Gap junctions are clusters of intercellular channels which enable direct cell-to cell cytoplasmic communication both electrically and chemically. Gap junctions play an important role in determining cell fate and function during development, underlining the functionality of their plasticity in developing systems. In the work presented here, Hirudo medicinalis, a European leech, was used to study gap junctional protein expression in both the whole embryonic and adult leech central nervous system (CNS) as well as in single neurons. First, quantitative studies revealed that Hm-inx1, a neural specific innexin, and Hm-inx2, a glial specific innexin, are both expressed at relatively higher levels in the embryo than in the adult. Aside from suggesting greater possible coupling among cells in the early leech CNS, this up-regulation may explain how glial cells regulate synapse numbers in neurons early in leech development. Second among 12 previously characterized innexins and two novel innexins, Hm-inxs 13 and 14, four (Hm-inxs 4, 11, 12, and 13) were found to be expressed in the adult CNS but not in the young embryo, while one (Hm-inx8) was found to be expressed in the young embryo but not in the adult. Finally, three specific types of neurons, Retzius (R) serotonergic neurons, anterior pagoda (AP) motor neurons, and pressure (P₁) neurons, were found to express unique subsets of innexins. All three cells express Hm-inxs 1 and 6. In addition, R cells express Hm-inx2, AP cells express Hm-inx4, and P1 cells express Hm-inx3 and Hm-inx11.