In the field of zoology, a spadix (pronounced //, "SPAY-dicks"; plural spadices //, "spay-DIE-seez") is a secondary sexual organ found in some cephalopods and hydrozoans. In the Nautilus genus, the spadix is a composite erectile organ in the male located in the oral region which is composed of four highly modified tentacles and which is paired with a somewhat smaller antispadix that is also composed of four tentacles. The spadix is normally a concealed organ but quickly becomes distended upon the animal's death. The exact function of the spadix and antispadix in Nautilus is not yet known.
- Arthur Giese (2 December 2012). Reproduction of Marine Invertebrates V4: Molluscs : Gastropods and Cephalopods. Elsevier Science. p. 233. ISBN 978-0-323-15305-8.