Onychophora

Abstract

AbstractOnychophorans, or velvet worms, are soft-bodied, terrestrial invertebrates that inhabit tropical and temperate forests of the southern hemisphere and around the equator, where they are mainly found in rotted logs and leaf litter. Together with tardigrades (water bears), onychophorans are regarded as the closest relatives of arthropods (spiders, centipedes, crustaceans, insects, and their kin). However, in contrast to arthropods, the anatomy of onychophorans has changed little since the Early Cambrian, making these animals a key group for understanding the evolution of arthropods, including the origins of their highly diverse nervous systems. This chapter summarizes our current knowledge on the organization and development of the nervous system in onychophorans, including details on the brain, the ventral nerve cords, the innervation of cephalic structures, and the foregut, midgut, and hindgut, as well as neurogenesis and neural development. Given the key phylogenetic position of Onychophora, different hypotheses on the evolutionary origin and homology of neural structures are presented and their implications on the phylogenetic relationship of Onychophora, Tardigrada, and Arthropoda are discussed.