View source for Biology:Cephalopod

{{Short description|Class of mollusks}}
{{Automatic taxobox
| name = Cephalopoda
| fossil_range = {{Geological range|Late Cambrian|Recent|earliest=Cambrian|[[Earth:Late Cambrian|Late Cambrian]] – present;<ref name="mollusca11">{{The Mollusca|volume=11}}</ref> possible Early Cambrian presence<ref name="ReferenceD">{{Cite journal |doi=10.1038/s42003-021-01885-w |title=A potential cephalopod from the early Cambrian of eastern Newfoundland, Canada |year=2021 |last1=Hildenbrand |first1=Anne |last2=Austermann |first2=Gregor |last3=Fuchs |first3=Dirk |last4=Bengtson |first4=Peter |last5=Stinnesbeck |first5=Wolfgang |journal=Communications Biology |volume=4 |issue=1 |page=388 |pmid=33758350 |pmc=7987959}}</ref>}}
| image = Cephalopoda diversity.jpg
| image_upright = 1.2
| image_caption = Extant and extinct cephalopods; clockwise from top-left: [[Biology:Common octopus|common octopus]] (''Octopus vulgaris''), [[Biology:Caribbean reef squid|Caribbean reef squid]] (''Sepioteuthis sepioidea''), chambered nautilus (''Nautilus pompilius''), ''Orthosphynctes'', ''[[Biology:Clarkeiteuthis|Clarkeiteuthis conocauda]]'', and common cuttlefish (''Sepia officinalis'')
| taxon = Cephalopoda
| authority = [[Biography:Georges Cuvier|Cuvier]], 1797
| subdivision_ranks = Subclasses
| subdivision = *[[Biology:Nautiloid|Nautiloid]]ea ''sensu lato'' (paraphyletic)
**[[Biology:Plectronoceratoidea|Plectronoceratoidea]]&nbsp;† (paraphyletic)
**[[Biology:Multiceratoidea|Multiceratoidea]]&nbsp;† (paraphyletic?)
**[[Biology:Nautiloid|Nautiloid]]ea ''sensu stricto''
**Endoceratoidea&nbsp;†
**[[Biology:Orthoceratoidea|Orthoceratoidea]]&nbsp;† (paraphyletic)
*Bactritoidea&nbsp;† (paraphyletic)
*[[Biology:Ammonoidea|Ammonoidea]]&nbsp;†
*[[Biology:Coleoidea|Coleoidea]]
*?Paracoleoidea&nbsp;† (disputed)
}}

A '''cephalopod''' {{IPAc-en|ˈ|s|ɛ|f|ə|l|ə|p|ɒ|d}} is any member of the [[Biology:Mollusca|mollusca]]n [[Biology:Taxonomic rank|class]] '''Cephalopoda''' {{IPAc-en|s|ɛ|f|ə|ˈ|l|ɒ|p|ə|d|ə}} ([[Social:Greek language|Greek]] plural {{lang|grc|κεφαλόποδες}}, {{transliteration|grc|kephalópodes}}; "head-feet")<ref>{{Cite book |last1=Queiroz |first1=K. |last2=Cantino |first2=P. D. |last3=Gauthier |first3=J. A. |title=Phylonyms: A Companion to the PhyloCode |publisher=CRC Press |year=2020 | page=1843 |isbn=978-1-138-33293-5}}</ref> such as a [[Biology:Squid|squid]], [[Biology:Octopus|octopus]], [[Biology:Cuttlefish|cuttlefish]], or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or [[Biology:Tentacle|tentacle]]s ([[Biology:Muscular hydrostat|muscular hydrostat]]s) modified from the primitive molluscan foot. Fishers sometimes call cephalopods "'''inkfish'''", referring to their common ability to squirt ink. The study of cephalopods is a branch of [[Biology:Malacology|malacology]] known as [[Biology:Teuthology|teuthology]].

Cephalopods became dominant during the [[Earth:Ordovician|Ordovician]] period, represented by primitive [[Biology:Nautiloid|nautiloid]]s. The class now contains two, only distantly related, extant subclasses: [[Biology:Coleoidea|Coleoidea]], which includes [[Biology:Octopus|octopus]]es, [[Biology:Squid|squid]], and [[Biology:Cuttlefish|cuttlefish]]; and [[Biology:Nautiloid|Nautiloid]]ea, represented by ''[[Biology:Nautilus (genus)|Nautilus]]'' and ''[[Biology:Allonautilus|Allonautilus]]''. In the Coleoidea, the molluscan shell has been internalized or is absent, whereas in the Nautiloidea, the external shell remains. About 800 living [[Biology:Species|species]] of cephalopods have been identified. Two important extinct [[Biology:Taxon|taxa]] are the [[Biology:Ammonoidea|Ammonoidea]] (ammonites) and [[Biology:Belemnoidea|Belemnoidea]] (belemnites). Extant cephalopods range in size from the 10&nbsp;mm (0.3&nbsp;in) ''Idiosepius thailandicus'' to the 14&nbsp;m (45.1&nbsp;ft) [[Biology:Colossal squid|colossal squid]], the largest extant [[Biology:Invertebrate|invertebrate]].

==Distribution==
{{multiple image
 | align    = right
 | footer   = '''Left:''' A pair of cuttlefish (''Sepia officinalis'') in shallow water<br />'''Right:''' An octopus (''Benthoctopus'' sp.) on the [[Earth:Davidson Seamount|Davidson Seamount]] at 2,422 m depth
 | image1   = Sepia officinalis Linnaeus, 1758 .jpg
 | width1   = 220
 | image2   = Benthoctopus sp.jpg
 | width2   = 190
}}
There are over 800 extant species of cephalopod,<ref>{{cite web |url=http://cephbase.eol.org/ |title=Welcome to CephBase |work=CephBase |access-date=29 January 2016 |archive-date=12 January 2016 |archive-url=https://web.archive.org/web/20160112225313/http://cephbase.eol.org/ |url-status=dead }}</ref> although new species continue to be described. An estimated 11,000 extinct [[Biology:Taxon|taxa]] have been described, although the soft-bodied nature of cephalopods means they are not easily fossilised.<ref name=mollusca12>{{The Mollusca|volume=12}}</ref>

Cephalopods are found in all the oceans of Earth. None of them can tolerate [[Earth:Fresh water|fresh water]], but the brief squid, ''[[Biology:Lolliguncula brevis|Lolliguncula brevis]]'', found in Chesapeake Bay, is a notable partial exception in that it tolerates [[Earth:Brackish water|brackish water]].<ref>{{cite journal |last1=Bartol |first1=I. K. |last2=Mann |first2=R. |last3=Vecchione |first3=M. |year=2002 |title=Distribution of the euryhaline squid ''Lolliguncula brevis'' in Chesapeake Bay: effects of selected abiotic factors |journal=Marine Ecology Progress Series |volume=226 |pages=235–247 |doi=10.3354/meps226235 |bibcode=2002MEPS..226..235B |doi-access=free }}</ref> Cephalopods are thought to be unable to live in fresh water due to multiple biochemical constraints, and in their >400&nbsp;million year existence have never ventured into fully freshwater habitats.<ref>{{cite web |url=http://www.abc.net.au/science/articles/2013/01/16/3670198.htm |title=Are there any freshwater cephalopods? |website=ABC Science |date=16 January 2013}}</ref>

Cephalopods occupy most of the depth of the ocean, from the [[Earth:Abyssal plain|abyssal plain]]s to the sea surface, and have also been found in the [[Earth:Hadal zone|hadal zone]].<ref>[https://www.labroots.com/trending/plants-and-animals/17819/cephalopods-observed-record-shattering-oceanic-depths Cephalopods Observed At Record-Shattering Oceanic Depths]</ref> Their diversity is greatest near the equator (~40 species retrieved in nets at 11°N by a diversity study) and decreases towards the poles (~5 species captured at 60°N).<ref name=Cephalopods/>

==Biology==
=== Nervous system and behavior ===
{{multiple image
 | align    = right
 | footer   = '''Left:''' An octopus opening a container with a screw cap<br />'''Right:''' Hawaiian bobtail squid, ''Euprymna scolopes'', burying itself in the sand, leaving only the eyes exposed
 | image1   = Oktopus opening a container with screw cap 01.jpg
 | width1   = 179
 | image2   = Euprymna_scolopes_(Bobtail_squid)_behavior.jpg
 | width2   = 202
}}
Cephalopods are widely regarded as the most intelligent of the [[Biology:Invertebrate|invertebrate]]s and have well developed senses and large [[Biology:Brain|brain]]s (larger than those of gastropods).<ref>{{cite book |last1=Tricarico |first1=E. |last2=Amodio |first2=P. |last3=Ponte |first3=G. |last4=Fiorito |first4=G. |year=2014 |chapter=Cognition and recognition in the cephalopod mollusc ''Octopus vulgaris'': coordinating interaction with environment and conspecifics |editor-last=Witzany |editor-first=G. |title=Biocommunication of Animals |publisher=Springer |pages=337–349 |isbn=978-94-007-7413-1}}</ref> The [[Biology:Nervous system|nervous system]] of cephalopods is the most complex of the invertebrates<ref>{{cite book |chapter-url=https://books.google.com/books?id=dW5e6FHOH-4C&pg=PA115 |chapter=The cephalopod nervous system: What evolution has made of the molluscan design |first=B. U. |last=Budelmann |title=The nervous systems of invertebrates: An evolutionary and comparative approach |isbn=978-3-7643-5076-5 |editor1-first=O. |editor1-last=Breidbach |editor2-first=W. |editor2-last=Kutsch |year=1995|publisher=Springer }}</ref><ref>{{Cite journal |last1=Chung |first1=Wen-Sung |last2=Kurniawan |first2=Nyoman D. |last3=Marshall |first3=N. Justin |date=2020 |title=Toward an MRI-Based Mesoscale Connectome of the Squid Brain |journal=iScience |language=en |volume=23 |issue=1 |pages=100816 |doi=10.1016/j.isci.2019.100816 |pmc=6974791 |pmid=31972515 |bibcode=2020iSci...23j0816C}}</ref> and their brain-to-body-mass ratio falls between that of [[Biology:Endotherm|endotherm]]ic and [[Biology:Ectotherm|ectotherm]]ic vertebrates.<ref name=Cephalopods/>{{Rp|14}} Captive cephalopods have also been known to climb out of their aquaria, maneuver a distance of the lab floor, enter another aquarium to feed on captive crabs, and return to their own aquarium.<ref>Raven, Peter et al. (2003). ''Biology'', p.&nbsp;669. McGraw-Hill Education, New York. {{ISBN|9780073383071}}.</ref>

The brain is protected in a [[Biology:Cartilage|cartilaginous]] cranium. The giant [[Biology:Nerve|nerve]] fibers of the cephalopod [[Biology:Mantle (mollusc)|mantle]] have been widely used for many years as experimental material in [[Biology:Neurophysiology|neurophysiology]]; their large diameter (due to lack of myelination) makes them relatively easy to study compared with other animals.<ref>{{cite journal |first1=I. |last1=Tasaki |first2=T. |last2=Takenaka |title=Resting and action potential of squid giant axons intracellularly perfused with sodium-rich solutions |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=50 |issue=4 |pages=619–626 |date=1963 |doi=10.1073/pnas.50.4.619|pmid=14077488 |pmc=221236 |bibcode=1963PNAS...50..619T |doi-access=free }}</ref>

Many cephalopods are social creatures; when isolated from their own kind, some species have been observed [[Biology:Shoaling and schooling|shoaling]] with fish.<ref name=Packard1972>{{cite journal |first1=A. |title=Cephalopods and fish: the limits of convergence |journal=Biological Reviews |volume=47 |issue=2 |last1=Packard |pages=241–307 |year=1972 |doi=10.1111/j.1469-185X.1972.tb00975.x |s2cid=85088231}}</ref>

Some cephalopods are able to fly through the air for distances of up to 50&nbsp;m. While cephalopods are not particularly aerodynamic, they achieve these impressive ranges by jet-propulsion; water continues to be expelled from the funnel while the organism is in the air.<ref name="Macia-Robinson-2004">{{cite journal |last1=Macia |first1=Silvia |last2=Robinson |first2=Michael P. |last3=Craze |first3=Paul |last4=Dalton |first4=Robert |last5=Thomas |first5=James D. |title=New observations on airborne jet propulsion (flight) in squid, with a review of previous reports |doi=10.1093/mollus/70.3.297 |journal=Journal of Molluscan Studies |volume=70 |issue=3 |pages=297–299 |year=2004 |doi-access=free }}</ref> The animals spread their fins and tentacles to form wings and actively control lift force with body posture.<ref name=MuramatsuEtAl2013>{{cite journal |last1=Muramatsu |first1=K. |last2=Yamamoto |first2=J. |last3=Abe |first3=T. |last4=Sekiguchi |first4=K. |last5=Hoshi |first5=N. |last6=Sakurai |first6=Y. |s2cid=84388744 |year=2013 |title=Oceanic squid do fly |journal=Marine Biology |volume=160 |issue=5 |pages=1171–1175 |doi=10.1007/s00227-013-2169-9}}</ref> One species, ''[[Biology:Japanese flying squid|Todarodes pacificus]]'', has been observed spreading tentacles in a flat fan shape with a mucus film between the individual tentacles,<ref name=MuramatsuEtAl2013/><ref>{{cite web |url=https://blog.nationalgeographic.org/2013/02/20/scientists-unravel-mystery-of-flying-squid/ |title=Scientists Unravel Mystery of Flying Squid |date=20 February 2013 |work=Ocean Views |publisher=National Geographic}}</ref> while another, ''[[Biology:Caribbean reef squid|Sepioteuthis sepioidea]]'', has been observed putting the tentacles in a circular arrangement.<ref>{{cite journal |last=Jabr |first=Ferris |date=2 August 2010 |title=Fact or Fiction: Can a Squid Fly out of Water? |journal=Scientific American |url=http://www.scientificamerican.com/article/can-squid-fly/}}</ref>

===Senses===
Cephalopods have advanced vision, can detect gravity with [[Biology:Statocyst|statocyst]]s, and have a variety of chemical sense organs.<ref name=Cephalopods/>{{Rp|34}} Octopuses use their arms to explore their environment and can use them for depth perception.<ref name=Cephalopods/>

