From ZooTerms (Dictionary of Invertebrate Zoology)
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septum (noun; plural -ta; Latin septum, partition): 1. Any dividing wall, membrane or partition separating cavities or masses of tissue; a dissepiment.

2. (Annelida) Two layers of peritoneal cells enclosing muscle fibers, and blood vessels that separate adjacent segments.

3. (Arthropoda: Crustacea) For barnacles, see transverse septum.

4. (Brachiopoda) Long, narrow, elevation of the secondary (shell) layer, usually bladelike. a. In Articulata, within underlying floor of valve with high, narrow deflections of fibrous calcite starting near the primary layer. b. In Inarticulata, comparable deflections of the shell lamellae.

5. (Mollusca: Cephalopoda) A calcareous tube that supports the siphuncle.

6. (Mollusca: Gastropoda) A transverse plate secreted with early formed whorls of shell.