The onychophoran walks at a rate of less than
1 cm/s with its 14–43 pairs of unjointed, hollow legs. Like the
muscles of annelids, its circular, longitudinal, and diagonal bodywall
muscles are smooth (nonstriated). These muscles work
against the hydraulic skeleton to move the velvet worm. Vascular
(hemal) channels that encircle the velvet worm body, like wire in
a vacuum cleaner hose, are unique to velvet worms. Hydrostatic
pressure maintains the firmness of the legs as leg muscles bend
and shorten the limbs; a valve at each leg base enables each leg to
be extended independently by altering the pressure in the hemocoel,
the main body cavity. Velvet worms are coelomates, but
their coeloms are vestigial, having been reduced to gonoducts
and tiny sacs surrounding the nephridia.