====Vision====
[[File:Nautilus pompilius (head).jpg|thumb|right|250px|The primitive nautilus eye functions similarly to a pinhole camera.]]
[[File:Sepia eyelid shape.theora.ogv|thumb|right|250px|The W-shaped pupil of the cuttlefish expanding when the lights are turned off]]
Most cephalopods rely on vision to detect predators and prey and to communicate with one another.<ref name=Serb2008>{{cite journal |first1=J. M. |first2=D. J. |last2=Eernisse |title=Charting Evolution's Trajectory: Using Molluscan Eye Diversity to Understand Parallel and Convergent Evolution |last1=Serb |s2cid=2881223 |journal=Evolution: Education and Outreach |volume=1 |issue=4 |pages=439–447 |year=2008 |doi=10.1007/s12052-008-0084-1 |doi-access=free}}</ref> Consequently, cephalopod vision is acute: training experiments have shown that the [[Biology:Common octopus|common octopus]] can distinguish the brightness, size, shape, and horizontal or vertical orientation of objects. The morphological construction gives cephalopod eyes the same performance as shark eyes; however, their construction differs, as cephalopods lack a cornea and have an everted retina.<ref name=Serb2008/> Cephalopods' eyes are also sensitive to the plane of [[Physics:Polarization (waves)|polarization]] of light.<ref>{{cite journal
|title=Part M, Chapter 4: Physiology of Coleoids
|journal=Treatise Online
|first=Martin J.
|last=Wells
|place=Lawrence, Kansas, USA
|url=http://129.237.145.244:591/FMPro?-db=treatiseonline&-format=treatiseonline%2fdetails.html&-lay=table&-sortfield=sortnumber&-recid=11&-findall=
|year=2011
|doi=10.17161/to.v0i0.4226
|access-date=2013-05-10
|archive-url=https://web.archive.org/web/20160822014821/http://129.237.145.244:591/FMPro?-db=treatiseonline&-format=treatiseonline%2fdetails.html&-lay=table&-sortfield=sortnumber&-recid=11&-findall=
|archive-date=2016-08-22
|url-status=dead
|doi-access=free
}} </ref> Unlike many other cephalopods, nautiluses do not have good vision; their eye structure is highly developed, but lacks a solid [[Biology:Lens (anatomy)|lens]]. They have a simple "[[Engineering:Pinhole camera|pinhole]]" eye through which water can pass. Instead of vision, the animal is thought to use [[Biology:Olfaction|olfaction]] as the primary sense for foraging, as well as locating or identifying potential mates.

[[File:Cuttlefish eye.jpg|thumb|250px|A cuttlefish with W-shaped pupils which may help them discern colors]]
Surprisingly, given their ability to change color, all octopuses<ref name=Boyle/> and most cephalopods<ref name="Messenger-17">{{cite book |last1=Messenger |first1=John B. |first2=Roger T. |last2=Hanlon |title=Cephalopod Behaviour |publisher=Cambridge University Press |year=1998 |location=Cambridge |pages=17–21 |isbn=978-0-521-64583-6}}</ref><ref>{{Cite journal |last1=Chung |first1=Wen-Sung |last2=Marshall |first2=N. Justin |date=2016-09-14 |title=Comparative visual ecology of cephalopods from different habitats |journal=Proceedings of the Royal Society B: Biological Sciences |language=en |volume=283 |issue=1838 |pages=20161346 |doi=10.1098/rspb.2016.1346 |issn=0962-8452 |pmc=5031660 |pmid=27629028}}</ref> are considered to be color blind. Coleoid cephalopods (octopus, squid, cuttlefish) have a single photoreceptor type and lack the ability to determine color by comparing detected photon intensity across multiple spectral channels. When [[Biology:Camouflage|camouflaging]] themselves, they use their chromatophores to change brightness and pattern according to the background they see, but their ability to match the specific color of a background may come from cells such as iridophores and leucophores that reflect light from the environment.<ref>Hanlon and Messenger, 68.</ref> They also produce visual pigments throughout their body and may sense light levels directly from their body.<ref>{{cite journal |last1=Mäthger |first1=L. |last2=Roberts |first2=S. |last3=Hanlon |first3=R. |title=Evidence for distributed light sensing in the skin of cuttlefish, ''Sepia officinalis'' |journal=Biology Letters |volume=6 |issue=5 |pages=600–603 |year=2010 |pmid=20392722 |pmc=2936158 |doi=10.1098/rsbl.2010.0223}}</ref> Evidence of [[Color vision|color vision]] has been found in the sparkling enope squid (''Watasenia scintillans'').<ref name="Messenger-17" /><ref name=Michinomae1994>{{cite journal |last1=Michinomae |first1=M. |last2=Masuda |first2=H. |last3=Seidou |first3=M. |last4=Kito |first4=Y. |title=Structural basis for wavelength discrimination in the banked retina of the firefly squid ''Watasenia scintillans'' |journal=Journal of Experimental Biology |volume=193 |issue=1 |pages=1–12 |year=1994 |doi=10.1242/jeb.193.1.1 |pmid=9317205|doi-access=free }}</ref> It achieves color vision with three [[Biology:Photoreceptor protein|photoreceptors]], which are based on the same [[Biology:Opsin|opsin]], but use distinct [[Chemistry:Retinal|retinal]] molecules as chromophores: A1 (retinal), A3 (3-dehydroretinal), and A4 (4-hydroxyretinal). The A1-photoreceptor is most sensitive to green-blue (484&nbsp;nm), the A2-photoreceptor to blue-green (500&nbsp;nm), and the A4-photoreceptor to blue (470&nbsp;nm) light.<ref name=Seidou1990>{{cite journal |last1=Seidou |first1=M. |last2=Sugahara |first2=M. |last3=Uchiyama |first3=H. |last4=Hiraki |first4=K. |last5=Hamanaka |first5=T. |last6=Michinomae |first6=M. |last7=Yoshihara |first7=K. |last8=Kito |first8=Y. |s2cid=25707481 |title=On the three visual pigments in the retina of the firefly squid, ''Watasenia scintillans'' |journal=Journal of Comparative Physiology A |volume=166 |issue=6 |year=1990 |doi=10.1007/BF00187321}}</ref>

In 2015, a novel mechanism for spectral discrimination in cephalopods was described. This relies on the exploitation of [[Physics:Chromatic aberration|chromatic aberration]] (wavelength-dependence of focal length). Numerical modeling shows that chromatic aberration can yield useful chromatic information through the dependence of image acuity on accommodation. The unusual off-axis slit and annular pupil shapes in cephalopods enhance this ability by acting as prisms which are scattering white light in all directions.<ref name="Stubbs">{{cite bioRxiv |last1=Stubbs |first1=A. L. |last2=Stubbs |first2=C. W. |year=2015 |title=A novel mechanism for color vision: Pupil shape and chromatic aberration can provide spectral discrimination for 'color blind' organisms |biorxiv=10.1101/017756}}</ref><ref>{{cite web |title=Octopus Eyes Are Crazier Than We Imagined |date=2016-07-06 |website=Gizmodo |archive-url=https://web.archive.org/web/20230430021550/https://gizmodo.com/octopus-eyes-are-crazier-than-we-imagined-1783195433 |archive-date=2023-04-30 |url-status=live |url=https://gizmodo.com/octopus-eyes-are-crazier-than-we-imagined-1783195433}}</ref>

====Photoreception====
In 2015, molecular evidence was published indicating that cephalopod chromatophores are photosensitive; reverse transcription polymerase chain reactions (RT-PCR) revealed [[Biology:Primary transcript|transcripts]] encoding [[Biology:Rhodopsin|rhodopsin]] and retinochrome within the retinas and skin of the [[Biology:Longfin inshore squid|longfin inshore squid]] (''Doryteuthis pealeii''), and the common cuttlefish (''Sepia officinalis'') and broadclub cuttlefish (''Sepia latimanus''). The authors claim this is the first evidence that cephalopod dermal tissues may possess the required combination of molecules to respond to light.<ref name="Kingston2015">{{cite journal |last1=Kingston |first1=A. C. |last2=Kuzirian |first2=A. M. |last3=Hanlon |first3=R. T. |last4=Cronin |first4=T. W. |s2cid=25431963 |year=2015 |title=Visual phototransduction components in cephalopod chromatophores suggest dermal photoreception |journal=Journal of Experimental Biology |volume=218 |issue=10 |pages=1596–1602 |doi=10.1242/jeb.117945 |pmid=25994635 |doi-access=free }}</ref>

====Hearing====
Some squids have been shown to detect sound using their [[Biology:Statocyst|statocyst]]s,<ref>{{cite news |url=http://news.bbc.co.uk/earth/hi/earth_news/newsid_8095000/8095977.stm |work=BBC News |title=The cephalopods can hear you |date=2009-06-15 |access-date=2010-04-28}}</ref> but, in general, cephalopods are deaf.

===Use of light===
[[Image:Cuttlefish color.jpg|thumb|This broadclub cuttlefish (''Sepia latimanus'') can change from camouflage tans and browns (top) to yellow with dark highlights (bottom) in less than a second.]]

Most cephalopods possess an assemblage of skin components that interact with light. These may include [[Biology:Chromatophore#Iridophores and leucophores|iridophores, leucophores]], [[Biology:Chromatophore|chromatophore]]s and (in some species) [[Biology:Photophore|photophore]]s. Chromatophores are colored pigment cells that expand and contract in accordance to produce color and pattern which they can use in a startling array of fashions.<ref name=Cephalopods/><ref name="Kingston2015" /> As well as providing camouflage with their background, some cephalopods bioluminesce, shining light downwards to disguise their shadows from any predators that may lurk below.<ref name=Cephalopods/> The [[Physics:Bioluminescence|bioluminescence]] is produced by bacterial symbionts; the host cephalopod is able to detect the light produced by these organisms.<ref>{{cite journal |last1=Tong |first1=D. |last2=Rozas |first2=S. |first3=H. |last3=Oakley |last5=Colley |last6=Mcfall-Ngai |last4=Mitchell |first4=J. |first5=J. |first6=J. |title=Evidence for light perception in a bioluminescent organ |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=106 |issue=24 |pages=9836–9841 |date=Jun 2009 |issn=0027-8424 |pmid=19509343 |pmc=2700988 |doi=10.1073/pnas.0904571106 |bibcode=2009PNAS..106.9836T |doi-access=free}}</ref> [[Physics:Bioluminescence|Bioluminescence]] may also be used to entice prey, and some species use colorful displays to impress mates, startle predators, or even communicate with one another.<ref name=Cephalopods/>

=== Coloration ===
{{Further|Biology:Animal coloration|Biology:Animals that can change color}}

Cephalopods can change their colors and patterns in milliseconds, whether for [[Biology:Signalling theory|signalling]] (both [[Biology:Sexual selection|within the species]] and for [[Earth:Aposematism|warning]]) or [[Engineering:Active camouflage|active camouflage]],<ref name=Cephalopods/> as their chromatophores are expanded or contracted.<ref name=encbrit>"Integument (mollusks)". Encyclopædia Britannica. 2009. Encyclopædia Britannica 2006 Ultimate Reference Suite DVD.</ref> Although color changes appear to rely primarily on vision input, there is evidence that skin cells, specifically chromatophores, can detect light and adjust to light conditions independently of the eyes.<ref>{{cite journal |last1=Ramirez |first1=M. D. |last2=Oakley |first2=T. H |title=Eye-independent, light-activated chromatophore expansion (LACE) and expression of phototransduction genes in the skin of ''Octopus bimaculoides'' |journal=Journal of Experimental Biology |date=2015 |volume=218 |issue=10 |pages=1513–1520 |doi=10.1242/jeb.110908 |doi-access=free |pmid=25994633 |url=http://jeb.biologists.org/content/jexbio/218/10/1513.full.pdf |archive-url=https://web.archive.org/web/20160809194051/http://jeb.biologists.org/content/jexbio/218/10/1513.full.pdf |archive-date=2016-08-09 |url-status=live |pmc=4448664}}</ref> The octopus changes skin color and texture during quiet and active sleep cycles.<ref>{{cite journal<!--|authors=Sylvia Lima de Souza Medeiros; Mizziara Marlen Matias de Paiva; Paulo Henrique Lopes; Wilfredo Blanco; Françoise Dantas de Lima; Jaime Bruno Cirne de Oliveira; Inácio Gomes Medeiros; Eduardo Bouth Sequerra; Sandro de Souza; Tatiana Silva Leite; Sidarta Ribeiro--> |author=Sylvia Lima de Souza Medeiros |display-authors=0 |title=Cyclic alternation of quiet and active sleep states in the octopus |journal=iScience |year=2021 |doi=10.1016/j.isci.2021.102223 |doi-access=free |pmc=8101055 |url=https://www.sciencedirect.com/science/article/pii/S2589004221001917 |url-status=live |archive-url=https://web.archive.org/web/20231030230840/https://www.sciencedirect.com/science/article/pii/S2589004221001917 |archive-date= Oct 30, 2023 }}</ref>

Cephalopods can use chromatophores like a muscle, which is why they can change their skin hue as rapidly as they do.
Coloration is typically stronger in near-shore species than those living in the open ocean, whose functions tend to be restricted to [[Biology:Camouflage#Disruptive patterning|disruptive camouflage]].<ref name=Cephalopods/>{{Rp|2}} These chromatophores are found throughout the body of the octopus, however, they are controlled by the same part of the brain that controls elongation during jet propulsion to reduce drag. As such, jetting octopuses can turn pale because the brain is unable to achieve both controlling elongation and controlling the chromatophores.<ref>{{Cite journal |last=Huffard |first=Christine L. |date=2006-10-01 |title=Locomotion by ''Abdopus aculeatus'' (Cephalopoda: Octopodidae):walking the line between primary and secondary defenses |journal=Journal of Experimental Biology |volume=209 |issue=19 |pages=3697–3707 |doi=10.1242/jeb.02435 |pmid=16985187 |s2cid=26862414 |s2cid-access=free |issn=0022-0949|doi-access=free }}</ref> Most octopuses mimic select structures in their field of view rather than becoming a composite color of their full background.<ref>{{cite journal |last1=Josef |first1=Noam |last2=Amodio |first2=Piero |last3=Fiorito |first3=Graziano |last4=Shashar |first4=Nadav |date=2012-05-23 |title=Camouflaging in a Complex Environment—Octopuses Use Specific Features of Their Surroundings for Background Matching |journal=PLOS ONE |volume=7 |issue=5 |pages=e37579 |doi=10.1371/journal.pone.0037579 |issn=1932-6203 |pmc=3359305 |pmid=22649542 |bibcode=2012PLoSO...737579J |bibcode-access=free |doi-access=free}}</ref>

