Final answer:
The helicotrema is the structure connecting the scala vestibuli and scala tympani, allowing fluid waves to pass between them in the inner ear.
Step-by-step explanation:
The structure that connects the scala vestibuli with the scala tympani is called the helicotrema. The scala vestibuli begins at the oval window and extends above the cochlear duct, curving over at the cochlea's apex to become the scala tympani.
The scala tympani then travels back down to the base of the cochlea under the cochlear duct and ends at the round window. It's at the helicotrema where the scala vestibuli meets the scala tympani, allowing the fluid waves to pass between them.