SUMMARY.1The locomotory mechanisms of the several groups of Chilopoda and of the Pauropoda are described, together with the factors which determine the choice of the gaits and the morphological features which make possible their execution.2The Epimorpha alone show small changes in body length associated with changes in the gait.3The gaits of the epimorphic and anamorphic Chilopoda are fundamentally different, and have evolved from the further development of types of gait seen in Peripatua ‘middle’ and ‘top gears’ respectively (pp. 121 and 158, and Part 1, p. 554).4Comparison of the gaits of the Onychophora, Chilopoda, Pauropoda and Diplopoda indicate that the locomotory mechanism and the structure of the body wall and limbs in the Onychophora are primitive, and not secondary derivations from jointed animals, p. 160.6Morphological characters facilitate the execution of the gaits and render them mutually exclusive. A phase difference between successive legs of>0.5 is shown by the Epimorpha correlated with short legs, and one of<0.5 is obligatory in the Anamorpha in consequence of their long legs. The footprints of all legs on one side of the body occupy common marks at each pace in the Epimorpha, but form forwardly directed ‘sets’ in the Anamorpha. Crossing of successive legs occurs in the recovery stroke in the Anamorpha and to a very small extent in the propulsive stroke of the Epimorpha.6Changes of speed are effected in the Epimorpha mainly by changes in pace duration and in gait, but in the Anamorpha in addition by changes in the angle of swing of the leg.7The gaits of the Geophilomorpha and Scolopendromorpha show the same wide range of prtttérn, approx. (5.0: 5.0) to (7.5: 2.6), at a phase difference between successive legs of 0.6–0.87, and form a series of even practicability. The gaits are elaborated for different purposes in the two groups. The Scolopendromorpha achieve fast speeds for surface running by steeply decreasing the pace duration to<0.04 sec. and the duration of the backstroke to<0.01 sec. The Geophilomorpha, increase the flexibility of the gait, varying the positions of the footholds, for a, burrowing or cryptic habit, and the pace durations do not steeply decrease.8Active burrowing in the Geophilomorpha is dependent upon the body becoming ‘fat’, and not upon the motive force exerted by the legs. A longitudinally contracted animal can increase its length by 66%.9Morphological specializations correlated with burrowing in the Geophilomorpha, are:—(i) shortness of legs, (ii) presence of intercalary sclerites, which are large dorsally and telescope freely, (iii) the staggered position of the dorsal and ventral joints of the body, (iv) the elastic pleural region with isolated sclerites, (v) the powerful longitudinal musculature, and (vi) the tendency to decrease the length of the segments and increase their number.10Morphological specializations correlated with running in the Scolopendro‐morpha are:—(i) the curtailment of the number of body segments, (ii) the longer legs of differential length, and (iii) alternate sized tergites.11The gaits of the Anamorpha show a smaller range of pattern than the Epimorpha, forLithobius(5.5: 4.6) to (6.54: 3.46) at a phase difference between successive legs of 0.16 to 0.154, and forScutigera(5.6: 4.6) to (6.43: 3.57) at a phase difference of 0.16 to 0.135. Great speed is achieved by the reduction of the pace duration to approx. 0.07 sec. and that of the backstroke to approx. 0.02 sec. Lateral undulations are considerably controlled.12The gaits are operated by either thirteen or fourteen pairs of legs, and result in an even mechanical performance, footfalls occurring at approximately equal intervals, the loads on each leg remaining constant.13The gaits of the young stages ofLithobiusare described. The second instar with eight pairs of legs employs only a slow type of gait (5.0: 5.0) at a phase difference of 0.25; a faster gait with an even mechanical performance is not possible until more legs are present.14Morphological features correlated with running in the Anamorpha are:— (i) alternate sized tergites and fusion of tergites as inScutigera, (ii) long legs of differential length, (iii) reduction in number of leg bearing segments to fifteen, and (iv) a tendency to shorten the trunk segments.16The methods of starting up and turning are described.16The main limiting factors to increase of speed are, in the Scolopendromorpha:— (i) sagging of the body between the propulsive legs, and (ii) lack of control of lateral undulations of the body, which results in a waste of energy and a loss of potential speed; and in the Anamorpha:—mainly an inability to quicken the pace still further.17The gaits of all stages ofPauropus gracilisare described. They bear a basio similarity to the epimorphic Chilopoda, but owing to their environment, the animals can effect a simultaneous transfer of the weight from leg to leg.18Morphological features associated with the gaits in the Pauropoda are:— the alternate sized tergites and the length of the segments.19The evolution of the body form of the main groups of Chilopoda is discussed., Support is given for the view that epimorphic development and a moderate number of segments may be primitive, and that anamorphic developments may be secondary. The nature of the locomotion has led to a shortening of the body in the Lithobio‐morpha and Scutigeromorpha and to an elongation of the body in many Geophilomorpha. The Scutigeromorpha represent the most advanced and swiftest, and not th