(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.