(Figure A). Paired
claws at the tip of each little foot are the basis of their phylum
name, Onychophora. Velvet worms usually walk on walking
pads; on rough, hard substrates, they extend their claws. Onychophorans
are just 14–200 mm long (Macroperipatus) and may be
mistaken for arthropods (A-19 through A-21) or annelid worms
(A-23). Females in one species may be 50 percent longer and
weigh twice as much as males. The onychophoran bodies may
be iridescent green, blue black, orange, red, or whitish, although
most are brown. 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.