Final answer:
The shape of the basilar membrane is spiral-shaped.
Step-by-step explanation:
The shape of the basilar membrane is spiral-shaped. It is thinner, floppier, and broader towards the apex or center of the cochlea, where it flexes back and forth in a wave-like fashion. Different regions of the basilar membrane vibrate according to the frequency of the sound wave, allowing the fluid-filled cochlea to detect different wave frequencies at different regions of the membrane.