Evidence of original coloration has been detected in cephalopod fossils dating as far back as the [[Earth:Silurian|Silurian]]; these orthoconic individuals bore concentric stripes, which are thought to have served as camouflage.<ref>{{cite journal |title=Minute Silurian oncocerid nautiloids with unusual color patterns |last1=Manda |first1=Štěpán |last2=Turek |first2=Vojtěch |s2cid=54043278 |s2cid-access=free |journal=Acta Palaeontologica Polonica |volume=54 |issue=3 |year=2009 |pages=503–512 |doi=10.4202/app.2008.0062 |doi-access=free }}</ref> Devonian cephalopods bear more complex color patterns, of unknown function.<ref>{{cite journal |title=Colour patterns in Early Devonian cephalopods from the Barrandian Area: Taphonomy and taxonomy |first=Vojtěch |last=Turek |s2cid=55851070 |s2cid-access=free |volume=54 |issue=3 |year=2009 |pages=491–502 |doi=10.4202/app.2007.0064 |journal=Acta Palaeontologica Polonica |doi-access=free }}</ref>

=== Chromatophores ===
Coleoids, a shell-less subclass of cephalopods (squid, cuttlefish, and octopuses), have complex pigment containing cells called chromatophores which are capable of producing rapidly changing color patterns. These cells store pigment within an elastic sac which produces the color seen from these cells. Coleoids can change the shape of this sac, called the cytoelastic sacculus, which then causes changes in the opacity, translucency, and opacity of the cell. By rapidly changing multiple chromatophores of different colors, cephalopods are able to change the color of their skin at astonishing speeds, an adaptation that is especially notable in an organism that sees in black and white. Chromatophores are known to only contain three pigments, red, yellow, and brown, which cannot create the full color spectrum.<ref>Hanlon, R. et al. (2011) Rapid adaptive camouflage in cephalopods. in Animal Camouflage: Mechanisms and Function. Cambridge University Press, Cambridge, UK. pp145-161.</ref> However, cephalopods also have cells called iridophores, thin, layered protein cells that reflect light in ways that can produce colors chromatophores cannot.<ref>Cloney, R. A., & Brocco, S. L. (1983). Chromatophore organs, reflector cells, iridocytes and leucophores in cephalopods. American Zoologist, 23(3), 581–592. https://doi.org/10.1093/icb/23.3.581</ref> The mechanism of iridophore control is unknown, but chromatophores are under the control of neural pathways, allowing the cephalopod to coordinate elaborate displays. Together, chromatophores and iridophores are able to produce a large range of colors and pattern displays.

==== Adaptive value ====
Cephalopods utilize chromatophores' color changing ability in order to camouflage themselves. Chromatophores allow Coleoids to blend into many different environments, from coral reefs to the sandy sea floor. The color change of chromatophores works in concert with papillae, epithelial tissue which grows and deforms through hydrostatic motion to change skin texture. Chromatophores are able to perform two types of camouflage, mimicry and color matching. Mimicry is when an organism changes its appearance to appear like a different organism. The squid  Sepioteuthis sepioide has been documented changing its appearance to appear as the non threatening herbivorous parrotfish to approach unaware prey. The octopus Thaumoctopus mimicus is known to mimic a number of different venomous organisms it cohabitates with to deter predators.<ref>Norman, M. D., Finn, J., & Tregenza, T. (2001). Dynamic mimicry in an Indo–Malayan octopus. Proceedings of the Royal Society of London. Series B: Biological Sciences, 268(1478), 1755–1758. https://doi.org/10.1098/rspb.2001.1708</ref> While background matching, a cephalopod changes its appearance to resemble its surroundings, hiding from its predators or concealing itself from prey. The ability to both mimic other organisms and match the appearance of their surroundings is notable given that cephalopods' vision is monochromatic.

Cephalopods also use their fine control of body coloration and patterning to perform complex signaling displays for both conspecific and intraspecific communication. Coloration is used in concert with locomotion and texture to send signals to other organisms. Intraspecifically this can serve as a warning display to potential predators. For example, when the octopus Callistoctopus macropus is threatened, it will turn a bright red brown color speckled with white dots as a high contrast display to startle predators. Conspecifically, color change is used for both mating displays and social communication. Cuttlefish have intricate mating displays from males to females. There is also male to male signaling that occurs during competition over mates, all of which are the product of chromatophore coloration displays.

==== Origin ====
There are two hypotheses about the evolution of color change in cephalopods. One hypothesis is that the ability to change color may have evolved for social, sexual, and signaling functions. Another explanation is that it first evolved because of selective pressures encouraging predator avoidance and stealth hunting.

For color change to have evolved as the result of social selection the environment of cephalopods' ancestors would have to fit a number of criteria. One, there would need to be some kind of mating ritual that involved signaling. Two, they would have to experience demonstrably high levels of sexual selection. And three, the ancestor would need to communicate using sexual signals that are visible to a conspecific receiver. For color change to have evolved as the result of natural selection different parameters would have to be met. For one, you would need some phenotypic diversity in body patterning among the population. The species would also need to cohabitate with predators which rely on vision for prey identification. These predators should have a high range of visual sensitivity, detecting not just motion or contrast but also colors. The habitats they occupy would also need to display a diversity of backgrounds.<ref>Stevens, M., & Merilaita, S. (Eds.). (2011). Animal camouflage: Mechanisms and function. Cambridge University Press.</ref> Experiments done in Dwarf chameleons testing these hypotheses showed that chameleon taxa with greater capacity for color change had more visually conspicuous social signals but did not come from more visually diverse habitats, suggesting that color change ability likely evolved to facilitate social signaling, while camouflage is a useful byproduct.<ref>Stuart-Fox, D., & Moussalli, A. (2008). Selection for social signalling drives the evolution of chameleon colour change. PLoS Biology, 6(1), e25. https://doi.org/10.1371/journal.pbio.0060025</ref> Because camouflage is used for multiple adaptive purposes in cephalopods, color change could have evolved for one use and the other developed later, or it evolved to regulate trade offs within both.

==== Convergent evolution ====
Color change is widespread in ectotherms including anoles, frogs, mollusks, many fish, insects, and spiders.<ref>Bagnara, J. T., & Hadley, M. E. (1973). Chromatophores and color change: The comparative physiology of animal pigmentation. Prentice-Hall.</ref> The mechanism behind this color change can be either morphological or physiological. Morphological change is the result of a change in the density of pigment containing cells and tends to change over longer periods of time. Physiological change, the kind observed in cephalopod lineages, is typically the result of the movement of pigment within the chromatophore, changing where different pigments are localized within the cell. This physiological change typically occurs on much shorter timescales compared to morphological change. Cephalopods have a rare form of physiological color change which utilizes neural control of muscles to change the morphology of their chromatophores. This neural control of chromatophores has evolved convergently in both cephalopods and teleosts fishes.<ref>Demski, L. S. (1992). Chromatophore systems in teleosts and cephalopods: A levels oriented analysis of convergent systems. Brain, Behavior and Evolution, 40(2–3), 141–156. https://doi.org/10.1159/000113909</ref>

===Ink===
With the exception of the [[Biology:Nautilidae|Nautilidae]] and the species of [[Biology:Octopus|octopus]] belonging to the [[Biology:Order|suborder]] [[Biology:Cirrina|Cirrina]],<ref>{{cite book |first1=Roger T. |last1=Hanlon |first2=John B. |last2=Messenger |title=Cephalopod Behaviour |page=2 |publisher=Cambridge University Press |year=1999 |isbn=978-0-521-64583-6}}</ref> all known cephalopods have an ink sac, which can be used to expel a cloud of dark ink to confuse predators.<ref name=Boyle>{{cite book |isbn=978-0-632-06048-1 |first1=Peter |last1=Boyle |first2=Paul |last2=Rodhouse |year=2004 |publisher=Blackwell |url=https://books.google.com/books?id=4UtCi2B4VnoC |doi=10.1002/9780470995310.ch2 |title=Cephalopods : ecology and fisheries }}</ref> This sac is a muscular bag which originated as an extension of the hindgut. It lies beneath the gut and opens into the anus, into which its contents – almost pure [[Chemistry:Melanin|melanin]] – can be squirted; its proximity to the base of the funnel means the ink can be distributed by ejected water as the cephalopod uses its jet propulsion.<ref name=Boyle/> The ejected cloud of melanin is usually mixed, upon expulsion, with [[Biology:Mucus|mucus]], produced elsewhere in the mantle, and therefore forms a thick cloud, resulting in visual (and possibly chemosensory) impairment of the predator, like a smokescreen. However, a more sophisticated behavior has been observed, in which the cephalopod releases a cloud, with a greater mucus content, that approximately resembles the cephalopod that released it (this decoy is referred to as a [[Earth:Pseudomorph#In other fields|pseudomorph]]). This strategy often results in the predator attacking the pseudomorph, rather than its rapidly departing prey.<ref name=Boyle/> For more information, see Inking behaviors.

The ink sac of cephalopods has led to a common name of "inkfish",<ref>{{cite encyclopedia |title=inkfish |url=https://www.merriam-webster.com/dictionary/inkfish |dictionary=Merriam-Webster |access-date=1 February 2018}}</ref> formerly the pen-and-ink fish.<ref>{{cite book |last1=Bickerdyke |first1=John |title=Sea Fishing |date=1895 |publisher=Longmans, Green, and Co. |location=London |page=114 |url=https://archive.org/stream/badmintonlibrar06watsgoog#page/n13/mode/2up/ |quote=the common squid or calamary (''Loligo vulgaris''). It is sometimes called the pen-and-ink fish, on account of its ink bag, and the delicate elongated shell which is found within it.}}</ref>

{{multiple image
|direction = vertical
|align     = right
|width     = 250
|image1    = Chtenopteryx sicula2.jpg
|caption1  = Viscera of ''[[Biology:Chtenopteryx sicula|Chtenopteryx sicula]]''
|image2    = Ocythoe tuberculata viscera.jpg
|caption2  = Viscera of ''[[Biology:Ocythoe tuberculata|Ocythoe tuberculata]]''
}}

===Circulatory system===
Cephalopods are the only molluscs with a closed circulatory system. Coleoids have two gill [[Biology:Heart|heart]]s (also known as [[Medicine:Branchial heart|branchial heart]]s) that move blood through the capillaries of the [[Biology:Gill|gill]]s. A single systemic heart then pumps the oxygenated blood through the rest of the body.<ref>{{cite journal |last=Wells |first=M.J. |title=Nervous control of the heartbeat in octopus |journal=Journal of Experimental Biology |volume=85 |issue=1 |pages=111–28 |date=1 April 1980 |doi=10.1242/jeb.85.1.111 |url=http://jeb.biologists.org/cgi/content/abstract/85/1/111 |pmid=7373208}}</ref>

Like most molluscs, cephalopods use [[Biology:Hemocyanin|hemocyanin]], a copper-containing protein, rather than [[Biology:Hemoglobin|hemoglobin]], to transport oxygen. As a result, their blood is colorless when deoxygenated and turns blue when bonded to oxygen.<ref>{{cite journal |last=Ghiretti-Magaldi |first=A. |s2cid=33290596 |title=The Pre-history of Hemocyanin. The Discovery of Copper in the Blood of Molluscs |journal=Cellular and Molecular Life Sciences |volume=48 |issue=10 |pages=971–972 |date=October 1992 |doi=10.1007/BF01919143 }}</ref> In oxygen-rich environments and in acidic water, hemoglobin is more efficient, but in environments with little oxygen and in low temperatures, hemocyanin has the upper hand.<ref>[https://www.newscientist.com/question/many-hearts-octopus/ How many hearts does an octopus have? - New Scientist]</ref><ref>[https://www.scienceabc.com/nature/animals/why-do-octopuses-have-three-hearts.html Why Do Octopuses Have Three Hearts? - Science ABC]</ref><ref>{{Cite web |url=https://www.illinoisscience.org/2018/02/where-would-we-be-without-blood/ |title=Where Would We Be Without Blood? - Illinois Science Council |access-date=2022-07-07 |archive-date=2023-04-25 |archive-url=https://web.archive.org/web/20230425044615/https://www.illinoisscience.org/2018/02/where-would-we-be-without-blood/ |url-status=dead }}</ref> The hemocyanin molecule is much larger than the hemoglobin molecule, allowing it to bond with 96 {{chem2|O2}} or {{CO2}} molecules, instead of the hemoglobin's just four. But unlike hemoglobin, which are attached in millions on the surface of a single red blood cell, hemocyanin molecules float freely in the bloodstream.<ref>[https://wilstar.com/science/why-is-octopus-blood-blue/ Why is Octopus Blood Blue? | Science | Wilstar.com]</ref>

===Respiration===
Cephalopods exchange gases with the seawater by forcing water through their gills, which are attached to the roof of the organism.<ref name=Gilbert1990/>{{Rp|488}}<ref name=Schipp1979/> Water enters the mantle cavity on the outside of the gills, and the entrance of the mantle cavity closes. When the mantle contracts, water is forced through the gills, which lie between the mantle cavity and the funnel. The water's expulsion through the funnel can be used to power jet propulsion. If respiration is used concurrently with jet propulsion, large losses in speed or oxygen generation can be expected.<ref name="ingentaconnect.com">{{Cite journal |last1=Wells |first1=M. J. |last2=O'Dor |first2=R. K. |date=1991-09-01 |title=Jet Propulsion and the Evolution of the Cephalopods |url=https://www.ingentaconnect.com/content/umrsmas/bullmar/1991/00000049/f0020001/art00038 |journal=Bulletin of Marine Science |volume=49 |issue=1–2 |pages=419–432}}</ref><ref>{{Cite journal |last1=Packard |first1=A. |last2=Trueman |first2=E. R. |date=October 1974 |title=Muscular activity of the mantle of Sepia and Loligo (Cephalopoda) during respiratory movements and jetting, and its physiological interpretation |url=https://pubmed.ncbi.nlm.nih.gov/4443736/ |journal=The Journal of Experimental Biology|volume=61 |issue=2 |pages=411–419 |doi=10.1242/jeb.61.2.411 |issn=0022-0949 |pmid=4443736}}</ref> The gills, which are much more efficient than those of other mollusks, are attached to the ventral surface of the mantle cavity.<ref name=Schipp1979>{{cite journal |title=Electron Microscopical and Histochemical Studies of Differentiation and Function of the Cephalopod Gill (''Sepia Officinalis'' L.) |doi=10.1007/BF00993999 |year=1979 |journal=Zoomorphologie |volume=93 |issue=3 |pages=193–207 |last1=Schipp |first1=Rudolf |last2=Mollenhauer |first2=Stephan |last3=Boletzky |first3=Sigurd |s2cid=20214206}}</ref>
There is a trade-off with gill size regarding lifestyle. To achieve fast speeds, gills need to be small – water will be passed through them quickly when energy is needed, compensating for their small size. However, organisms which spend most of their time moving slowly along the bottom do not naturally pass much water through their cavity for locomotion; thus they have larger gills, along with complex systems to ensure that water is constantly washing through their gills, even when the organism is stationary.<ref name=Gilbert1990/> The water flow is controlled by contractions of the radial and circular mantle cavity muscles.<ref name=Bone1994>{{cite journal |url=http://jeb.biologists.org/cgi/reprint/194/1/153.pdf |archive-url=https://web.archive.org/web/20090304172648/http://jeb.biologists.org/cgi/reprint/194/1/153.pdf |archive-date=2009-03-04 |url-status=live |year=1994 |journal=Journal of Experimental Biology |pages=153–165 |volume=194 |title=On the respiratory flow in the cuttlefish ''Sepia Officinalis'' |first1=Q. |last1=Bone |first2=E. R. |last2=Brown |first3=G. |last3=Travers |issue=1 |doi=10.1242/jeb.194.1.153 |pmid=9317534}}</ref>

