Olfactory systems: common design, uncommon origins?

In both vertebrates and invertebrates, odorant molecules reach the dendrite s of olfactory receptor cells through an aqueous medium, which reflects the evolutionary origin of these systems in a marine environment. Important re cent advances, however, have demonstrated striking interphyletic difference s between the structure of vertebrate and invertebrate olfactory receptor p roteins, as well as the organization of the genes encoding them. While thes e disparities support independent origins for odor-processing systems in cr aniates and protostomes (and even between the nasal and vomeronasal systems of craniates), olfactory neuropils share close neuroanatomical and physiol ogical characters. Whereas there is a case to be made for homology among me mbers of the two great protostome clades (the ecdysozoans and lophotrochozo ans), the position of the craniates remains ambiguous.