Velvet wormOnychophora (velvet worms) are segmented, caterpillar-like, terrestrial animals somewhat resembling both arthropods and annelid worms. They are generally regarded either as a class of arthropods or as a separate phylum. About 110 modern species are known separated into two families, with 10 genera. They live in many tropical and subtropical regions worldwide including Mexico, Central America, South America, Africa, Australia and New Zealand. Modern species are fairly small. At least one species reaches lengths of 20cm (about 8 inches). The best known modern form is 'Peripatus' which was described by Guilding in 1825.
Onychophores are apparently descended from some segmented form but have lost their overt segmentation except for the head which is composed of three segments. There are two antennae on the first head segment and usually a pair of eyes. The second head segment has a mouth. Unlike arthropods, the animal does not have a stiff exoskeleton. Their skin (cuticle) is covered with fine hairs (papillae) giving a velvet-like feel and accounting for their popular name of "velvet worms". The cuticle is shed periodically to allow growth. Although the body has several dozen repeated leg pairs, it is not actually segmented. Onychophores are coelomates with a haemocoel. The entire structure is supported by blood pumped by a heart. Legs are tipped by chitinous claws and walking pads used on smooth surfaces. Onychopores breath through always open passages in the skin trachae. As a result, all known species require a humid environment to avoid desiccation.
Modern Onychophores are predators that are able to immobilize animals several times their own size with a gluey substance that they eject from glands in their head. They can immobilize targets up to 30cm away.
Onychophores are thought to be closely related to the arthropods. Possible marine Onychopores are known from the Lower Cambrian -- Aysheaia, Hallucigenia -- and possibly the late Pre-Cambrian -- Xenusion. A single fossil Terrestrial genus, is known from the Pennsylvanian of Illinois, and some Tertiary specimens are reported to be known from Caribbean island amber.