Phylum Onychophora

Common Name: Velvet Worms

Etymology: "Claw-bearer", from the Greek onychos for talon (a bird of prey's claw) and phoros for bearer, in reference to the claws at the tip of each leg.

Representative examples:

Click images to enlarge.
An onychophoran from the Osa Peninsula, Costa Rica	Opisthopatus roseus, a rare species from Natal, S. Africa	Peripatopsis moseleyi, from Natal	An undescribed, pure white species of Opisthopatus, from Natal	An onychophorna.

Symmetry: Bilateral Tissue Layers: 3 Coelom: Eucoelomate Digestive Tract: Complete Special Features: Slime Glands = velvet worm are able to eat prey several times larger than themselves by immobilising it with a gluey secretion from glands in its head, projected up to 30cm. The secretion holds the prey while the animal approaches it, bites through the cuticle, and injects a toxic, digestive saliva into the wound. click to enlarge Unjointed "sac-like" legs Some features intermediate between those of annelids and arthropods

Additional Information:

Presently considered to be closely related to, if not included in, the Phylum Arthropoda (see phylogeny above right), onychophorans are an ancient group (at least 500 million years old) now restricted in species diversity and habitat. Although onychophorans have an open circulatory system, breathe via tracheae, and have a pair of legs for each internal body segment (all features shared with many arthropods), they also exhibit a lack of external segmentation, unjointed, sac-like legs, layered (as opposed to bundled) muscles, and pairs of nephridia for each internal segment, all features that are distinct from those of arthropods. In addition, onychophorans have a chitinous exoskeleton which is molted in response to the same hormones seen in arthropods, although the onychophoran exoskeleton is very thin and more permeable than that of typical arthropods. Velvet worms also display convergence with spiders in their feeding ecology - using sticky secretions ejected from glands near their mouths to ensnare prey, immobilizing it until they can deliver a toxic, digestive saliva with their bite. Living onychophorans are terrestrial, tropical predators. Some extinct species are believed to have been marine organisms.

Links:

• Introduction to the Onychophora (Univ. of California Museum of Paleontology)
• Diagrams of onychophoran anatomy (BIODIDAC)
• Onychophora (Eartlife.net)