SUMMARY1The habits ofPolyxenus lagurusand the locomotory mechanism and skeleto‐museular systems which make these habits possible are described.2It is suggested that the circumstance and the habit which have been of greatest significance in directing the evolution of the Pselaphognatha are: small size (which renders burrowing by pushing less effective than in larger millipedes); and an ability to live on smooth ceilings of crevices (where many predators cannot easily follow), although this habit is not always exercised, pp. 180 and 183.3The structure and mode of adhesion by the tarsal claw are described. The adhesive lappet ofPolyxenusis considered to be a refinement enabling glass‐smooth surfaces to be negotiated, and not to be a feature upon which the whole evolution of the Pselaphognatha has depended.4The components of the exo‐ and endoskeleton are very light, basically diplo‐podan in nature, and highly specialized in detail. The internal transverse segmental tendons corresponding to every pair of legs is a feature commonly found in Crustacea, but restricted to the head in other Diplopoda (mandibular adductor tendon, etc.) and the V‐ and Y‐shaped skeletal bars are unique.5The sheets of trunk muscles seen in other Diplopoda are represented inPolyxenusby an elaborate series of penicilli which appear to have evolved from the basic diplopodan pattern. Some cause movement, others by their tone stabilize skeletal units, and the muscular system is quite unlike that of a primitively soft‐bodied arthropod.6The structure of the cuticle is described; the soft body is shown to be a specialization enabling the rings to be telescoped (in contrast to other Diplopoda), and the pleural lobes and posterior spine bundles to be moved.7Provisions for defence against predators and water are described; they include the flatter shape, the structure and distribution of spines and spinules, telescoping and local body movements.8Specializations connected with the ability to live on the ceiling of a hide involve details of the whole trunk region and are listed under 1–18 on pp. 180–181.9The movable part of the leg obtains an advantageous lateral origin by the form and immobility of the coxa. Unique extrinsic promotor and remotor muscles operate from the pre‐femur. The limbs are not as short as in some Juliformia yet are under cover by the spines at all times.10Polyxenusis relatively fast moving, and obtains speed by using a larger angle of swing and quicker stepping than other Diplopoda.11The pattern of the gait (40: 6‐0)‐(0–5: 9‐5) or slower, at phase differences between successive legs of 0–8‐0‐6, is remarkable‐in that speed is obtained from a slow pattern of gait, and that the phase differences between successive legs resembles those of the epimorphic Chilopoda and not the Diplopoda. Both features are shown to be obligate mechanical necessities.12The phase difference between legs of a pair is 0 in the slower gaits and increases to 0–5 in the faster gaits. Functional reasons are given for this curious combination of paired legs moving out of phase and a ‘bottom gear’ gait.13It is concluded that the Pselaphognatha are one of the most specialized orders of Diplopoda, showing neither primitive di