geophilomorpha is mitigated by through conduction tracheae; fleet running and crevice living in Scolopendromorpha is assisted by limited through conduction tracheae and by sinuses; extreme hydrostatic changes inCraterostigmusare resisted by numerous little‐branched narrow tracheae; and inScutigerathe utilization of the blood for the entire transport of respiratory gases takes place from the dorsal tracheal system to the tissues.11The Anamorpha can locate prey at a distance and pounce upon it; the Epi‐morpha do not pounce, and they find food without ocelli. Anamorphic gaits facilitate a more rapid take off than epimorphic gaits. The differences in the anatomy of the poison claw segment and in the manner of use of the poison claws are described. It is suggested that ancestral divergence of the Anamorpha from the Epimorpha was associated with the advancement of a pouncing habit in the Anamorpha, with which was associated the evolution of: acuity of vision; wide and rapid action of the poison claws which can secure spiders and flies; strongly biting mandibles; longer legs; anamorphic gaits; and the facilitating morphology, including segment number and lay out of muscles and tendons. The crevice living of the Epimorpha favours short legs, and the only way of obtaining speedy running with short legs is by obtaining the long stride provided by the epimorphic patterns of gait. Prey is dealt with more strongly but less speedily by the greater specializations of the poison claw segment, and the mandibles bite less strongly.12Within the Scolopendromorpha fast running is achieved in the Cryptopidae by rapid stepping with little trunk specialization. Such stepping is probably inappropriate for larger chilopods, and fast running in the Scolopendridae is obtained by the use of fast patterns of gait, with very few legs on the ground at one moment, and slower stepping.These accomplishments depend on heteronomy of trunk organization (sclerites, muscles and joints) which gives a zone of maximum stability at leg‐bearing segments 7 and 8 and inhibited or controllable mobility at the anterior ends of the long tergites. These features reduce lateral undulations during fast running to manageable proportions, and the described modifications in the musculature ensure the absence of sagging of the long stretches of unsupported segments of the body.13In the Anamorpha a greater degree of tergite heteronomy results in proportionately greater control of intersegmental flexures and of body undulations, permitting the use of longer legs. It is suggested that the characteristics of the trunk segmental tendons and musculature of the Anamorpha, which contrast with those of the Epimorpha, facilitate the performance of the anamorphic patterns of gait.14The muscles showing heteronomy in the Scolopendromorpha and the Anamorpha exhibit this phenomenon only in their dorsal insertions, sizes and degrees of subdivision. These muscles exhibit a uniform series of origins from the ventral segmental tendons, intersegments or limb bases. There is no evidence to support the suggested partial exterpolation of segments as an explanation of tergite heteronomy, all muscles being accountable for in terms of ventral and limb metamerism.15The form of the cuticle and the extrinsic and intrinsic musculature of the legs in the four orders is described and related to functional needs. In all the basic structure of the leg appears to have served fairly speedy movements, in contrast to the form and functions of diplopod legs. Features facilitating strong leg movements and the geophiloerites encircling the leg base, appear to be secondary developments. From 13 (Geophilomorpha) to 34 (Scutigeromorpha) extrinsic leg muscles operate each leg, in contrast to the Diplopoda with 2 or 4 pairs of such muscles.16A basic chilopodan mandibular mechanism is seen in Scolopendromorpha and in Lithobiomorpha, together with the associated mobile tentorium. The differences between the mandibles and tentorial apodemes in the two orders are correlated in the Lithobiomorpha with the ability to catch fleet prey with the poison claws and with the need for stronger biting mandibles, and in the Scolopendromorpha with the ability to break open slower moving prey more strongly with the poison claws and with the need to feed in very confined spaces.17The mandibles ofScutigeraare exaggerations of the lithobiomorph type. They are very large and cut more strongly than the mandibles of all other Chilopoda. The head morphology is correlated in great detail with the working of such large mandibles. The specializations include: acuity of vision; the secondary dome shape of the head; the large immobile tentorium; the form of the head tendons; the musculature of the mandibles and tentorium; the form and functions of the paw‐like maxilla 1; the strongly suctorial pharynx, with armoured musculated lateral pouches; and the elaborate glands secreting salivary and grooming fluids. The view that head structure inScutigerais primitive is rejected.18The mandible and head of the Geophilomorpha are highly specialized along their own lines and far removed from the basic chilopodan type. The mandibular mechanism is correlated with the ability of the head to change shape a little, and play its part in burrowing by making the initial penetration. The specializations include: the form of the cuticle and musculature of the head and of the poison claw segment; the immobile tentorium, structurally and functionally different from the immobile tentorium ofScutigera;the non‐biting mandible and its modified manner of use, being secondarily independent of the tentorium, of necessity lacking a transverse mandibular tendon, and possessing other peculiarities; the strong suctorial feeding (different from that ofScutigera)and the abundant extrusion of salivary juice.19The mandible and tentorium together with their muscles inCraterostigmus tasmanianusare typically scolopendromorph in detail. As in the Geophilomorpha, the mandible is minute and not used for biting. Chewing of prey is done by the huge poison claws. The complement of glands is of the scolopendromorph type and they are very large in size. External digestion results from extrusion of abundant salivary juice, and fluid material is sucked into the mouth.20The feeding mechanism ofCraterostigmusis associated with hydrostatic protraction and muscular retraction on the head on the poison claw segment. It is suggested that the importance of hydrostatic body movements ofCraterostigmushas been great enough to account for the loss of normal branching tracheae and for their replacement by numerous little branched onychophoran‐like tracheae. Numerous and narrow tracheae in the Onychophora are also associated with extensive shape changes of the body.21The systematic position and affinities ofCraterostigmusare considered. The basic organization of trunk, poison claw and head resembles the Epimorpha and not the Ana‐morpha. Hatching with 12 pairs of legs is intermediate between Epi‐ and Anamorpha. Closest affinity lies with the Scolopendromorpha, family Scolopendridae. The peculiarities of structure shown byCraterostigmusare associated with habits. It is suggested that the Craterostigmidae should form a third family, additional to the Cryptopidae and Scolopendridae, within the Scolopendromorpha.Craterostigmusis not an annectant or an archaic but a very highly specialized chilopod.22The probable evolution of the Chilopoda and of its four orders is considered in the light of the data presented.23The pressures exerted by the body surface against a resistance has been determined for the earthwormAllolobophoraandArenicolafor comparison with the Geo‐philomorpha. In the septateAllolobophorathe pushing musculature is largely situated at the site of the thrust and 80 to 110 g./cm.2can be exerted. In the non‐septate part ofArenicolamuscles at a distance inflate a group of segments which deliver a thrust of 130 to 200 g./cm.2. The significance of the minimal force of 230 and probable force of 360 to 400 g./cm.2exerted byOryaof comparable size is considered in relation to the possible circumstances of haemocoel evolution in the ancestors of t