Editing Nautilus (section)

===Shell===
[[Image:NautilusCutawayLogarithmicSpiral.jpg|thumb|Nautilus half-shell showing the camerae in a [[logarithmic spiral]]|alt=]][[File:Nautilus Section cut.jpg|thumb|Section cut of a nautilus shell]]
{{multiple image
| align = right
| total_width = 220
| image1 = NautilusTop.jpg
| alt1 = 
| caption1 = 
| image2 = NautilusBottom.jpg
| alt2 = 
| caption2 = 
| footer = A nautilus shell viewed from above (left), and from underneath  (right)
}}

Nautili are the sole living cephalopods whose bony body structure is externalized as a planispiral [[Exoskeleton|shell]]. The animal can withdraw completely into its shell and close the opening with a leathery hood formed from two specially folded [[tentacle]]s. The shell is coiled, [[aragonite|aragonitic]],<ref>{{Cite journal| last1 = Buchardt| first1 = B.| title = Diagenesis of aragonite from Upper Cretaceous ammonites: a geochemical case-study| journal = Sedimentology| volume = 28| issue =  3| pages = 423–438| year = 1981| doi = 10.1111/j.1365-3091.1981.tb01691.x| last2 =  Weiner| first2 = S.|bibcode = 1981Sedim..28..423B }}</ref> [[nacre]]ous and pressure-resistant, imploding at a depth of about {{cvt|800|m|ft|-2}}. The nautilus shell is composed of two layers: a [[Gloss (material appearance)|matte]] white outer layer with dark orange stripes,<ref>{{cite web |url= https://marinesanctuary.org/blog/sea-wonder-chambered-nautilus/ |title= Sea Wonder: Chambered Nautilus |date= 21 August 2021 |website= National Marine Sanctuary Foundation |access-date= 26 November 2022}}</ref> and a striking white [[iridescence|iridescent]] inner layer. The innermost portion of the shell is a pearlescent blue-gray. The [[osmeña pearl]], contrarily to its name, is not a [[pearl]], but a [[jewellery]] product derived from this part of the shell.{{Citation needed|date=August 2024}}

Internally, the shell divides into [[Camera (cephalopod)|camerae]] (chambers), the chambered section being called the [[phragmocone]]. The divisions are defined by [[Septa (biology)|septa]], each of which is pierced in the middle by a duct, the [[siphuncle]]. As the nautilus matures, it creates new, larger camerae and moves its growing body into the larger space, sealing the vacated chamber with a new septum. The camerae increase in number from around 4 at the moment of [[egg (biology)|hatching]] to 30 or more in adults.

The shell coloration also keeps the animal [[cryptic (zoology)|cryptic]] in the water. When seen from above, the shell is darker in color and marked with irregular stripes, which helps it blend into the dark water below. The underside is almost completely white, making the animal indistinguishable from brighter waters near the surface. This mode of [[camouflage]] is called [[countershading]].

The nautilus shell presents one of the finest natural examples of a [[logarithmic spiral]], although it is not a [[golden spiral]]. The use of nautilus shells in art and literature is covered at [[Chambered Nautilus#In literature and art|nautilus shell]].