Penetration of action potentials during collision in the median and   lateral giant axons of invertebrates
  The collisions of two simultaneously generated impulses in the giant axons of both earthworms and lobster propagating in orthodromic and antidromic direction were investigated. The experiments have been performed on the extracted ventral cords of Lumbricus terrestris and the abdominal ventral cord of lobster, Homarus americanus, by using external stimulation and recording. The collision of two nerve impulses of orthodromic and antidromic propagation didn't result in the annihilation of the two signals contrary to the common notion that is based on the existence of a refractory period in the well-known Hodgkin-Huxley theory. However, the results are in agreement with the electromechanical soliton theory for nerve pulse propagation as suggested by Heimburg and Jackson (Proc. Natl. Acad. Sci. USA 102, 9790 (2005)). 
A. Gonzalez-Perez, R. Budvytyte, L. D. Mosgaard, M. T. Stauning, S.   Nissen and T. Heimburg
physics.bio-ph q-bio.NC