Cochlea is a coiled structure that looks like a snail’s shell. It is a fluid filled structure present in the inner ear (of the membranous labyrinth) and it contains the sound detecting “organ of corti (spiral organ).”
The organ of corti is made up of sensory cells called hair cells (hearing receptors). These hair cells are in contact with the fibres of cochlear nerve (a part of 8th cranial nerve).
Thus, the blank can be filled with hair cells; cochlea.