Fig. 4. Lophotrochin and trochin expression in the nemertean Maculaura alaskensis.

a Diagram of a torus-stage pilidium larva, cilia in green. Only one complement of the paired structures is shown for clarity (i.e., left lappet, left ciliary ridge, and left anterior and posterior axil). AX: axil, JR: juvenile rudiment, ROE: ciliated ridge of esophagus, ST: stomach. b The first expression of lophotrochin (purple) is noted at gastrula stage—in the developing archenteron, but not in ectodermal ciliated cells. c In later developmental stages, more structures with lophotrochin expression are found: stomach, ciliated ridges of esophagus, juvenile rudiment, the axils—recesses between lobes and lappets, the apical organ, and the primary ciliated band adjacent to the axils. Scale bars: 50 μm. For each in situ hybridization, at least 20 animals were stained and all had the pattern shown.