The gills of cephalopods are supported by a skeleton of robust fibrous proteins; the lack of mucopolysaccharides distinguishes this matrix from cartilage.<ref name="Cole2009">{{cite journal |doi=10.1016/j.zool.2008.01.003 |pmid=18722759 |year=2009 |last1=Cole |first1=A. |last2=Hall |first2=B. |title=Cartilage differentiation in cephalopod molluscs |volume=112 |issue=1 |pages=2–15 |journal=Zoology}}</ref><ref>See also http://tolweb.org/articles/?article_id=4200 {{Webarchive |url=https://web.archive.org/web/20100616120826/http://tolweb.org/articles/?article_id=4200 |date=2010-06-16 }}</ref> The gills are also thought to be involved in excretion, with NH<sub>4</sub><sup>+</sup> being swapped with K<sup>+</sup> from the seawater.<ref name=Schipp1979/>

=== Locomotion and buoyancy ===
[[Image:Octopus3.jpg|thumb|right|Octopuses swim headfirst, with arms trailing behind]]
While most cephalopods can move by jet propulsion, this is a very energy-consuming way to travel compared to the tail propulsion used by fish.<ref name="mollusca12-11"/> The efficiency of a [[Engineering:Propeller|propeller]]-driven [[Physics:Jet (fluid)|waterjet]] (i.e. [[Physics:Propulsive efficiency#Propeller engines|Froude efficiency]]) is greater than a [[Engineering:Rocket|rocket]].<ref name="pmid10952883">{{cite journal |last1=Anderson |first1=E. |last2=Demont |first2=M. |title=The mechanics of locomotion in the squid ''Loligo pealei'': Locomotory function and unsteady hydrodynamics of the jet and intramantle pressure |journal=Journal of Experimental Biology |volume=203 |issue=18 |pages=2851–2863 |year=2000 |doi=10.1242/jeb.203.18.2851 |pmid=10952883 |url=http://jeb.biologists.org/content/203/18/2851}}</ref> The relative efficiency of [[Physics:Jet propulsion|jet propulsion]] decreases further as animal size increases; [[Biology:Paralarva|paralarva]]e are far more efficient than juvenile and adult individuals.<ref name="Bartol2008">{{cite journal |last1=Bartol |first1=I. K. |last2=Krueger |first2=P. S. |last3=Thompson |first3=J. T. |last4=Stewart |first4=W. J. |title=Swimming dynamics and propulsive efficiency of squids throughout ontogeny |doi=10.1093/icb/icn043 |journal=Integrative and Comparative Biology |volume=48 |issue=6 |pages=720–733 |year=2008 |pmid=21669828 |doi-access=free }}</ref> Since the Paleozoic era, as competition with [[Biology:Fish|fish]] produced an environment where efficient motion was crucial to survival, jet propulsion has taken a back role, with fins and tentacles used to maintain a steady velocity.<ref name="mollusca12" />
Whilst jet propulsion is never the sole mode of locomotion,<ref name="mollusca12"/>{{rp|208}} the stop-start motion provided by the jets continues to be useful for providing bursts of high speed – not least when capturing prey or avoiding predators.<ref name="mollusca12"/> Indeed, it makes cephalopods the fastest marine invertebrates,<ref name=Cephalopods>{{cite book |isbn=978-0-19-852761-9 |first1=Marion |last1=Nixon |first2= J. Z. |last2=Young |year=2003 |publisher=Oxford University Press |location=New York |title=The Brains and Lives of Cephalopods}}</ref>{{Rp|Preface}}
and they can out-accelerate most fish.<ref name=Gilbert1990>{{cite book |title=Squid as Experimental Animals |first1=Daniel L. |last1=Gilbert |first2=William J. |last2=Adelman |first3=John M. |last3=Arnold |edition=illustrated |publisher=Springer |year=1990 |isbn=978-0-306-43513-3 |url=https://archive.org/details/squidasexperimen00gilb}}</ref>
The jet is supplemented with fin motion; in the squid, the fins flap each time that a jet is released, amplifying the thrust; they are then extended between jets (presumably to avoid sinking).<ref name="Bartol2008"/>
Oxygenated water is taken into the [[Biology:Mantle (mollusc)|mantle cavity]] to the [[Biology:Gill|gill]]s and through muscular contraction of this cavity, the spent water is expelled through the hyponome, created by a fold in the mantle. The size difference between the posterior and anterior ends of this organ control the speed of the jet the organism can produce.<ref name="Shea2002">{{cite journal |doi=10.1007/s00227-001-0772-7 |title=Quantification of ontogenetic discontinuities in three species of oegopsid squids using model II piecewise linear regression |year=2002 |last1=Shea |first1=E. |last2=Vecchione |first2=M. |s2cid=84822175 |journal=Marine Biology |volume=140 |issue=5 |pages=971–979 }}</ref> The velocity of the organism can be accurately predicted for a given mass and morphology of animal.<ref>{{cite journal |title=A study in jet propulsion: an analysis of the motion of the squid, ''Loligo vulgaris'' |url=http://jeb.biologists.org/content/56/1/155 |journal=Journal of Experimental Biology |volume=56 |pages=155–165 |issue=1972 |first1=W. |last1=Johnson |first2=P. D. |last2=Soden |first3=E. R. |last3=Trueman |date=February 1972 |doi=10.1242/jeb.56.1.155 }}</ref> Motion of the cephalopods is usually backward as water is forced out anteriorly through the hyponome, but direction can be controlled somewhat by pointing it in different directions.{{sfnp|Campbell|Reece|Mitchell|1999|p=612}} Some cephalopods accompany this expulsion of water with a gunshot-like popping noise, thought to function to frighten away potential predators.<ref name="Guerra2007">{{cite journal |doi=10.1017/S0025315407058225 |title=A new noise detected in the ocean |year=2007 |last1=Guerra |first1=A. |last2=Martinell |first2=X. |last3=González |first3=A. F. |last4=Vecchione |first4=M. |last5=Gracia |first5=J. |last6=Martinell |first6=J. |journal=Journal of the Marine Biological Association of the United Kingdom |volume=87 |issue=5 |pages=1255–1256 |hdl=10261/27009 |s2cid=85770435 |hdl-access=free}}</ref>

Cephalopods employ a similar method of propulsion despite their increasing size (as they grow) changing the dynamics of the water in which they find themselves. Thus their paralarvae do not extensively use their fins (which are less efficient at low [[Reynolds number]]s) and primarily use their jets to propel themselves upwards, whereas large adult cephalopods tend to swim less efficiently and with more reliance on their fins.<ref name="Bartol2008"/>

[[Image:Nautilus front.jpg|thumb|left|''Nautilus belauensis'' seen from the front, showing the opening of the hyponome]]

Early cephalopods are thought to have produced jets by drawing their body into their shells, as ''Nautilus'' does today.<ref name=Wells1991>{{cite journal |title=Jet Propulsion and the Evolution of the Cephalopods |journal=Bulletin of Marine Science |volume=49 |issue=1 |date=July 1991 |pages=419–432(14) |last1=Wells |first1=Martin J. |last2=O'Dor |first2=R. K. }}</ref> ''Nautilus'' is also capable of creating a jet by undulations of its funnel; this slower flow of water is more suited to the extraction of oxygen from the water.<ref name=Wells1991/> When motionless, ''Nautilus'' can only extract 20% of oxygen from the water.<ref name="ingentaconnect.com"/> The jet velocity in ''Nautilus'' is much slower than in coleoids, but less musculature and energy is involved in its production.<ref name="CHAMBERLAINJR1993">{{cite journal |doi=10.1016/S0016-6995(06)80360-8 |title=Locomotion in ancient seas: Constraint and opportunity in cephalopod adaptive design |year=1993 |last1=Chamberlain |first1=J. Jr. |journal=Geobios |volume=26 |issue=Suppl. 1 |pages=49–61 |bibcode=1993Geobi..26...49C }}</ref> Jet thrust in cephalopods is controlled primarily by the maximum diameter of the funnel orifice (or, perhaps, the average diameter of the funnel)<ref name=oDor1988/>{{Rp|440}} and the diameter of the mantle cavity.<ref name=oDor2000>{{cite journal |last1=O'Dor |first1=R. K. |last2=Hoar |first2=J. A. |year=2000 |title=Does geometry limit squid growth? |journal=ICES Journal of Marine Science |volume=57 |pages=8–14 |doi=10.1006/jmsc.1999.0502|doi-access=free }}</ref> Changes in the size of the orifice are used most at intermediate velocities.<ref name=oDor1988/> The absolute velocity achieved is limited by the cephalopod's requirement to inhale water for expulsion; this intake limits the maximum velocity to eight body-lengths per second, a speed which most cephalopods can attain after two funnel-blows.<ref name=oDor1988>{{cite journal |last1=O'Dor |first1=R. K. |title=The forces acting on swimming squid |url=http://jeb.biologists.org/content/137/1/421 |journal=Journal of Experimental Biology |volume=137 |pages=421–442 |year=1988 |doi=10.1242/jeb.137.1.421 }}</ref> Water refills the cavity by entering not only through the orifices, but also through the funnel.<ref name=oDor1988/> Squid can expel up to 94% of the fluid within their cavity in a single jet thrust.<ref name="pmid10952883"/> To accommodate the rapid changes in water intake and expulsion, the orifices are highly flexible and can change their size by a factor of twenty; the funnel radius, conversely, changes only by a factor of around 1.5.<ref name=oDor1988/>

Some octopus species are also able to walk along the seabed. Squids and cuttlefish can move short distances in any direction by rippling of a flap of [[Biology:Muscle|muscle]] around the mantle.

While most cephalopods float (i.e. are [[Physics:Neutral buoyancy|neutrally buoyant]] or nearly so; in fact most cephalopods are about 2–3% denser than seawater<ref name=Packard1972/>), they achieve this in different ways.<ref name="mollusca12-11">{{The Mollusca|chapter=11: Evolution of Buoyancy and Locomotion in recent cephalopods|volume=12}}</ref>
Some, such as ''Nautilus'', allow gas to diffuse into the gap between the mantle and the shell; others allow purer water to ooze from their kidneys, forcing out denser salt water from the body cavity;<ref name="mollusca12-11"/> others, like some fish, accumulate oils in the liver;<ref name="mollusca12-11"/> and some octopuses have a gelatinous body with lighter [[Chemistry:Chloride|chloride]] [[Physics:Ion|ion]]s replacing [[Chemistry:Sulfate|sulfate]] in the body chemistry.<ref name="mollusca12-11"/>

Squids are the primary sufferers of negative buoyancy in cephalopods. The negative buoyancy means that some squids, especially those whose habitat depths are rather shallow, have to actively regulate their vertical positions. This means that they must expend energy, often through jetting or undulations, in order to maintain the same depth. As such, the cost of transport of many squids are quite high. That being said, squid and other cephalopod that dwell in deep waters tend to be more neutrally buoyant which removes the need to regulate depth and increases their locomotory efficiency.<ref>{{Cite journal |last1=Seibel |first1=B. A. |last2=Thuesen |first2=E. V. |last3=Childress |first3=J. J. |last4=Gorodezky |first4=L. A. |date=April 1997 |title=Decline in Pelagic Cephalopod Metabolism With Habitat Depth Reflects Differences in Locomotory Efficiency |url=https://pubmed.ncbi.nlm.nih.gov/28581868/ |journal=The Biological Bulletin |volume=192 |issue=2 |pages=262–278 |doi=10.2307/1542720 |jstor=1542720 |issn=1939-8697 |pmid=28581868}}</ref><ref name="ingentaconnect.com"/>

The ''Macrotritopus defilippi'', or the sand-dwelling octopus, was seen mimicking both the coloration and the swimming movements of the sand-dwelling flounder ''Bothus lunatus'' to avoid predators. The octopuses were able to flatten their bodies and put their arms back to appear the same as the flounders as well as move with the same speed and movements.<ref>{{cite journal |last1=Hanlon |first1=Roger T. |last2=Watson |first2=Anya C. |last3=Barbosa |first3=Alexandra |date=2010-02-01 |title=A 'Mimic Octopus' in the Atlantic: Flatfish Mimicry and Camouflage by ''Macrotritopus defilippi'' |journal=The Biological Bulletin |volume=218 |issue=1 |pages=15–24 |doi=10.1086/BBLv218n1p15 |pmid=20203250 |issn=0006-3185 |hdl=1912/4811 |s2cid=12935620 |hdl-access=free}}</ref>

Females of two species, ''Ocythoe tuberculata'' and ''Haliphron atlanticus'', have evolved a true [[Biology:Swim bladder|swim bladder]].<ref>{{Cite web |url=https://www.researchgate.net/publication/44613041 |title=The argonaut shell: Gas-mediated buoyancy control in a pelagic octopus}}</ref>

==== Octopus vs. squid locomotion ====
Two of the categories of cephalopods, octopus and squid, are vastly different in their movements despite being of the same class. Octopuses are generally not seen as active swimmers; they are often found scavenging the sea floor instead of swimming long distances through the water. Squids, on the other hand, can be found to travel vast distances, with some moving as much as 2000&nbsp;km in 2.5&nbsp;months at an average pace of 0.9&nbsp;body lengths per second.<ref name=Gosline-deMont-1985>{{cite magazine |last1=Gosline |first1=John M. |last2=de&nbsp;Mont |first2=M. Edwin |date=1985 |title=Jet-propelled swimming in squids |url=https://www.jstor.org/stable/24967551 |magazine=Scientific American |volume=252 |issue=1 |pages=96–103 |doi=10.1038/scientificamerican0185-96 |jstor=24967551 |issn=0036-8733}}</ref> There is a major reason for the difference in movement type and efficiency: anatomy.

