Representative examples of gap junctions from vertebrates (ciliary epithelium from the eye of a Macaca mulatta, upper) and invertebrates (inverted gap junctions from cells of the mid-gut of the horseshoe crab, Limulus polyphemus, lower). Gap junctions in vertebrates consist of closely aggregated 8-9nm particles on the P-face, whereas those in invertebrate tissues have somewhat larger intramembranous particles associated with the E-face in freeze fracture preparations. Figures 104 (upper, by Giuseppina Raviola) and 105 (lower, from Lane, J. Cell Sci. 32: 293-305, 1978,reprinted with permission in Chapter 3 (Junctional Specializations) of 'The Cell, 2nd Ed.' by Don W. Fawcett M.D. A PDF copy of the accompanying chapter is available on the ASCB's BioEDUCATE website.