Abstract
Oral opening of multicellular organisms is generally formed at the anterior end of the animal. Phylum Cnidaria is so far the only exception where oral opening is formed at the posterior end of the animal expressing Wnt-3a homologues (1), a typical gene which is expressed specifically at the posterior end. Why this opposite oral-aboral polarity relative to A-P polarity appeared only in this phylum remains unknown. Expression of Hox-1 homologues at the oral end has been proposed as evidence that oral end represents anterior end even in Cnidaria (2). Here we report that the aboral end of Hydra bears several similarities to the oral end of higher organisms. [J Physiol Sci. 2006;56 Suppl:S101]
