Siebolds organ see crista acoustica

sieve area (PORIF) The area containing inhalant apertures.

sieve plate 1. (echinod) see madreporite. 2. (porif) A plate-like porous structure below the osculum.

sieve tracheae (ARTHRO: Chelicerata) In Ricinulei, bundles of tracheae that arise from a tubule of an ectodermal invagination on the 8th somite. see tube tracheae.

sigillum n.; pl. sigilla (ARTHRO: Chelicerata) 1. In some spiders, the impressed, suboval, clear areas on the sternum. 2. In Acari, the external mark of a muscle insertion.

sigma n. [Gr. the 16th letter, sigma] 1. (ARTHRO: Insecta) See furca. 2. (PORIF) A C-shaped diactinal microsclere.

sigmaspires [Gr. the 16th letter, sigma; speira, twist] (PORIF) Diactinal microsclere sigmas that are spirally twisted.

sigmoid a. [Gr. the 16th letter, sigma; eidos, form] Shaped like the letter S; sigmoidal.

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