Expression of annelid otx and six3 genes. In the polychaete annelids Platynereis (a-f) and Pristina (g-k), six3 orthologues (a, c-e, g, i) are expressed anterior of otx orthologues (b-d, f, h, k). Single (a, b, e-k) or two-colour (c, d) whole-mount in situ hybridisations. Twenty-four hours (a-d) or 48 h (e, f) old Platynereis larvae. Pristina early (g, h) and late (i, k) fission stage. Asterisks in (a, b) point out stomodaeal expression (out of focus). Dashed line: Prototroch ciliary band. (c,d) Blue: nuclear DAPI stain. (i, k) Dotted line: Boundary of two forming worms dividing by fission; continuous line: Plo-six3/Plo-otx expression boundary. Arrows: Tentacles protruding dorsally from the anterior tip of the forming worm.