onychophoran

Onychophora, or velvet worms, are a phylum of legged invertebrates that inhabit moist, terrestrial environments in tropical and temperate regions. They are elongate, soft-bodied animals with a pair of antennae and a ventral series of lobopodous appendages, typically 13 to 43 pairs depending on species. Their bodies are covered with a velvety cuticle and lack the hardened exoskeleton of arthropods.

Velvet worms are predators that hunt by secreting adhesive slime through oral papillae, which they project onto prey to entangle it. Once captured, they secrete digestive fluids and ingest the softened tissue. They move slowly and rely on a hydrostatic skeleton and flexible cuticle for locomotion. Gas exchange occurs via a network of tracheal-like tubes opening to the surface by spiracles; the circulatory system is open with a tubular heart.

Reproduction varies: many species are ovoviviparous or viviparous, with some laying eggs. In some southern Asian and Australian taxa, female provision of nutrition to developing embryos resembles a placenta. Most peripatids are tropical and do not survive desiccation; peripatopsids occur in temperate zones of the Southern Hemisphere.

Onychophora represent a key lineage in the broader group Panarthropoda, as a close relative of arthropods and tardigrades, and they are of interest for understanding the evolution of key arthropod features. The fossil record includes Cambrian and later forms, but modern diversity is limited.