Fig. 4. Schematic diagram of towing-tank. The model of the antennule was attached to a computer-controlled motorized rail table (Daedal MC6023) positioned over the top of the tank and towed through the mineral oil. Neutrally buoyant marker particles (Potter Industries© 11 µm silver-coated hollow glass spheres) in the mineral oil were illuminated by a sheet of laser light and imaged using a Redlake Motionscope PCI 1000s video camera at 60 frames per second. The camera was mounted directly above the tank on the same towing rig as the model, and thus captured images of the particle motion relative to the model. The model was towed in both a horizontal (as shown) and vertical orientation. Towing the model from right to left simulates an antennule in the `downward' flicking orientation, while a slower tow from left to right simulates an antennule in the `upstroke' return orientation.