Editing Nautilus (section)

==Anatomy==
[[File:Nautilus diagram-en.svg|upright=1.8|thumb|Diagram of the anatomical structure of a female ''N. pompilius'' including most of its internal organs.]]

===Tentacles===
The arm crown of modern nautilids (genera ''[[Nautilus (genus)|Nautilus]]'' and ''[[Allonautilus]]'') is very distinct in comparison to coleoids. Unlike the ten-armed [[Squid|Decabrachia]] or the eight-armed [[Octopodiformes]], nautilus may possess any number of tentacles (cirri) from 50 to  over 90 tentacles depending on the sex and individual.<ref>{{cite journal |last1=Kröger |first1=Björn |last2=Vinther |first2=Jakob |last3=Fuchs |first3=Dirk |title=Cephalopod origin and evolution: A congruent picture emerging from fossils, development and molecules: Extant cephalopods are younger than previously realised and were under major selection to become agile, shell-less predators |journal=BioEssays |date=August 2011 |volume=33 |issue=8 |pages=602–613 |doi=10.1002/bies.201100001|pmid=21681989 |s2cid=2767810 }}</ref><ref name=":1">{{cite book |last1=Sasaki |first1=T |last2=Shigeno |first2=S|last3=Tanabe |first3=K |date=2010 |title=Cephalopods - Present and Past |publisher=Tokai University Press |chapter=Anatomy of living Nautilus: reevaluation of primitiveness and comparison with Coeloidea |name-list-style=amp}}</ref> These tentacles are classified into three distinct categories: ocular, digital, and labial (buccal).<ref name=":2">{{cite book |last1=Owen |first1=Richard |title=Memoir on the pearly nautilus (Nautilus pompilius, Linn). With illustrations of its external form and internal structure |date=1832 |publisher=Richard Taylor |location=London |url=https://www.biodiversitylibrary.org/bibliography/10441}}</ref> There are two sets of ocular tentacles: one set in front of the eye (pre-ocular) and one set behind the eye (post-ocular). The digital and labial tentacles are arrayed circularly around the mouth, with the digital tentacles forming the outermost ring and the labial tentacles in between the digital tentacles and the mouth. There are 19 pairs of digital tentacles that, together with the ocular tentacles, make up the 42 appendages that are visible when observing the animal (not counting the modified tentacles that form the hood). The labial tentacles are generally not visible, being smaller than the digital tentacles, and more variable both in number and in shape.<ref name=":1" /><ref name=":2" /><ref>{{Cite journal |last=Griffin |first=Lawrence E. |date=1899 |title=XXVI.— Notes on the tentacles of Nautilus pompilius |journal=Annals and Magazine of Natural History |language=en |volume=3 |issue=14 |pages=170–176 |doi=10.1080/00222939908678098 |issn=0374-5481|url=https://www.biodiversitylibrary.org/part/58804 }}</ref> Males modify three of their labial tentacles into the [[Spadix (zoology)|spadix]], which delivers [[spermatophore]]s into the female during copulation.<ref name=":1" />

