Vestibular system (Wiki)
This article on how humans maintain their balance is provided for reference.
The infundibular balance organ in amphioxus larvae (Goog)
Only abstract available online. I got the PDF through the library.
Also discussed in Anterior Cerebral Vesicle.
"The balance organ lies just in front of the infundibular cells and consists of 10 electron-dense cells with long, bulbous cilia, each surrounded by a ring of accessory cells. The ciliary bulb cells have axons that terminate in vesicle-filled swellings that lack identifiable synapses. The accessory cells have short basal processes that are minor contributors to the adjacent neuropile. Based on morphology, we suggest a mechanosensory function for the ciliary bulb cells, possibly related to balance or motion detection."
The infundibular balance organ in amphioxus larvae and related aspects of cerebral vesicle organization
Link to abstract only, but I have the PDF.