FMRFamidergic nervous system development in P. lamarckii. Scale bars = 50 μm. Apical is to the top of the figure in each panel unless indicated. A. Early gastrula, lateral view. A FMRFamidergic region can be seen in the apical organ (ao). B. Early trochophore, left lateral view. The mouth (m) has reached its final location, but no further FMRFamidergic structures are visible. C. Early trochophore, left lateral view. A FMRFamidergic nerve ring is visible around the opening of the stomach to the intestine (ir), and a ventral process can be seen leading posteriorly from the apical organ (arrowhead). D. Late trochophore, left lateral view. A nerve ring around the mouth is visible (arrowhead). E. Elongating trochophore, left lateral view. Elements of the cerebral ganglion (cg) are now developing, and are connected to the ventral nerve cords (vnc) by the circumoesophageal connectives (cc). F. Metatrochophore, ventral view. The cerebral ganglion is now fully formed and additional FMRFamidergic fibres are seen lateral to it. Elements of the apical organ are still visible (*). Commissures are apparent between the ventral nerve cords. A medial nerve (mn) runs between the ventral nerve cords and is more evident at the posterior pole of the larva. Fibres can be seen encircling the intestine (arrowhead). Chaetae and uncini are autofluorescent. G. Juvenile, ventral view, anterior is towards the top. The FMRFamidergic nervous system is similar to that in the metatrochophore. Fibres can be seen along each side of the intestine (arrowhead). Remnants of the mucous tube (mu) are apparent.