Both octopuses and squids have mantles (referenced above) which function towards respiration and locomotion in the form of jetting. The composition of these mantles differs between the two families, however. In octopuses, the mantle is made up of three muscle types: longitudinal, radial, and circular. The longitudinal muscles run parallel to the length of the octopus and they are used in order to keep the mantle the same length throughout the jetting process. Given that they are muscles, it can be noted that this means the octopus must actively flex the longitudinal muscles during jetting in order to keep the mantle at a constant length. The radial muscles run perpendicular to the longitudinal muscles and are used to thicken and thin the wall of the mantle. Finally, the circular muscles are used as the main activators in jetting. They are muscle bands that surround the mantle and expand/contract the cavity. All three muscle types work in unison to produce a jet as a propulsion mechanism.<ref name=Gosline-deMont-1985/>

Squids do not have the longitudinal muscles that octopus do. Instead, they have a tunic.<ref name=Gosline-deMont-1985/> This tunic is made of layers of collagen and it surrounds the top and the bottom of the mantle. Because they are made of collagen and not muscle, the tunics are rigid bodies that are much stronger than the muscle counterparts. This provides the squids some advantages for jet propulsion swimming. The stiffness means that there is no necessary muscle flexing to keep the mantle the same size. In addition, tunics take up only 1% of the squid mantle's wall thickness, whereas the longitudinal muscle fibers take up to 20% of the mantle wall thickness in octopuses.<ref name=Gosline-deMont-1985/> Also because of the rigidity of the tunic, the radial muscles in squid can contract more forcefully.

The mantle is not the only place where squids have collagen. Collagen fibers are located throughout the other muscle fibers in the mantle. These collagen fibers act as elastics and are sometimes named "collagen springs".<ref name=Gosline-deMont-1985/> As the name implies, these fibers act as springs. When the radial and circular muscles in the mantle contract, they reach a point where the contraction is no longer efficient to the forward motion of the creature. In such cases, the excess contraction is stored in the collagen which then efficiently begins or aids in the expansion of the mantle at the end of the jet. In some tests, the collagen has been shown to be able to begin raising mantle pressure up to 50ms before muscle activity is initiated.<ref name=Gosline-deMont-1985/>

These anatomical differences between squid and octopuses can help explain why squid can be found swimming comparably to fish while octopuses usually rely on other forms of locomotion on the sea floor such as bipedal walking, crawling, and non-jetting swimming.<ref>{{Cite journal |last=Huffard |first=Christine L. |date=2006-10-01 |title=Locomotion by ''Abdopus aculeatus'' (Cephalopoda: Octopodidae): Walking the line between primary and secondary defenses |journal=Journal of Experimental Biology |volume=209 |issue=19 |pages=3697–3707 |doi=10.1242/jeb.02435 |pmid=16985187 |s2cid=26862414 |issn=0022-0949|doi-access=free }}</ref>

===Shell===
{{multiple image
|direction = vertical
|width = 220
|image1 = Spirula spirula.jpg
|caption1 = Cross section of ''Spirula spirula'', showing the position of the shell inside the mantle
|image2 = Herklots 1859 I 2 Sepia officinalis - schelp.jpg
|caption2 = Cuttlebone of ''Sepia officinalis''
|image3 = Sepioteuthis lessoniana gladius.jpg
|caption3 = Gladius of ''Sepioteuthis lessoniana''
}}

Nautiluses are the only extant cephalopods with a true external shell. However, all molluscan shells are formed from the [[Biology:Ectoderm|ectoderm]] (outer layer of the embryo); in [[Biology:Cuttlefish|cuttlefish]] (''Sepia'' spp.), for example, an invagination of the ectoderm forms during the embryonic period, resulting in a shell (cuttlebone) that is internal in the adult.<ref name="Baratte2007">{{cite journal |last1=Baratte |first1=S. |last2=Andouche |first2=A. |last3=Bonnaud |first3=L. |s2cid=22241391 |title=Engrailed in cephalopods: a key gene related to the emergence of morphological novelties |journal=Development Genes and Evolution |volume=217 |issue=5 |pages=353–362 |year=2007 |pmid=17394016 |doi=10.1007/s00427-007-0147-2}}</ref> The same is true of the chitinous [[Biology:Gladius (cephalopod)|gladius]] of squid<ref name="Baratte2007"/> and octopuses.<ref>{{cite journal |doi=10.1016/j.geobios.2003.01.009 |trans-title='Nude ammonoids': a challenge to cephalopod phylogeny? |title='Ammonoïdes nus': un défi pour la phylogénie des céphalopodes ? |year=2004 |last1=von Boletzky |first1=S. |journal=Geobios |volume=37 |pages=117–118 }}</ref> [[Biology:Cirrina|Cirrate]] octopods have [[Biology:Cirrate shell|arch-shaped cartilaginous fin supports]],<ref>{{cite book |url=https://books.google.com/books?id=R7-TfdYeLEgC&pg=PA288 |page=288 |title=Oceanography and Marine Biology: An Annual Review |editor1-first=R. N. |editor1-last=Gibson |editor2-first=R. J. A. |editor2-last=Atkinson |editor3-first=J. D. M. |editor3-last=Gordon |publisher=CRC Press |year=2006 |isbn=978-1420006391}}</ref> which are sometimes referred to as a "shell vestige" or "gladius".<ref>{{cite journal |year=1983 |doi=10.1098/rstb.1983.0021 |title=''Cirrothauma murrayi'' Chun, a finned octopod |last1=Aldred |first1=R. G. |last2=Nixon |first2=M. |last3=Young |first3=J. Z. |journal=Philosophical Transactions of the Royal Society B: Biological Sciences |volume=301 |issue=1103 |pages=1–54 |bibcode=1983RSPTB.301....1A |doi-access=free }}</ref> The Incirrina have either a pair of rod-shaped stylets or no vestige of an internal shell,<ref>{{cite journal |last1=Fuchs |first1=D. |last2=Ifrim |first2=C. |last3=Stinnesbeck |first3=W. |year=2008 |title=A new ''Palaeoctopus'' (Cephalopoda: Coleoidea) from the Late Cretaceous of Vallecillo, north-eastern Mexico, and implications for the evolution of Octopoda |journal=Palaeontology |volume=51 |issue=5 |pages=1129–1139 |doi=10.1111/j.1475-4983.2008.00797.x |bibcode=2008Palgy..51.1129F |doi-access=free }}</ref> and some squid also lack a gladius.<ref>{{cite journal |title=The terminal spine of sepiolid hatchlings: its development and functional morphology (Mollusca, Cephalopoda) |last=von Boletzky |first=Sigurd |journal=Bulletin of Marine Science |date=July 1991 |pages=107–112 |volume=49}}</ref> The shelled coleoids do not form a clade or even a paraphyletic group.<ref name="Strugnell2007"/> The ''Spirula'' shell begins as an organic structure, and is then very rapidly mineralized.<ref>{{cite journal |doi=10.1007/s10347-005-0054-9 |title=''Spirula'' – a window to the embryonic development of ammonoids? Morphological and molecular indications for a palaeontological hypothesis |year=2005 |last1=Warnke |first1=K. |last2=Keupp |first2=H. |s2cid=85026080 |journal=Facies |volume=51 |issue=1–4 |pages=60–65 |url=http://doc.rero.ch/record/13832/files/PAL_E731.pdf}}</ref> Shells that are "lost" may be lost by resorption of the calcium carbonate component.<ref>{{cite journal |first5=A. |last5=Beran |first4=M. |title=Molluscan shell evolution with review of shell calcification hypothesis |volume=154 |journal=Comparative Biochemistry and Physiology B |pages=351–371 |issue=3 |last4=Smrz |first3=I. |last1=Furuhashi |year=2009 |pmid=19665573 |first1=T. |last2=Schwarzinger |last3=Miksik |first2=C. |doi=10.1016/j.cbpb.2009.07.011}}</ref>

Females of the octopus genus ''[[Biology:Argonaut (animal)|Argonauta]]'' secrete a specialized paper-thin egg case in which they reside, and this is popularly regarded as a "shell", although it is not attached to the body of the animal and has a separate evolutionary origin.

The largest group of shelled cephalopods, the [[Biology:Ammonite|ammonite]]s, are extinct, but their shells are very common as [[Biology:Fossil|fossil]]s.

The deposition of carbonate, leading to a mineralized shell, appears to be related to the acidity of the organic shell matrix (see [[Biology:Mollusc shell|Mollusc shell]]); shell-forming cephalopods have an acidic matrix, whereas the gladius of squid has a basic matrix.<ref>{{cite journal |year=1996 |last=Dauphin |first=Y. |title=The organic matrix of coleoid cephalopod shells: molecular weights and isoelectric properties of the soluble matrix in relation to biomineralization processes |journal=Marine Biology |volume=125 |issue=3 |pages=525–529 |doi=10.1007/BF00353265 |s2cid=83400480 |url=https://link.springer.com/article/10.1007/BF00353265}}</ref> The basic arrangement of the cephalopod outer wall is: an outer (spherulitic) prismatic layer, a laminar (nacreous) layer and an inner prismatic layer. The thickness of every layer depends on the taxa.<ref>{{Cite book |title=Les subdivisions majeures de la classe des céphalopodes : bases de la systématique actuelle : apport de l'analyse microstructurale |last=Dauphin |first=Y. |date=1983 |publisher=These Doct. Etat, Université Paris Sud |oclc=972899981}}</ref> In modern cephalopods, the Ca carbonate is aragonite. As for other mollusc shells or coral skeletons, the smallest visible units are irregular rounded granules.<ref>{{Cite journal |last=Dauphin |first=Y. |s2cid=126900936 |date=2001 |title=Nanostructures de la nacre des tests de céphalopodes actuels |journal=Paläontologische Zeitschrift |volume=75 |issue=1 |pages=113–122 |doi=10.1007/bf03022601 |issn=0031-0220}}</ref>

{{multiple image
 | align    = left
 | footer   = '''Left:''' A giant squid found in Logy Bay, Newfoundland, in 1873. The two long feeding tentacles are visible on the extreme left and right.<br />'''Right:''' Detail of the tentacular club of ''Abraliopsis morisi''
 | image1   = Logy bay giant squid 1873.png
 | width1   = 220
 | image2   = Tentacule Abraliopsis morisi.jpg
 | width2   = 133
}}

===Head appendages===
Cephalopods, as the name implies, have muscular appendages extending from their heads and surrounding their mouths. These are used in feeding, mobility, and even reproduction. In [[Biology:Coleoidea|coleoids]] they number eight or ten. Decapods such as cuttlefish and squid have five pairs. The longer two, termed ''[[Biology:Tentacle|tentacle]]s'', are actively involved in capturing prey;<ref name="mollusca11" />{{Rp|225}} they can lengthen rapidly (in as little as 15 milliseconds<ref name="mollusca11"/>{{Rp|225}}). In [[Biology:Giant squid|giant squid]] they may reach a length of 8 metres. They may terminate in a broadened, sucker-coated club.<ref name="mollusca11"/>{{Rp|225}} The shorter four pairs are termed ''arms'', and are involved in holding and manipulating the captured organism.<ref name="mollusca11"/>{{Rp|225}} They too have suckers, on the side closest to the mouth; these help to hold onto the prey.<ref name="mollusca11"/>{{Rp|226}} Octopods only have four pairs of sucker-coated arms, as the name suggests, though developmental abnormalities can modify the number of arms expressed.<ref name="Toll1991">{{cite journal |last1=Toll |first1=R. B. |last2=Binger |first2=L. C. |s2cid=34858474 |title=Arm anomalies: Cases of supernumerary development and bilateral agenesis of arm pairs in Octopoda (Mollusca, Cephalopoda) |journal=Zoomorphology |volume=110 |issue=6 |pages=313–316 |year=1991 |doi=10.1007/BF01668021}}</ref>

The tentacle consists of a thick central nerve cord (which must be thick to allow each sucker to be controlled independently)<ref>{{cite book |year=1912 |title=Anatomy of the Common Squid}}</ref> surrounded by circular and radial muscles. Because the volume of the tentacle remains constant, contracting the circular muscles decreases the radius and permits the rapid increase in length. Typically a 70% lengthening is achieved by decreasing the width by 23%.<ref name="mollusca11"/>{{Rp|227}} The shorter arms lack this capability.

The size of the tentacle is related to the size of the buccal cavity; larger, stronger tentacles can hold prey as small bites are taken from it; with more numerous, smaller tentacles, prey is swallowed whole, so the mouth cavity must be larger.<ref>Nixon 1988 in {{cite journal |first1=M. G. E. |first2=J. |title=''Allocrioceras'' from the Cenomanian (mid-Cretaceous) of the Lebanon and its bearing on the palaeobiological interpretation of heteromorphic ammonites |last1=Wippich |journal=Palaeontology |volume=47 |issue=5 |pages=1093–1107 |year=2004 |doi=10.1111/j.0031-0239.2004.00408.x |last2=Lehmann |bibcode=2004Palgy..47.1093W |doi-access=free}}</ref>

Externally shelled [[Biology:Nautilidae|nautilids]] (''Nautilus'' and ''[[Biology:Allonautilus|Allonautilus]]'') have on the order of 90 finger-like appendages, termed ''tentacles'', which lack suckers but are sticky instead, and are partly retractable.

