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ABSTRACT
Anatomical information on four systems of presumed non-chemosensory innervation are discussed in the context of possible functions in the lancelet (amphioxus) as well as evolutionary relationships to somatosensory pathways in vertebrates. New observations provided by DiI tracing of nerves and by scanning electron microscopic observation of nerve-like structures are consistent with the results of earlier studies. The systems are categorized as: (1) free nerve endings similar to the Rohon-Beard system of anamniotic vertebrates; (2) ciliated primary receptor cells; (3) corpuscles of de Quatrefages associated with the nerves to the rostrum; and (4) specialized encapsulated nerve fibers in the metapleural folds.