The tentacle is composed of two distinct structures: the first structure, a fleshy sheath that contains the second structure: an extendable [[cirrus (biology)|cirrus]] (plural: cirri).<ref name=":2" /><ref name=":3">{{Citation |last=Kier |first=William M. |title=The Functional Morphology of the Tentacle Musculature of Nautilus pompilius |date=2010 |url=https://doi.org/10.1007/978-90-481-3299-7_18 |work=Nautilus: The Biology and Paleobiology of a Living Fossil, Reprint with additions |pages=257–269 |editor-last=Saunders |editor-first=W. Bruce |access-date=2023-11-28 |series=Topics in Geobiology |volume=6 |place=Dordrecht |publisher=Springer Netherlands |language=en |doi=10.1007/978-90-481-3299-7_18 |isbn=978-90-481-3299-7 |editor2-last=Landman |editor2-first=Neil H.|url-access=subscription }}</ref> The sheaths of the digital tentacles are fused at their base into a single mass referred to as the cephalic sheath. The digital cirri can be fully withdrawn into the sheath and are highly flexible, capable of extending just over double their fully retracted length and show a high degree of allowable bendability and torsion.<ref name=":3" /> Despite not having suckers, the digital tentacles show strong adhesive capabilities. Adhesion is achieved through the secretion of a neutral (rather than acidic) [[Glycosaminoglycan|mucopolysaccharide]] from secretory cells in the ridges of the digital cirri.<ref>{{Citation |last=Fukuda |first=Yoshio |title=Histology of the Long Digital Tentacles |date=2010 |url=https://doi.org/10.1007/978-90-481-3299-7_17 |work=Nautilus: The Biology and Paleobiology of a Living Fossil, Reprint with additions |pages=249–256 |editor-last=Saunders |editor-first=W. Bruce |access-date=2023-11-28 |series=Topics in Geobiology |volume=6 |place=Dordrecht |publisher=Springer Netherlands |language=en |doi=10.1007/978-90-481-3299-7_17 |isbn=978-90-481-3299-7 |editor2-last=Landman |editor2-first=Neil H.|url-access=subscription }}</ref><ref>{{Cite journal |last1=Muntz |first1=W. R. A. |last2=Wentworth |first2=S. L. |date=1995 |title=Structure of the Adhesive Surface of the Digital Tentacles of Nautilus Pompilius |journal=Journal of the Marine Biological Association of the United Kingdom |language=en |volume=75 |issue=3 |pages=747–750 |doi=10.1017/S0025315400039163 |bibcode=1995JMBUK..75..747M |s2cid=83497566 |issn=1469-7769}}</ref><ref name=":4">{{Cite journal |last1=von Byern |first1=Janek |last2=Wani |first2=Ryoji |last3=Schwaha |first3=Thomas |last4=Grunwald |first4=Ingo |last5=Cyran |first5=Norbert |date=2012-02-01 |title=Old and sticky—adhesive mechanisms in the living fossil Nautilus pompilius (Mollusca, Cephalopoda) |journal=Zoology |volume=115 |issue=1 |pages=1–11 |doi=10.1016/j.zool.2011.08.002 |issn=0944-2006 |pmc=3311398 |pmid=22221553|bibcode=2012Zool..115....1V }}</ref> Release is triggered through contraction of the tentacle musculature rather than the secretion of a chemical solvent, similar to the adhesion/release system in ''[[Euprymna]]'', though it is unclear whether these adhesives are [[Homology (biology)|homologous]].<ref name=":4" /><ref>{{Cite journal |last1=von Byern |first1=Janek |last2=Cyran |first2=Norbert |last3=Klepal |first3=Waltraud |last4=Nödl |first4=Marie Therese |last5=Klinger |first5=Lisa |date=February 2017 |title=Characterization of the adhesive dermal secretion of Euprymna scolopes Berry, 1913 (Cephalopoda) |url=https://linkinghub.elsevier.com/retrieve/pii/S0944200616300836 |journal=Zoology |language=en |volume=120 |pages=73–82 |doi=10.1016/j.zool.2016.08.002|pmid=27646066 |bibcode=2017Zool..120...73V |url-access=subscription }}</ref> The ocular tentacles show no adhesive capability but operate as sensory organs. Both the ocular tentacles and the eight lateral digital tentacles show chemoreceptive abilities; the preocular tentacles detect distant odor and the lateral digital tentacles detect nearby odor.<ref>{{Cite journal |last1=Basil |first1=Jennifer |last2=Bahctinova |first2=Irina |last3=Kuroiwa |first3=Kristine |last4=Lee |first4=Nandi |last5=Mims |first5=Desiree |last6=Preis |first6=Michael |last7=Soucier |first7=Christian |date=2005 |title=The function of the rhinophore and the tentacles of Nautilus pompilius L. (Cephalopoda, Nautiloidea) in orientation to odor |journal=Marine and Freshwater Behaviour and Physiology |language=en |volume=38 |issue=3 |pages=209–221 |doi=10.1080/10236240500310096 |bibcode=2005MFBP...38..209B |s2cid=33835096 |issn=1023-6244}}</ref><ref>{{Cite journal |last1=Ruth |first1=Peter |last2=Schmidtberg |first2=Henrike |last3=Westermann |first3=Bettina |last4=Schipp |first4=Rudolf |date=March 2002 |title=The sensory epithelium of the tentacles and the rhinophore of Nautilus pompilius L. (cephalopoda, nautiloidea) |journal=Journal of Morphology |volume=251 |issue=3 |pages=239–255 |doi=10.1002/jmor.1086 |issn=0362-2525 |pmid=11835362|s2cid=38377665 }}</ref>

===Digestive system===
The [[radula]] is wide and distinctively has nine teeth.