===Feeding===
[[File:Architeuthis beak.jpg|thumb|The two-part beak of the giant squid, ''Architeuthis'' sp.]]
All living cephalopods have a two-part beak;<ref name=Cephalopods/>{{Rp|7}} most have a [[Biology:Radula|radula]], although it is reduced in most octopus and absent altogether in ''Spirula''.<ref name=Cephalopods/>{{Rp|7}}<ref>{{The Mollusca|volume=12|chapter=5}}</ref>{{rp|110}} They feed by capturing prey with their tentacles, drawing it into their mouth and taking bites from it.<ref name=Boyle/> They have a mixture of toxic digestive juices, some of which are manufactured by symbiotic algae, which they eject from their salivary glands onto their captured prey held in their mouths. These juices separate the flesh of their prey from the bone or shell.<ref name=Boyle/> The salivary gland has a small tooth at its end which can be poked into an organism to digest it from within.<ref name=Boyle/>

The digestive gland itself is rather short.<ref name=Boyle/> It has four elements, with food passing through the crop, stomach and caecum before entering the intestine. Most digestion, as well as the absorption of nutrients, occurs in the digestive gland, sometimes called the liver. Nutrients and waste materials are exchanged between the gut and the digestive gland through a pair of connections linking the gland to the junction of the stomach and caecum.<ref name=Boyle/> Cells in the digestive gland directly release pigmented excretory chemicals into the lumen of the gut, which are then bound with mucus passed through the anus as long dark strings, ejected with the aid of exhaled water from the funnel.<ref name=Boyle/> Cephalopods tend to concentrate ingested heavy metals in their body tissue.<ref>C.Michael Hogan. 2011. [http://www.eoearth.org/article/Celtic_Sea?topic=49523 ''Celtic Sea''. eds. P.Saundry & C.Cleveland. Encyclopedia of Earth. National Council for Science and the Environment. Washington DC.]</ref> However, octopus arms use a family of cephalopod-specific chemotactile receptors (CRs) to be their "taste by touch" system.<ref>{{cite news<!--|authors=Lena van Giesen; Peter B. Kilian; Corey A.H. Allard; Nicholas W. Bellono--> |author=Lena van Giesen |display-authors=0 |title=Molecular Basis of Chemotactile Sensation in Octopus |publisher=Cell |year=2020 |volume=183 |issue=3 |pages=594–604 |doi=10.1016/j.cell.2020.09.008}}</ref>

==== Radula ====
[[File:Veined Octopus - Amphioctopus Marginatus eating a Crab.jpg|thumb|''[[Biology:Amphioctopus marginatus|Amphioctopus marginatus]]'' eating a crab]]
The cephalopod radula consists of multiple symmetrical rows of up to nine teeth<ref>{{tolweb|2035|title=Cephalopod radula |first1=Richard E. |last1=Young |first2=Michael |last2=Vecchione |first3=Katharina M. |last3=Mangold}}</ref> – thirteen in fossil classes.<ref name=Nixon1995>{{cite journal |doi=10.1111/j.1469-7998.1995.tb01785.x |title=A nomenclature for the radula of the Cephalopoda (Mollusca) – living and fossil |year=1995 |last1=Nixon |first1=M. |journal=Journal of Zoology |volume=236 |pages=73–81}}</ref> The organ is reduced or even vestigial in certain octopus species and is absent in ''[[Biology:Spirula|Spirula]]''.<ref name=Nixon1995/> The teeth may be homodont (i.e. similar in form across a row), heterodont (otherwise), or ctenodont (comb-like).<ref name=Nixon1995/> Their height, width and number of cusps is variable between species.<ref name=Nixon1995/> The pattern of teeth repeats, but each row may not be identical to the last; in the octopus, for instance, the sequence repeats every five rows.<ref name=Nixon1995/>{{rp|79}}

Cephalopod radulae are known from fossil deposits dating back to the Ordovician.<ref name="Gabbott1999">{{cite journal |last1=Gabbott |first1=S. E. |title=Orthoconic cephalopods and associated fauna from the late Ordovician Soom Shale Lagerstatte, South Africa |journal=Palaeontology |volume=42 |pages=123–148 |year=1999 |issue=1 |doi=10.1111/1475-4983.00065 |bibcode=1999Palgy..42..123G |doi-access=free }}</ref> They are usually preserved within the cephalopod's body chamber, commonly in conjunction with the mandibles; but this need not always be the case;<ref>{{cite book |url=https://books.google.com/books?id=Q19AwHOqOZ0C&q=fossil+cephalopod+radula&pg=PA223 |title=Cephalopods present and past: new insights and fresh perspectives |publisher=Springer |isbn=978-1-4020-6461-6 |editor1-last=Landman |editor1-first=Neil H. |editor2-last=Davis |editor2-first=Richard Arnold |editor3-last=Mapes |editor3-first=Royal H. |year=2007}}</ref> many radulae are preserved in a range of settings in the Mason Creek.<ref>{{cite book |author=Richardson & ... |year=1977 |title=Fossils of the Mason Creek}}</ref>
Radulae are usually difficult to detect, even when they are preserved in fossils, as the rock must weather and crack in exactly the right fashion to expose them; for instance, radulae have only been found in nine of the 43 ammonite genera,<ref name="Kruta2011">{{cite journal |last1=Kruta |first1=I. |last2=Landman |first2=N. |last3=Rouget |first3=I. |last4=Cecca |first4=F. |last5=Tafforeau |first5=P. |s2cid=206530342 |title=The role of ammonites in the Mesozoic marine food web revealed by jaw preservation |doi=10.1126/science.1198793 |journal=Science |volume=331 |issue=6013 |pages=70–72 |year=2011 |pmid=21212354 |bibcode=2011Sci...331...70K }}</ref>{{Clarify|date=March 2011}} and they are rarer still in non-ammonoid forms: only three pre-Mesozoic species possess one.<ref name="Gabbott1999" />

=== <span id="Renal appendage"></span> Excretory system ===
Most cephalopods possess a single pair of large [[Biology:Nephridium|nephridia]]. Filtered nitrogenous waste is produced in the pericardial cavity of the [[Medicine:Branchial heart|branchial heart]]s, each of which is connected to a nephridium by a narrow canal. The canal delivers the excreta to a bladder-like renal sac, and also resorbs excess water from the filtrate. Several outgrowths of the lateral vena cava project into the renal sac, continuously inflating and deflating as the branchial hearts beat. This action helps to pump the secreted waste into the sacs, to be released into the mantle cavity through a pore.<ref name=IZ>{{cite book |last=Barnes |first=Robert D. |year=1982 |title=Invertebrate Zoology |publisher=Holt-Saunders International |location=Philadelphia, PA |pages=450–460 |isbn=978-0-03-056747-6}}</ref>

''Nautilus'', unusually, possesses four nephridia, none of which are connected to the pericardial cavities.

The incorporation of [[Chemistry:Ammonia|ammonia]] is important for shell formation in terrestrial molluscs and other non-molluscan lineages.<ref name=Loest1979>{{cite journal |jstor=30155937 |pages=461–469 |title=Ammonia Volatilization and Absorption by Terrestrial Gastropods_ a Comparison between Shelled and Shell-Less Species |journal=Physiological Zoology |volume=52 |issue=4 |year=1979 |last1=Loest |first1=R. A. |doi=10.1086/physzool.52.4.30155937 |s2cid=87142440 }}</ref> Because [[Biology:Protein|protein]] (i.e. flesh) is a major constituent of the cephalopod diet, large amounts of ammonium ions are produced as waste. The main organs involved with the release of this excess ammonium are the gills.<ref name="Boucher-Rodoni1994">{{cite journal |doi=10.1080/10236249409378907 |title=Ammonia production in cephalopods, physiological and evolutionary aspects |year=1994 |last1=Boucher-Rodoni |first1=R. |last2=Mangold |first2=K. |journal=Marine and Freshwater Behaviour and Physiology |volume=25 |issue=1–3 |pages=53–60}}</ref> The rate of release is lowest in the shelled cephalopods ''Nautilus'' and ''[[Biology:Sepia (genus)|Sepia]]'' as a result of their using [[Chemistry:Nitrogen|nitrogen]] to fill their shells with gas to increase buoyancy.<ref name="Boucher-Rodoni1994"/> Other cephalopods use ammonium in a similar way, storing the [[Physics:Ion|ion]]s (as [[Chemistry:Ammonium chloride|ammonium chloride]]) to reduce their overall density and increase buoyancy.<ref name="Boucher-Rodoni1994"/>

=== Reproduction and life cycle ===
[[File:Papierboot Argonauta 200705181139.jpg|220px|thumb|Female ''[[Biology:Argonauta argo|Argonauta argo]]'' with eggcase and eggs]]
[[File:Ocythoe tuberculata hectocotylus.jpg|220px|thumb|Detail of the hectocotylus of ''Ocythoe tuberculata'']]

[[File: Onykia ingens with non-erect penis.jpg|250px|thumb|A dissected male specimen of ''[[Biology:Onykia ingens|Onykia ingens]]'', showing a non-erect penis (the white tubular structure located below most of the other organs)]]
[[File: Onykia ingens with erect penis.jpg|250px|thumb|A specimen of the same species exhibiting an elongation of the penis to 67&nbsp;cm in length]]

Cephalopods are a diverse group of species, but share common life history traits, for example, they have a rapid growth rate and short life spans.<ref name="ReferenceA">{{cite book |last1=Vidal |first1=Erica A. G. |title=Advances in Cephalopod Science: Biology, Ecology, Cultivation and Fisheries.}}</ref> Stearns (1992) suggested that in order to produce the largest possible number of viable offspring, spawning events depend on the ecological environmental factors of the organism. The majority of cephalopods do not provide parental care to their offspring, except, for example, octopus, which helps this organism increase the survival rate of their offspring.<ref name="ReferenceA"/> Marine species' life cycles are affected by various environmental conditions.<ref name="ReferenceB">{{cite journal |last1=Rodrigues |first1=M. |last2=Guerra |last3=Troncoso |s2cid=41577834 |title=The embryonic phase and its implication in the hatchling size and condition of Atlantic bobtail squid ''Sepiola Atlantica'' |journal=Helgoland Marine Research |volume=65 |issue=2 |pages=211–216 |doi=10.1007/s10152-010-0217-0 |year=2010 |bibcode=2011HMR....65..211R |doi-access=free}}</ref> The development of a cephalopod embryo can be greatly affected by temperature, oxygen saturation, pollution, light intensity, and salinity.<ref name="ReferenceA"/> These factors are important to the rate of embryonic development and the success of hatching of the embryos. Food availability also plays an important role in the reproductive cycle of cephalopods. A limitation of food influences the timing of spawning along with their function and growth.<ref name="ReferenceB"/> Spawning time and spawning vary among marine species; it's correlated with temperature, though cephalopods in shallow water spawn in cold months so that the offspring would hatch at warmer temperatures. Breeding can last from several days to a month.<ref name="ReferenceA"/>

==== Sexual maturity ====
Cephalopods that are sexually mature and of adult size begin spawning and reproducing. After the transfer of genetic material to the following generation, the adult cephalopods then die.<ref name="ReferenceA"/> Sexual maturation in male and female cephalopods can be observed internally by the enlargement of gonads and accessory glands.<ref name="ReferenceC">{{cite journal |last1=Arkhipkin |first1=A. I. |title=Reproductive system structure, development and function in cephalopods with a new general scale for maturity stages |journal=Journal of Northwest Atlantic Fishery Science |date=1992 |volume=12 |pages=63–74 |doi=10.2960/j.v12.a7 |doi-access=free}}</ref> Mating would be a poor indicator of sexual maturation in females; they can receive sperm when not fully reproductively mature and store them until they are ready to fertilize the eggs.<ref name="ReferenceB"/> Males are more aggressive in their pre-mating competition when in the presence of immature females than when competing for a sexually mature female.<ref>{{cite journal |last1=Mohanty |first1=Sobhi |last2=Ojanguren |first2=Alfredo F. |last3=Fuiman |first3=Lee A. |s2cid=85256742 |date=2014-07-01 |title=Aggressive male mating behavior depends on female maturity in ''Octopus bimaculoides'' |journal=Marine Biology |volume=161 |issue=7 |pages=1521–1530 |doi=10.1007/s00227-014-2437-3 |issn=0025-3162}}</ref> Most cephalopod males develop a hectocotylus, an arm tip which is capable of transferring their spermatozoa into the female mantle cavity. Though not all species use a hectocotylus; for example, the adult nautilus releases a spadix.<ref>{{cite journal |last1=Saunders |first1=W. B |last2=Spinosa |first2=C. |title=Sexual dimorphism in ''Nautilus'' from Palau |journal=Paleobiology |date=1978 |volume=4 |issue=3 |pages=349–358 |doi=10.1017/S0094837300006047 |bibcode=1978Pbio....4..349S |s2cid=85899974 }}</ref> An indication of sexual maturity of females is the development of brachial photophores to attract mates.<ref>{{cite journal |last1=Young |first1=R. B. |s2cid=145374345 |title=A Systematic Approach to Planning Occupational Programs |journal=Community College Review |date=1975 |pages=19–25 |doi=10.1177/009155217500300204 |volume=3 |issue=2}}</ref>

==== Fertilization ====
Cephalopods are not broadcast spawners. During the process of fertilization, the females use sperm provided by the male via [[Biology:External fertilization|external fertilization]]. Internal fertilization is seen only in octopuses.<ref name="ReferenceC"/> The initiation of copulation begins when the male catches a female and wraps his arm around her, either in a "male to female neck" position or mouth to mouth position, depending on the species. The males then initiate the process of fertilization by contracting their mantle several times to release the spermatozoa.<ref name=SquiresEtAl2013>{{cite journal |last1=Squires |first1=Z. E |last2=Norman |first2=M. D |last3=Stuart-Fox |first3=D. |title=Mating behaviour and general spawning patterns of the southern dumpling squid ''Euprymna tasmanica'' |journal=Journal of Molluscan Studies |date=2013 |volume=79 |issue=3 |pages=263–269 |doi=10.1093/mollus/eyt025 |doi-access=free}}</ref> Cephalopods often mate several times, which influences males to mate longer with females that have previously, nearly tripling the number of contractions of the mantle.<ref name=SquiresEtAl2013/> To ensure the fertilization of the eggs, female cephalopods release a sperm-attracting peptide through the gelatinous layers of the egg to direct the spermatozoa. Female cephalopods lay eggs in clutches; each egg is composed of a protective coat to ensure the safety of the developing embryo when released into the water column. Reproductive strategies differ between cephalopod species. In giant Pacific octopus, large eggs are laid in a den; it will often take several days to lay all of them.<ref name="ReferenceC"/> Once the eggs are released and attached to a sheltered substrate, the females then die,<ref name="ReferenceC"/> making them semelparous. In some species of cephalopods, egg clutches are anchored to substrates by a mucilaginous adhesive substance. These eggs are swelled with perivitelline fluid (PVF), a hypertonic fluid that prevents premature hatching.<ref>{{cite journal |last1=Marthy |first1=H. J. |last2=Hauser |first2=R |last3=Scholl |first3=A. |s2cid=8693207 |title=Natural tranquilizer in cephalopod eggs |journal=Nature |volume=261 |issue=5560 |pages=496–7 |date=1976 |doi=10.1038/261496a0 |pmid=945466 |bibcode=1976Natur.261..496M}}</ref> Fertilized egg clusters are neutrally buoyant depending on the depth that they were laid, but can also be found in substrates such as sand, a matrix of corals, or seaweed.<ref name="ReferenceB"/> Because these species do not provide parental care for their offspring, egg capsules can be injected with ink by the female in order to camouflage the embryos from predators.<ref name="ReferenceB"/>

