This article on human hearing is provided for reference.
Molecular evolution of the vertebrate mechanosensory cell and ear (Goog)
"However, recent years have shown that an ancestral global
patterning of the body exists combined with a diffuse nerve net
and singly distributed sensory cells. Moreover, the basiepithelial nerve net or skin brain maybe the ancestral condition in deuterostomes out of which the central nervous system evolved through concentration in a specific region ...
Comparable to this focal formation of a central nervous system is the local formation of all major sensory organs in chordates. ... the evolutionary precursor of the brain is the diffuse nerve net whereas the evolutionary precursors of the ear might be single, diffusely distributed mechanosensory cells. ... Indeed, in many chordates and also in amphioxus there is distribution of single sensory cells in the skin..."
Cells, molecules and morphogenesis: the making of the vertebrate ear (PubMed)
"The development and evolution of mechanosensory cells and the vertebrate ear is reviewed ... Outgroup comparisons suggests that mechanosensory cells are ancient features of multicellular organisms. Molecular evidence suggests that key genes involved in mechanosensory cell function and development are also conserved among metazoans. The divergent morphology of mechanosensory cells across phyla is interpreted here as 'deep molecular homology' that was in parallel shaped into different forms in each lineage."