Gut formation during P. caudatus embryogenesis. z projections of confocal stacks of embryos at 5, 6, 7, and 8 days post-fertilization, hatching larva, and the first lorica larva, stained with phallacidin (green) and propidium iodide (magenta). (A) Post-gastrula embryos exhibit a parenchymatous endomesoderm (em), a ventrally forming mouth (mo), and a narrowed blastopore (bp) that will give rise to the anus. (B) With the onset of organogenesis, the mouth moves to an anterior terminal position, and a digestive tract (dg) connecting the mouth and the anus (an) is visible as a strongly actin-positive bundle. Muscle (mc) differentiation also starts at this stage. (C,
D) After organogenesis, the introvert retracts into the trunk, pulling down the digestive tract and mouth to the posterior end of the trunk. The scalids (sc), which develop at the introvert-trunk boundary (B, B’), are located at the anterior end of the introvert as it retracts. (E) The hatching larva exhibits a fully developed digestive system, despite the lack of a mouth and anal opening in the cuticle. (F) With the first molting, the first lorica larva exhibit greater body complexity, and the digestive system increases its number of cells generally. (A’-F’) Schematic drawings of the studied embryonic and larval stages, placed in the general context of priapulid embryonic development. The pairs of arrowheads indicate the position of the introvert-trunk boundary. Drawings are not to scale. All panels and drawings are oriented with the animal/anterior pole to the top. In A, ventral side to the left. Scale bars in A-D 50 μm; and E and F, 100 μm.