==== Male–male competition{{anchor|Male-male_competition}} ====
Most cephalopods engage in aggressive sex: a protein in the male capsule sheath stimulates this behavior. They also engage in male–male aggression, where larger males tend to win the interactions.<ref name="ReferenceA"/> When a female is near, the males charge one another continuously and flail their arms. If neither male backs away, the arms extend to the back, exposing the mouth, followed by the biting of arm tips.<ref name=NormanEtAl1997>{{cite journal |last1=Norman |first1=M. D. |last2=Lu |first2=C. C. |title=Redescription of the southern dumpling squid ''Euprymna tasmanica'' and a revision of the genus ''Euprymna'' (Cephalopoda: Sepiolidae) |journal=Journal of the Marine Biological Association of the United Kingdom |date=1997 |volume=77 |issue=4 |pages=1109–1137 |doi=10.1017/s0025315400038662 |s2cid=85748957 }}</ref> During mate competition males also participate in a technique called flushing. This technique is used by the second male attempting to mate with a female. Flushing removes spermatophores in the buccal cavity that was placed there by the first mate by forcing water into the cavity.<ref name="ReferenceA"/> Another behavior that males engage in is sneaker mating or mimicry – smaller males adjust their behavior to that of a female in order to reduce aggression. By using this technique, they are able to fertilize the eggs while the larger male is distracted by a different male.<ref name=NormanEtAl1997/> During this process, the sneaker males quickly insert drop-like sperm into the seminal receptacle.<ref>{{cite journal |last1=Iwata |first1=Y. |last2=Ito |first2=K. |last3=Sakurai |first3=Y. |s2cid=43094931 |title=Effect of low temperature on mating behavior of squid ''Loligo bleekeri'' |journal=Fisheries Science |date=2008 |volume=74 |issue=6 |pages=1345–1347 |doi=10.1111/j.1444-2906.2008.01664.x}}</ref>

==== Mate choice ====
[[Philosophy:Mate choice|Mate choice]] is seen in cuttlefish species, where females prefer some males over others, though characteristics of the preferred males are unknown.<ref name="ReferenceA"/> A hypothesis states that females reject males by olfactory cues rather than visual cues.<ref name="ReferenceA"/> Several cephalopod species are polyandrous – accepting and storing multiple male spermatophores, which has been identified by DNA fingerprinting.<ref name=SquiresEtAl2013/> Females are no longer receptive to mating attempts when holding their eggs in their arms. Females can store sperm in two places (1) the buccal cavity where recently mated males place their spermatophores, and (2) the internal sperm-storage receptacles where sperm packages from previous males are stored.<ref name="ReferenceA"/> Spermatophore storage results in sperm competition; which states that the female controls which mate fertilizes the eggs. In order to reduce this sort of competition, males develop agonistic behaviors like mate guarding and flushing.<ref name="ReferenceA"/> The ''Hapalochlaena lunulata'', or the blue-ringed octopus, readily mates with both males and females.<ref>{{cite journal |last1=Cheng |first1=Mary W. |last2=Caldwell |first2=Roy L. |s2cid=32899443 |date=July 2000 |title=Sex identification and mating in the blue-ringed octopus, ''Hapalochlaena lunulata'' |journal=Animal Behaviour |volume=60 |issue=1 |pages=27–33 |doi=10.1006/anbe.2000.1447 |pmid=10924200 |issn=0003-3472}}</ref>

==== Sexual dimorphism ====
In a variety of marine organisms, it is seen that females are larger in size compared to the males in some closely related species. In some lineages, such as the [[Biology:Blanket octopus|blanket octopus]], males become structurally smaller and smaller resembling a term, "dwarfism" dwarf males usually occurs at low densities.<ref name="Princeton University Press">{{cite book |last1=Fairbairn |first1=D. |chapter=Blanket Octopus: Drifting Females and Dwarf Males |title=Odd couples: Extraordinary differences between the sexes in the animal kingdom |date=2013 |publisher=Princeton University Press |pages=104–115}}</ref> The blanket octopus male is an example of sexual-evolutionary dwarfism; females grow 10,000 to 40,000 times larger than the males and the sex ratio between males and females can be distinguished right after hatching of the eggs.<ref name="Princeton University Press"/>

[[File:SquidEggCases-MontereryAquarium-April2-07.png|thumb|left|Egg cases laid by a female squid]]

==== Embryology ====
Cephalopod eggs span a large range of sizes, from 1 to 30&nbsp;mm in diameter.<ref name=Boletzky2009/> The fertilised ovum initially divides to produce a disc of germinal cells at one pole, with the yolk remaining at the opposite pole. The germinal disc grows to envelop and eventually absorb the yolk, forming the embryo. The tentacles and arms first appear at the hind part of the body, where the foot would be in other molluscs, and only later migrate towards the head.<ref name="IZ"/><ref name="Shigeno2008">{{cite journal |last2=Sasaki |first1=S. |last5=Vecchione |first2=T. |last4=Kasugai |last3=Moritaki |first3=T. |last6=Agata |first4=T. |first5=M. |first6=K. |title=Evolution of the cephalopod head complex by assembly of multiple molluscan body parts: Evidence from ''Nautilus'' embryonic development |journal=Journal of Morphology |volume=269 |issue=1 |pages=1–17 |date=Jan 2008 |pmid=17654542 |last1=Shigeno |doi=10.1002/jmor.10564 |s2cid=13109195}}</ref>

The funnel of cephalopods develops on the top of their head, whereas the mouth develops on the opposite surface.<ref>{{cite book |isbn=978-0-306-43513-3 |first1=Daniel L. |last1=Gilbert |first2=William J. |last2=Adelman |first3=John M. |last3=Arnold |year=1990 |publisher=Plenum Press |location=New York |title=Squid as experimental animals |url=https://archive.org/details/squidasexperimen00gilb }}</ref>{{Rp|86}} The early embryological stages are reminiscent of ancestral gastropods and extant [[Biology:Monoplacophora|Monoplacophora]].<ref name=Shigeno2008/>

The shells develop from the ectoderm as an organic framework which is subsequently mineralized.<ref name="Baratte2007"/> In ''Sepia'', which has an internal shell, the ectoderm forms an invagination whose pore is sealed off before this organic framework is deposited.<ref name="Baratte2007"/>

==== Development ====
[[Image:Chtenopteryx sicula paralarvae.jpg|thumb|right|400px|''Chtenopteryx sicula'' paralarvae. '''Left:''' Two very young paralarvae. The circular tentacular clubs bear approximately 20 irregularly arranged suckers. Two chromatophores are present on each side of the mantle. '''Centre:''' Ventral, dorsal and side views of a more advanced paralarva. An equatorial circulet of seven large yellow-brown chromatophores is present on the mantle. Posteriorly the expanded vanes of the gladius are visible in the dorsal view. '''Right:''' Ventral and dorsal views of a very advanced paralarva.]]
{{multiple image
 | align    =right
 | footer   ='''Left:''' Immature specimens of ''Chiroteuthis veranyi''. In this paralarval form, known as the doratopsis stage, the pen is longer than the mantle and 'neck' combined<br />
'''Right:''' A mature ''Chiroteuthis veranyi''. This species has some of the longest tentacles in proportion to its size of any known cephalopod.
 | image1   =Chiroteuthis veranyi immature.jpg
 | width1   =100
 | image2   =Chiroteuthis_veranyi.jpg
 | width2   =226
}}
The length of time before hatching is highly variable; smaller eggs in warmer waters are the fastest to hatch, and newborns can emerge after as little as a few days. Larger eggs in colder waters can develop for over a year before hatching.<ref name="Boletzky2009">{{cite book 
| pmid=12846042 
| isbn=978-0-12-026144-4 
| year=2003 
| last1=Von Boletzky | first1=S. 
| title=Biology of early life stages in cephalopod molluscs 
| volume=44 
| pages=143–203 
| doi=10.1016/S0065-2881(03)44003-0
| series=Advances in Marine Biology 
}}</ref>

The process from spawning to hatching follows a similar trajectory in all species, the main variable being the amount of yolk available to the young and when it is absorbed by the embryo.<ref name=Boletzky2009/>

Unlike most other molluscs, cephalopods do not have a morphologically distinct [[Biology:Larva|larva]]l stage. Instead, the juveniles are known as paralarvae. They quickly learn how to hunt, using encounters with prey to refine their strategies.<ref name=Boletzky2009/>

Growth in juveniles is usually allometric, whilst adult growth is [[Biology:Allometry#Isometric scaling and geometric similarity|isometric]].<ref>{{cite journal |title=Understanding the process of growth in cephalopods |journal=Marine and Freshwater Research |issue=4 |year=2004 |first=Natalie A. |last=Moltschaniwskyj |pages=379–386 |volume=55 |doi=10.1071/MF03147|url=https://figshare.com/articles/journal_contribution/22847906 }}</ref>

== Evolution ==
The traditional view of cephalopod evolution holds that they evolved in the Late Cambrian from a monoplacophoran-like ancestor<ref name=Lemche1959>{{cite journal |last1=Lemche |first1=H. |last2=Wingstrand |first2=K. G. |url=http://www.zmuc.dk/inverweb/Galathea/index.html |format=Link to free full text + plates |year=1959 |title=The anatomy of ''Neopilina galatheae'' Lemche, 1957 (Mollusca, Tryblidiacea) |journal=Galathea Report |volume=3 |pages=9–73}}</ref> with a curved, tapering shell,<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=K. G. |year=1985 |title=On the anatomy and relationships of Recent Monoplacophora |journal=Galathea Report |volume=16 |pages=7–94 |access-date=2009-03-23 |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> which was closely related to the gastropods (snails).<ref name='Boyle3'>{{cite book |doi=10.1002/9780470995310.ch3 |isbn=9780470995310 |title=Cephalopods |year=2005 |last1=Boyle |chapter=Origin and Evolution |first1=P. |last2=Rodhouse |first2=P. |pages=36}}</ref> The similarity of the early shelled cephalopod ''[[Biology:Plectronoceras|Plectronoceras]]'' to some gastropods was used in support of this view. The development of a siphuncle would have allowed the shells of these early forms to become gas-filled (thus buoyant) in order to support them and keep the shells upright while the animal crawled along the floor, and separated the true cephalopods from putative ancestors such as ''Knightoconus'', which lacked a siphuncle.<ref name=Boyle3/> Neutral or positive buoyancy (i.e. the ability to float) would have come later, followed by swimming in the [[Biology:Plectronocerida|Plectronocerida]] and eventually jet propulsion in more derived cephalopods.<ref name=Kroger2007>{{cite journal |last1=Kröger |first1=B. R. |title=Some Lesser Known Features of the Ancient Cephalopod Order Ellesmerocerida (Nautiloidea, Cephalopoda) |year=2007 |journal=Palaeontology |volume=50 |issue=3 |pages=565–572 |doi=10.1111/j.1475-4983.2007.00644.x |bibcode=2007Palgy..50..565K |doi-access=free }}</ref>

Possible early Cambrian remains have been found in the Avalon Peninsula, matching genetic data for a pre-Cambrian origin.<ref name="ReferenceD">{{Cite journal |doi=10.1038/s42003-021-01885-w |title=A potential cephalopod from the early Cambrian of eastern Newfoundland, Canada |year=2021 |last1=Hildenbrand |first1=Anne |last2=Austermann |first2=Gregor |last3=Fuchs |first3=Dirk |last4=Bengtson |first4=Peter |last5=Stinnesbeck |first5=Wolfgang |journal=Communications Biology |volume=4 |issue=1 |page=388 |pmid=33758350 |pmc=7987959}}</ref>

However, some morphological evidence is difficult to reconcile with this view, and the redescription of ''[[Biology:Nectocaris|Nectocaris pteryx]]'', which did not have a shell and appeared to possess jet propulsion in the manner of "derived" cephalopods, complicated the question of the order in which cephalopod features developed – provided ''Nectocaris'' is a cephalopod at all.<ref name=SmithAndCaron2010>{{cite journal |title=Primitive soft-bodied cephalopods from the Cambrian |last1=Smith |first1=Martin R. |last2=Caron |first2=Jean-Bernard |s2cid=205055896 |year=2010 |journal=Nature |pages=427–428 |volume=465 |pmid=20505713 |issue=7297 |doi=10.1038/465427a |bibcode=2010Natur.465..427B }}</ref>

Early cephalopods were likely predators near the top of the food chain.<ref name=Boyle/> After the [[Earth:Cambrian–Ordovician extinction event|late Cambrian extinction]] led to the disappearance of many [[Biology:Anomalocaridid|Anomalocaridid]]s, predatory niches became available for other animals.<ref>{{cite book |last=Jain |first=Sreepat |title=Fundamentals of Invertebrate Palaeontology: Macrofossils |url=https://books.google.com/books?id=pTygDQAAQBAJ&pg=PA73 |year=2016 |publisher=Springer |isbn=978-81-322-3658-0 |page=73}}</ref> During the Ordovician period the primitive cephalopods underwent pulses of diversification<ref name="Kroger2009">{{cite journal |last2=Yun-bai |first1=B. |first2=Y. B. |title=Pulsed cephalopod diversification during the Ordovician |journal=Palaeogeography, Palaeoclimatology, Palaeoecology |volume=273 |issue=1–2 |pages=174–201 |year=2009 |last1=Kröger |doi=10.1016/j.palaeo.2008.12.015 |bibcode=2009PPP...273..174K}}</ref> to become diverse and dominant in the [[Earth:Paleozoic|Paleozoic]] and [[Earth:Mesozoic|Mesozoic]] seas.<ref name="Dzik1981">{{cite journal |last=Dzik |first=J. |year=1981 |title=Origin of the Cephalopoda |journal=Acta Palaeontologica Polonica |volume=26 |issue=2 |pages=161–191 |url=http://www.paleo.pan.pl/people/Dzik/Publications/Cephalopoda.pdf |archive-url=https://web.archive.org/web/20081216231345/http://www.paleo.pan.pl/people/Dzik/Publications/Cephalopoda.pdf |archive-date=2008-12-16 |url-status=live}}</ref>

