From ZooTerms (Dictionary of Invertebrate Zoology)
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osphradium (noun; plural -dia; Greek osphradion, strong smell): (Mollusca) 1. In Gastropoda, a small sensory organ on the posterior margin of each afferent gill membrane that functions as a chemoreceptor and also determines the amount of sediment in the inhalant current. 2. In Bivalvia, located in the exhalant chamber, doubtfully homologous to Gastropods.