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an organ of chemoreception that is part of the olfactory system of amphibians, reptiles, and mammals, although it does not occur in all tetrapod groups. It is a patch of sensory cells within the main nasal chamber that detects heavy moistureborne odour particles. Airborne odours, in contrast, are detected by the olfactory sensory cells located in the main nasal chambers. Some groups of mammals also initiate a behaviour known as the flehmen response, in which the animal facilitates the exposure of the vomeronasal organ to a scent or pheromone by opening the mouth and curling the upper lip during inhalation.