The oral nerve plexus in amphioxus larvae: function, cell types and phylogenetic significance

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Abstract
Serial electron microscope reconstructions were used to examine the organization and cell types of the nerve plexus that surrounds the mouth in amphioxus larvae. The plexus is involved in a rejection response that occurs during feeding: a number of oral spines project across the mouth, and debris impinging on them triggers a contraction of the gill slit and pharyngeal musculature that forces water through the mouth, dislodging the debris. The oral spine cells are secondary sense cells that synapse with neurites belonging to a class of peripheral interneurons intrinsic to the oral nerve plexus. These in turn synapse with a second class of peripheral neurons with large axons that we interpret as sensory cells and which probably transmit signals to the nerve cord. The intrinsic cells also appear to synapse with each other, implying that local integrative activities of some complexity occur in the oral plexus. In comparative terms, the intrinsic neurons most closely resemble the Merkel–like accessory cells of vertebrate taste buds, and we postulate a homology between oral spine cells and taste buds, despite differences in function. There are also similarities between the amphioxus oral plexus and adoral nerves and ganglia of echinoderm larvae, suggesting homology of both the oral nerve plexus and the mouth itself between lower deuterostome phyla and chordates.