The gut of protochordates consists of a simple, straight tube through which food is propelled by ciliary action. An early elaboration of the gut is seen in lampreys where an infolding (typhlosole) of the gut wall presumably increases the absorptive area of the gut. A similar, but spiral, infolding of the hindgut occurs in sharks, rays, and the coelocanth (Latimeria) in the form of the spiral valve (spiral intestine). The gut wall in lampreys also contains diagonal muscle fibres, although true peristalsis (travelling wave of contraction) is thought not to occur. Teleost fish have a gut which is typical of the higher vertebrates in many respects, although the midgut villi (absorptive papilli) of mammals are absent in fish