Onychophoran cephalic nerves and their bearing on our understanding of head segmentation and stem-group evolution of Arthropoda

The extant Onychophora are basal representatives of the Arthropoda, resembl ing Cambrian arthropod stem-group fossils such as Aysheaia and other so-cal led lobopodians. They thus provide an important model system for early stag es in arthropod evolution, especially those of the head. However, homologie s between onychophoran and euarthropod head structures have remained uncert ain, and previous descriptions of the onychophoran head are contradictory. Our investigation examines the cephalic nerves of two species of onychophor an using immunohistochemical and confocal laser scanning microscopical tech niques. Our new reconstruction for the distribution of onychophoran cephali c nerves differs significantly from previous accounts. The so-called "labru m" of the Onychophora appears to be part of the pharynx, and any affinities to the various pre-oral outgrowths Of crown-group arthropods referred to a s hypostomata and/or labra can be ruled out. The innervation of the mouth i s particularly noteworthy, as its nerves originate from three different reg ions of the brain, including the dorsum. This suggests that the mouth was p rimitively terminal/frontal, although subsequent ventral rotation of the mo uth may have been parallelly achieved in Onychophora and in Euarthropoda. O ur model allows the onychophoran cephalic nervous system Co be interpreted as a modified circumoral nerve ring, similar to that seen in the nematodes and other cycloneuralians. We regard this as supporting the Ecdysozoa model of arthropod relationships. (C) 2001 Elsevier Science Ltd. All rights rese rved.