Figure 3: Mouth development in the onychophoran E. rowelli.

Note that three consecutive mouth openings arise during onychophoran ontogeny (indicated by green, red and white dotted lines, respectively). (a–e) Confocal micrographs of embryos at subsequent developmental stages⁵² in ventral view, labelled with the DNA-marker Hoechst as described previously⁶⁰. Anterior is up. Presumptive jaws are coloured artificially in yellow. (a) Embryonic disc of a late stage 1 embryo. Note the first embryonic mouth (em) arising at the anterior end of the slit-like blastopore. Developing segments are numbered. Arrowheads point to closing blastoporal lips. (b) Anterior end of an early stage 4 embryo. Note the position of the stomodaeum (second mouth) at the posterior border of the antennal segment. (c) Anterior end of a late stage 4 embryo. Note the beginning formation of lips that originate from three different segments (white dotted lines). Asterisks indicate openings of presumptive salivary glands. (d) Anterior end of a stage 5 embryo showing the formation of the definitive (third) mouth. Note that the stomodaeum and the anlagen of jaws and tongue are incorporated into the mouth cavity. (e) Anterior end of a stage 7 embryo. Mouth development is completed and lips surround the circular mouth opening. (f) Diagram of an adult onychophoran illustrating regions of digestive tract that correspond to the three consecutive embryonic mouth openings (dotted lines with Roman numerals I–III). Dorsal is up, anterior is right. an, developing anus; at, antenna; em, first embryonic mouth (derivative of the blastopore); jw, jaw; le1–le4, legs 1–4; lp, developing lips; mc, mouth cavity; mg, midgut; oe, oesophagus; ph, pharynx; so, stomodaeum; sp, slime papilla; to, presumptive tongue. Scale bars, 200 μm in a–e.