Equatorial Velvet Worms

Phylum Onychophora, or velvet worms, contains approximately 180 species of bilaterally symmetrical, coelomate organisms that somewhat resemble caterpillars. Onychophorans range from 5 mm to 15 cm in length, with homonomous bodies and small heads. The head carries a pair of annulated, fleshy antennae and a pair of small eyes at their bases, with large, chitinous lenses and a well-developed retinal layer, as well as a pair of jaws surrounded by circular lips, and a pair of fleshy oral papillae, also known as slime papillae.

The sticky secretions produced by the latter structures are used to capture prey, which includes other small invertebrates. Living species are divided into two families, Peripatidae and Peripatopsidae, which live in mutually exclusive geographical regions; peripatids are circumtropical, while peripatopsids are circumaustral. Members of the two families differ in their number of legs (generally greater in peripatids), the position of their gonopore (on a more posterior body segment in peripatopsids), and their reproductive habits.

All of the currently known living species are terrestrial, living mainly in dark, moist microhabitats. Research indicates that they are either modified arthropods or represent a link between annelids and arthropods, most recent molecular phylogenies favor the first hypothesis. (Brusca and Brusca, 2003; Monge-Nájera, 1995; Shapiro, 2012; Zhang, 2011)