Amphioxus Balance Organ

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.

Lacalli: 2001 
The infundibular balance organ in amphioxus larvae and related aspects of cerebral vesicle organization
Acta Zoologica
Link to abstract only, but I have the PDF.