Many animals in the ocean use appendages bearing arrays of hairs to capture molecules from the surrounding fluid (e.g. feathery gills take up oxygen; olfatory antennae capture odorants), to capture food particles (e.g. hairy suspension-feeding appendages catch single-celled algae), or to move the fluid around them (e.g. setulose appendages are used to swim or create ventilatory currents). Since hairy little appendages serve such important biological functions in animals from so many phyla, we have been trying to elucidate the basic rules governing how they all work.