From ZooTerms (Dictionary of Invertebrate Zoology)
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ostium (noun; plural ostia; Latin ostium, door): 1. Any opening to a passage, usually associated with a valve or circular muscle.

2. (Arthropoda) The paired slitlike openings in the heart.

3. (Arthropoda: Insecta) In male Lepidoptera, the opening or area through which the internal pouch is everted during copulation.

4. (Mollusca: Bivalvia) One of many tiny holes in the gill walls allowing currents of water through the gills.

5. (Porifera) Pore openings entering the interior cavity (the spongocoel or atrium) of sponges; incurrent pores; inhalent pores.