Abstract: A vehicle adapted to be advanced in a fluid by deforming itself progressively along its length, the vehicle comprising a head containing a fluid compressor and a body connected to the head and having a flexible and deformable envelope. An assembly of longitudinally displaceable rings is mounted in the body and the rings are connected to longitudinal and toroidal muscle elements for applying radial and longitudinal forces to the envelope at spaced locations along the length thereof to form a continuously generated progressive wave along the envelope which serves to propel the vehicle in the fluid.