Fig. 2. Diagram of the plumbing circuits associated with the reversible-flow olfactometer. A standard crayfish head preparation was secured in a recording chamber (not shown), and the lateral antennular flagellum on one side was inserted into the olfactometer and sealed at the base with Vaseline^TM. Fluid could be introduced to the olfactometer either through port A, at the base of the antennule, or alternatively through port B, beyond the tip of the flagellum. Solenoid switches were controlled so that when either water or odorant entered through one port, the exhaust to the opposite port was simultaneously opened. The diagram shows an instance when water is entering port A, flowing through the olfactometer past the flagellum in the PD direction, and out through exhaust B (arrows).