Mauthner's xon

one of the two giant neurons linked to parts of the acoustico-lateralis, optic, and cerebellar systems, and to somatic motor neurons by way of decussating (intersecting) giant nerve fibres that run the length of the spinal chord. This short circuits ordinary reflex paths and provides the rapid and co-ordinated response of swimming muscles for rapid escape. Found in larvae of Petromyzontidae, in Holocephali, Teleostomi and in tadpoles and aquatic urodeles, absent in Selachii and Rajiformes