The mouth consists of a [[cephalopod beak|parrot-like beak]] made up of two interlocking jaws capable of ripping the animal's food— mostly crustaceans— from the rocks to which they are attached.<ref name=grif>{{Cite book
|title=The anatomy of ''Nautilus pompilius''
|last=Griffin
|first=Lawrence E.
|volume=8
|year=1900
|publisher=Government Printing Office
|place=Washington, D.C.
|url={{google books |plainurl=y |id=IHJBAQAAIAAJ}}
|oclc=18760979
|doi=10.5962/bhl.title.10466
}}</ref>{{rp|p. 105}} Males can be superficially differentiated from females by examining the arrangement of tentacles around the buccal cone: males have a [[Spadix (zoology)|spadix]] organ (shaped like a spike or shovel) located on the left side of the cone making the cone look irregular, whereas the buccal cone of the female is [[Symmetry in biology|bilaterally symmetrical]].<ref name=grif/>{{rp|pp. 115–130}}

The crop is the largest portion of the digestive tract, and is highly extensible. From the crop, food passes to the small muscular stomach for crushing, and then goes past a digestive caecum before entering the relatively brief intestine.

===Circulatory system===
Like all cephalopods, the blood of the nautilus contains [[hemocyanin]], which is blue in its oxygenated state. There are two pairs of [[gill]]s which are the only remnants of the ancestral [[Metamerism (biology)|metamerism]] to be visible in extant cephalopods.<ref name=Wingstrand1985>{{cite journal
|url=http://www.zmuc.dk/inverweb/Galathea/Galathea_p5.html
|format=Link to free full text + plates
|last=Wingstrand
|first=KG
|year=1985
|title=On the anatomy and relationships of Recent Monoplacophora
|journal=Galathea Rep.
|volume=16
|pages=7–94
|access-date=2009-04-21
|archive-url=https://web.archive.org/web/20160303182836/http://www.zmuc.dk/inverweb/Galathea/Galathea_p5.html
|archive-date=2016-03-03
|url-status=dead
}}</ref>{{rp|56}} Oxygenated blood arrives at the heart through four ventricles and flows out to the animal's organs through distinct aortas but returns through veins which are too small and varied to be specifically described. The one exception to this is the vena cava, a single large vein running along the underside of the crop into which nearly all other vessels containing deoxygenated blood empty. All blood passes through one of the four sets of filtering organs (composed of one pericardial appendage and two renal appendages) upon leaving the vena cava and before arriving at the gills for re-oxygenation. Blood waste is emptied through a series of corresponding pores into the [[pallial cavity]].

===Nervous system===
The central component of the nautilus nervous system is the oesophageal [[nerve ring]] which is a collection of [[ganglia]], [[commissure]]s, and connectives that together form a ring around the animal's oesophagus. From this ring extend all of the nerves forward to the mouth, tentacles, and funnel; laterally to the eyes and [[rhinophore]]s; and posteriorly to the remaining organs.

The nerve ring does not constitute what is typically considered a cephalopod "brain": the upper portion of the nerve ring lacks differentiated lobes, and most of the nervous tissue appears to focus on finding and consuming food (i.e., it lacks a "higher learning" center). Nautili also tend to have rather short memory spans, and the nerve ring is not protected by any form of brain case.<ref>{{cite journal
 | last =Young
 | first =J. Z.
 | title =The Central Nervous System of Nautilus
 | journal =Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences
 | volume =249
 | issue =754
 | pages =1–25
 | date =27 May 1965
 | jstor =2416631
 | doi =10.1098/rstb.1965.0006
 | bibcode =1965RSPTB.249....1Y
 | s2cid =83951270
 | doi-access =
 }}</ref>

===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]].

===Size===
''N. pompilius'' is the largest species in the genus. One form from [[Indonesia]] and northern [[Australia]], once called ''N. repertus'', may reach {{cvt|25.4|cm}} in diameter.<ref>Pisor, D.L. (2008). ''[[Registry of World Record Size Shells]]''. Fifth edition. ConchBooks, Hackenheim. 207&nbsp;pp. {{ISBN|0615194753}}.</ref> However, most nautilus species never exceed {{cvt|20|cm|in|0}}. ''[[Bellybutton Nautilus|Nautilus macromphalus]]'' is the smallest species, usually measuring only {{cvt|16|cm|in|frac=2}}. A dwarf population from the [[Sulu Sea]] (''[[Nautilus pompilius suluensis]]'') is even smaller, with a mean shell diameter of {{cvt|11.56|cm}}.<ref name="Dunstan et al. 2011" />