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Abstract

Nervous systems are complex networks that allow animals to sense their environment and coordinate specific reactions. However, little is known about their origin and how the first neuronal circuits functioned. The sea anemone Exaiptasia diaphana, a cnidarian and sister group to all bilaterians, holds a key position for comparative studies. Alongside ctenophores, cnidarians are the only non-bilaterian phyla with a nervous system. Here, I present the first comprehensive volume electron microscopy analysis of Exaiptasia’s planula. This marine, ciliated, free-swimming larva likely reflects characteristics of the common metazoan ancestor. I reconstructed and annotated 1,285 cells in a whole-body serial EM dataset, identified 16 cell types and mapped their anatomical distribution. I describe specific features of epithelial, gland, stinging, sensory cells and neurons. Different kinds of synapses and their circuits are analyzed with respect to their putative functional architecture. The connectome reveals that interneurons serve as central integrators, while direct sensory-to-motor pathways are absent. Only a limited number of cell types communicate via chemical synapses, suggesting neuropeptides are still the primary communication pathway. My work provides an essential foundation for understanding the evolution of nervous systems. Together with future cross-species comparisons, it will help uncover mechanisms underlying the emergence of neuronal systems.