Onychophorans are commonly
known as the velvet worms. They got this name because their skin is
covered in small bumps many of which have a small filament sticking out of the
There are about 120 species, and they range in length from 1.5 - 15 cm. They are found in tropical and sub-tropical forests.
Velvet worm evolutionary relationships
For many years they were thought to be the missing link between the worms and
the Arthropods, because they share characteristics with both. The fossil record
shows that they have changed little in the last 500 million years.
is now believed that their Arthropod-like characteristics are examples of
parallel evolution, and that they are not an Arthropod ancestor. The main
reason for this is that their tracheae form numerous branches near the spiracle
opening, but rarely ever branch again. Also the spiracles appear to be randomly
located on the body surface and have no closing mechanism. This lack of closing
mechanism confines them to the most humid of environments such as the forest floor, leaf litter and under stones. In fact in an ordinary room Peripatus (below) can lose as much as one third of its body weight in moisture in just 4 hours.
It is now believed that they evolved from the nematodes some time during the Cambrian.
Velvet worm body
Velvet worm cuticle never
hardens like Arthropod cuticle, and is not moulted all at one time, but in
patches. On the head they have a pair
of antennae each with an eye at the
base, the eye is similar to Annelid eyes (see Peripatopsis capensis below right).
Their legs are unjointed, operated hydraulically, and end in a pair of claws. The colour of the upper body varies from dark grey, green, brown or red; with the underneath whitish or light coloured.
mouth has a pair of claw-like mandibles and papillae. The papillae are the
exits for a pair of slime glands.