From ZooTerms (Dictionary of Invertebrate Zoology)
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diaphragm (noun; Greek diaphragma, partition, wall): 1. Any of the horizontal dividing membranes of a body cavity.

2. A structure controlling admission of light through an aperture.

3. (Arthropoda: Insecta) a. In Heteroptera, separating the general body cavity from the genital chamber. b. In Lepidoptera, that which closes the body cavity caudally, comprised of dorsally the fultura superior and ventrally the fultura inferior.

4. (Bryozoa) a. In Stenolaemata, the membranous or skeletal partition which extends transversly across the entire zooidal chamber. b. In Gymnolaemata autozooids, the muscular ring of the body wall.

5. (Cnidaria: Hydrozoa) A delicate chitinous floor that supports the hydranth.

6. (Echiura) A thin-walled, funnel-like septum incompletely separating an anterior or peripharyngeal coelom from the general body cavity.