In the Early Palaeozoic, their range was far more restricted than today; they were mainly constrained to sublittoral regions of shallow shelves of the low latitudes, and usually occurred in association with [[Biology:Thrombolite|thrombolite]]s.<ref name="Kroger2009b">{{cite journal |year=2009 |title=The origin and initial rise of pelagic cephalopods in the Ordovician |journal=PLOS ONE  |volume=4 |issue=9 |pages=e7262 |doi=10.1371/journal.pone.0007262 |pmid=19789709 |pmc=2749442 |last1=Kröger |first1=B. R. |last2=Servais |first2=T. |last3=Zhang |first3=Y. |last4=Kosnik |first4=M. |bibcode=2009PLoSO...4.7262K |doi-access=free }}</ref> A more pelagic habit was gradually adopted as the Ordovician progressed.<ref name="Kroger2009b"/> Deep-water cephalopods, whilst rare, have been found in the Lower Ordovician – but only in high-latitude waters.<ref name="Kroger2009b"/>
The mid-Ordovician saw the first cephalopods with septa strong enough to cope with the pressures associated with deeper water, and could inhabit depths greater than 100–200&nbsp;m.<ref name="Kroger2009" /> The direction of shell coiling would prove to be crucial to the future success of the lineages; endogastric coiling would only permit large size to be attained with a straight shell, whereas exogastric coiling – initially rather rare – permitted the spirals familiar from the fossil record to develop, with their corresponding large size and diversity.<ref name=Holland1986>{{cite journal |first1=C. H. |title=The nautiloid cephalopods: a strange success: President's anniversary address 1986 |journal=Journal of the Geological Society |volume=144 |issue=1 |last1=Holland |s2cid=128629737 |pages=1–15 |year=1987 |doi=10.1144/gsjgs.144.1.0001 |bibcode=1987JGSoc.144....1H}}</ref> (Endogastric means the shell is curved so as the ventral or lower side is longitudinally concave (belly in); exogastric means the shell is curved so as the ventral side is longitudinally convex (belly out) allowing the funnel to be pointed backward beneath the shell.)<ref name=Holland1986/>

[[File:1212amma.jpg|thumb|left|upright=1.2|An ammonoid with the body chamber missing, showing the septal surface (especially at right) with its undulating lobes and saddles]]

The ancestors of coleoids (including most modern cephalopods) and the ancestors of the modern nautilus, had diverged by the Floian Age of the Early Ordovician Period, over 470 million years ago.<ref name="Kroger2009b"/><ref name="Kröger2006">{{cite journal |last=Kröger |first=Björn |year=2006 |title=Early growth-stages and classification of orthoceridan cephalopods of the Darriwillian (Middle Ordovician) of Baltoscandia |journal=Lethaia |volume=39 |issue=2 |pages=129–139 |doi=10.1080/00241160600623749}}</ref> The [[Biology:Bactritida|Bactritida]], a Devonian–Triassic group of orthocones, are widely held to be paraphyletic without the coleoids and ammonoids, that is, the latter groups arose from within the Bactritida.<ref name=Young1998>{{cite journal |title=The evolution of coleoid cephalopods and their present biodiversity and ecology |last1=Young |first1=R. E. |last2=Vecchione |first2=M. |last3=Donovan |first3=D. T. |journal=South African Journal of Marine Science |volume=20 |issue=1 |year=1998 |pages=393–420 |doi=10.2989/025776198784126287}}</ref>{{rp|393}} An increase in the diversity of the coleoids and ammonoids is observed around the start of the Devonian period and corresponds with a profound increase in fish diversity. This could represent the origin of the two derived groups.<ref name=Young1998/>

Unlike most modern cephalopods, most ancient varieties had protective shells. These shells at first were conical but later developed into curved nautiloid shapes seen in modern nautilus species.
Competitive pressure from fish is thought to have forced the shelled forms into deeper water, which provided an evolutionary pressure towards shell loss and gave rise to the modern coleoids, a change which led to greater metabolic costs associated with the loss of buoyancy, but which allowed them to recolonize shallow waters.<ref name="Boyle3"/>{{Rp|36}} However, some of the straight-shelled [[Biology:Nautiloid|nautiloid]]s evolved into [[Biology:Belemnitida|belemnites]], out of which some evolved into [[Biology:Squid|squid]] and [[Biology:Cuttlefish|cuttlefish]].{{Verify source|date=November 2008}} The loss of the shell may also have resulted from evolutionary pressure to increase maneuverability, resulting in a more fish-like habit.<ref name="mollusca11"/>{{Rp|289}}

There has been debate on the [[Biology:Embryogenesis|embryological origin]] of cephalopod appendages.<ref name=Tanabe>{{cite book |title=Cephalopods – Present and Past |last=Tanabe |first=K. |publisher=Tokai University Press |year=2008 |location=Tokyo }}{{page needed|date=December 2017}}</ref> Until the mid-twentieth century, the "Arms as Head" hypothesis was widely recognized. In this theory, the arms and tentacles of cephalopods look similar to the head appendages of gastropods, suggesting that they might be [[Biology:Homology|homologous structures]]. Cephalopod appendages surround the mouth, so logically they could be derived from embryonic head tissues.<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 |s2cid=33835096 |date=2005-09-01 |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 |volume=38 |issue=3 |pages=209–221 |doi=10.1080/10236240500310096}}</ref> However, the "Arms as Foot" hypothesis, proposed by [[Biography:Adolf Naef|Adolf Naef]] in 1928, has increasingly been favoured;<ref name=Tanabe/> for example, [[Biology:Fate mapping|fate mapping]] of limb buds in the chambered nautilus indicates that limb buds originate from "foot" embryonic tissues.<ref>{{cite journal |last1=Shigeno |first1=Shuichi |last2=Sasaki |first2=Takenori |last3=Moritaki |first3=Takeya |last4=Kasugai |first4=Takashi |last5=Vecchione |first5=Michael |last6=Agata |first6=Kiyokazu |date=January 2008 |title=Evolution of the cephalopod head complex by assembly of multiple molluscan body parts: Evidence from Nautilus embryonic development |journal=Journal of Morphology |volume=269 |issue=1 |pages=1–17 |doi=10.1002/jmor.10564 |pmid=17654542|s2cid=13109195 }}</ref>

=== Genetics ===
The sequencing of a full Cephalopod genome has remained challenging to researchers due to the length and repetition of their DNA.<ref name="O'Brien 700">{{Cite journal |last1=O'Brien |first1=Caitlin E. |last2=Roumbedakis |first2=Katina |last3=Winkelmann |first3=Inger E. |date=2018-06-06 |title=The Current State of Cephalopod Science and Perspectives on the Most Critical Challenges Ahead From Three Early-Career Researchers |journal=Frontiers in Physiology |volume=9 |pages=700 |doi=10.3389/fphys.2018.00700 |pmid=29962956 |pmc=6014164 |issn=1664-042X |doi-access=free}}</ref> The characteristics of Cephalopod genomes were initially hypothesized to be the result of entire [[Biology:Gene duplication|genome duplications]]. Following the full sequencing of a [[Biology:California two-spot octopus|California two-spot octopus]], the genome showed similar patterns to other marine invertebrates with significant additions to the genome assumed to be unique to Cephalopods. No evidence of full genome duplication was found.<ref name="Albertin 220–224">{{Cite journal |last1=Albertin |first1=Caroline B. |last2=Simakov |first2=Oleg |last3=Mitros |first3=Therese |last4=Wang |first4=Z. Yan |last5=Pungor |first5=Judit R. |last6=Edsinger-Gonzales |first6=Eric |last7=Brenner |first7=Sydney |last8=Ragsdale |first8=Clifton W. |last9=Rokhsar |first9=Daniel S. |date=August 2015 |title=The octopus genome and the evolution of cephalopod neural and morphological novelties |journal=Nature |language=en |volume=524 |issue=7564 |pages=220–224 |doi=10.1038/nature14668 |pmid=26268193 |pmc=4795812 |bibcode=2015Natur.524..220A |issn=0028-0836}}</ref>

Within the California two-spot octopus genome there are substantial replications of two gene families. Significantly, the expanded gene families were only previously known to exhibit replicative behaviour within vertebrates.<ref name="Albertin 220–224"/> The first gene family was identified as the [[Biology:Protocadherin|Protocadherin]]s which are attributed to neuron development. Protocadherins function as cell adhesion molecules, essential for [[Biology:Synaptogenesis|synaptic specificity]]. The mechanism for Protocadherin gene family replication in vertebrates is attributed to complex splicing, or cutting and pasting, from a locus. Following the sequencing of the California two-spot octopus, researchers found that the Prorocadherin gene family in Cephalopods has expanded in the genome due to [[Biology:Tandem exon duplication|tandem gene duplication]]. The different replication mechanisms for Protocadherin genes indicate an independent evolution of Protocadherin gene expansion in vertebrates and invertebrates.<ref name="Albertin 220–224"/> Analysis of individual Cephalopod Protocadherin genes indicate independent evolution between species of Cephalopod. A species of shore squid ''[[Biology:Longfin inshore squid|Doryteuthis pealeii]]'' with expanded Protocadherin gene families differ significantly from those of the California two-spot octopus suggesting gene expansion did not occur before [[Biology:Speciation|speciation]] within Cephalopods. Despite different mechanisms for gene expansion, the two-spot octopus Protocadherin genes were more similar to vertebrates than squid, suggesting a [[Biology:Convergent evolution|convergent evolution]] mechanism. The second gene family known as {{chem2|C2H2}} are small proteins that function as [[Biology:Zinc finger transcription factor|zinc transcription factors]]. {{chem2|C2H2}} are understood to moderate DNA, RNA and protein functions within the cell.<ref name="O'Brien 700"/>

The sequenced California two spot octopus genome also showed a significant presence of [[Biology:Transposable element|transposable element]]s as well as transposon expression. Although the role of transposable elements in marine vertebrates is still relatively unknown, significant expression of transposons in nervous system tissues have been observed.<ref name="Gehring 117–142">{{cite book |last=Gehring |first=Mary A. |chapter=Imprinted Gene Expression and the Contribution of Transposable Elements |date=2013-02-04 |title=Plant Transposons and Genome Dynamics in Evolution |pages=117–142 |publisher=Wiley-Blackwell |doi=10.1002/9781118500156.ch7 |isbn=978-1-118-50015-6}}</ref> In a study conducted on vertebrates, the expression of transposons during development in the fruitfly ''[[Biology:Drosophila melanogaster|Drosophila melanogaster]]'' activated genomic diversity between neurons.<ref>{{Cite journal |last1=Erwin |first1=Jennifer A. |last2=Marchetto |first2=Maria C. |last3=Gage |first3=Fred H. |date=August 2014 |title=Mobile DNA elements in the generation of diversity and complexity in the brain |journal=Nature Reviews Neuroscience |language=en |volume=15 |issue=8 |pages=497–506 |doi=10.1038/nrn3730 |pmid=25005482 |pmc=4443810 |issn=1471-003X}}</ref> This diversity has been linked to increased memory and learning in mammals. The connection between transposons and increased neuron capability may provide insight into the observed intelligence, memory and function of Cephalopods.<ref name="Gehring 117–142"/>

Using long-read sequencing, researchers have decoded the cephalopod genomes and discovered they have been churned and scrambled. The genes were compared to those of thousands of other species and while blocks of three or more genes co-occurred between squid and octopus, the blocks of genes were not found together in any other animals'. Many of the groupings were in the nervous tissue, suggesting the course they adapted their intelligence.<ref>Caroline B. Albertin et al. May 4th, 2022 [https://www.nature.com/articles/s41467-022-29748-w "Genome and transcriptome mechanisms driving cephalopod evolution"] Nature Communications</ref><ref>Hannah Schimdbour et al. April 21, 2022 [https://www.nature.com/articles/s41467-022-29694-7 "Emergence of novel cephalopod gene regulation and expression through large-scale genome reorganization"] Nature Communications</ref>

===Phylogeny===

The approximate consensus of extant cephalopod phylogeny, after Whalen & Landman (2022), is shown in the [[Biology:Cladogram|cladogram]].<ref>{{Cite journal |last1=Whalen |first1=Christopher D. |last2=Landman |first2=Neil H. |date=2022-03-08 |title=Fossil coleoid cephalopod from the Mississippian Bear Gulch Lagerstätte sheds light on early vampyropod evolution |journal=Nature Communications |language=en |volume=13 |issue=1 |pages=1107 |doi=10.1038/s41467-022-28333-5 |pmid=35260548 |pmc=8904582 |bibcode=2022NatCo..13.1107W |issn=2041-1723}}</ref> Mineralized taxa are in '''bold'''.  
{{clade
   |label1=Cephalopoda
   |1= {{clade
      |label1= [[Biology:Nautiloid|Nautiloid]]s &nbsp; 
      |1= {{clade
          |1= '''''[[Biology:Nautilida|Nautilida]]''''' [[File:Nautilus Palau.JPG|70px]]
          }}
      |label2= Coleoids
         |2= {{clade
            |label1=[[Biology:Octopodiformes|Octopodiformes]] &nbsp; 
            |1= {{clade
                |1= [[Biology:Vampyromorphida|Vampyromorphida]] (vampire squid) [[File:Vampire des abysses.jpg|70px]]
                |2= Octopoda (octopus) [[File:Octopus vulgaris Merculiano.jpg|70px]]
                }}
         |label2= [[Decapodiformes]] &nbsp; 
         |2= {{clade
             |1={{clade 
                 |1={{extinct}}[[Biology:Belemnoidea|Belemnoidea]]
                 |2={{clade
             |1= '''''[[Biology:Spirulida|Spirulida]]''''' [[File:Spirula spirula illustration.jpg|70px]]
             |2={{clade
                |1= '''''[[Biology:Sepiida|Sepiida]]''''' (cuttlefish) [[File:