SummaryThe methods employed for the work recorded in subsequent Parts are described. The gaits of the Arthropodaveraappear to have been evolved from those of soft bodied ancestors using a locomotory mechanism such as is seen in Peripatus. Each group has restricted the versatility of its gaits and perfected the type adopted. It is suggested that habits determined the general nature of the gaits employed, and that perfection of the gaits occurred along with the evolution of associated morphological features. Many of the latter are diagnostic features of the larger groups.It is suggested that the evolution of many of the larger groups of the Arthropoda has been determined by differentiation and persistence of habits, and that adaptations to particular environments have been of lesser importance.The arthropodan type of integument and muscle has led to an improvement in leg structure. The majority of terrestrial Arthropoda do not ‘stand up’ on their legs, aa do the vertebrates, but ‘hand down’ from them, and most groups have adopted an advantageous method of stapping which is impossible to the Onychophora.The effects of leg length and leg number on the fields of leg movement are considered in relation to the evolution of body form.The mechanism for changing speed in the Arthropoda differs from that of the Onychophora. Change in pace duration has become a major factor additional to the effects of changes in the gait and in the angle of swing of the leg.A relationship is shown between the shape and number of the trunk segments and the type of gait employed; ‘bottom gear’ gaits requiring short, and often numerous segments, and ‘top gear’ gaits requiring longer and fewer segments.The phase difference between the paired legs, which may be used in the same or in opposite phase, or in some other phase relationship, is shown to be dependent upon the nature of the gait in animals with many legs. The conditions in animals with few legs is considered.Undulations of the body, either lateral or dorso‐ventral, tend to occur at fast speeds and are disadavantageous. Morphological features which minimize or prevent these undulations are:—wide, short segments, alternate sized tergites, diplo‐segments, a small number of long legs borne on consecutive fused segments (t