can also be deflected by solid objects or water flow. They are located on different body regions depending on the animal, such as on the tail for crayfish and lobsters. Neural excitation occurs when setae are moved in one direction, while inhibition occurs with movement in the opposite direction. Fish Fish and some aquatic amphibians detect hydrodynamic stimuli via a lateral line. This system consists of an array of sensors called neuromasts along the length of the fish's body. Neuromasts can be free-standing (superficial neuromasts) or within fluid-filled canals (canal neuromasts). The sensory cells within neuromasts are polarized hair cells contained within a gelatinous cupula. The cupula, and the stereocilia within, are moved by a certain amount depending on the movement of the surrounding water. Afferent nerve fibers are excited or inhibited depending on whether the hair cells they arise from are deflected in the preferred or opposite direction. Lateral line receptors form somatotopic maps within the brain informing the fish of amplitude and direction of flow at different points along the body. These maps are located in the medial octavolateral nucleus (MON) of the medulla and in higher areas such as the torus semicircularis. Mammals Detection of hydrodynamic stimuli in mammals typically occurs through use of hairs (vibrissae) or “push-rod” mechanoreceptors, as in platypuses. When hairs are used, they are often in the form of whiskers and contain a follicle-sinus complex (F-SC), making them different from the hairs with which humans are most familiar. Pinnipeds Pinnipeds, including sea lions and seals, use their mystacial vibrissae (whiskers) for active touch, including size and shape discrimination, and texture discrimination in seals. When used for touch, these vibrissae are moved to the forward position and kept still while the head moves, thus moving the vibrissae on the surface of an object. This is in contrast to rodents, which move the whiskers themselves to explore objects. More recently, research has been done to see if pinnipeds can use these same whiskers to detect hydrodynamic stimuli in addition to tactile stimuli. While this ability has been verified behaviorally, the specific neural circuits involved have not yet been determined. Seals Research on the ability of pinnipeds to detect hydrodynamic stimuli was first done on harbor seals (Phoca vitulina). It had been unclear how seals could find food in dark waters. It was found that a harbor seal that could use only its whiskers for sensory