(A) Connectivity map of all possible known neuropeptide-GPCR signaling, colors represent modules defined by randomized community detection analysis in Gephi and node size represents authority. (B) Reconstructed INRGWaand INNOS projection neurons (ANS) (shades of brown and cyan) in a full body transmission electron microscopy (ssTEM) dataset of a 3-day-old larva, shown against a framework of ANS neurons (light grey). Axons and dendrites appear as lines and cell body positions are represented by spheres. (C) ssTEM reconstruction of INRGWa projection neurons and Ser-h1 serotonergic neurons. Red spheres indicate INRGWa presynaptic sites. (D) ssTEM reconstruction of INRGWa projection neurons and a presynaptic ciliomotor neuron, cMNvl. (E) ssTEM reconstruction of INRGWa neurons and a presynaptic sensory neuron, pygPDbicil. (F) Synaptic connectivity graph of INRGWa, Ser-h1, pygPDbicil, and cMNvl neurons. Synaptic inputs from the synaptic nervous system to the INRGWa neuron are in red. Peptidergic inputs to the INRGWa neurons are indicated with dashed arrows. (G) ssTEM reconstruction of INRGWa projection neurons, Ser-h1 neurons, and the cholinergic ciliomotor MC neuron, anterior view (H) Confocal microscopy image of correlated pixels of GCaMP6s signal in a 2-day-old Platynereis larva after the addition of 25 µM D-achatin neuropeptide, anterior view. Cells showing correlated activity with the serotonergic neurons (red) and the MC cell (cyan) are shown. INRGWa-dcl2 could not be identified in this larva and is likely obscured by the MC cell, Ser-h1l and/or INRGWa-dcl1. (I) Neuronal activity patterns of individually identified neurons in a 2-day-old larva treated with 25 µM achatin.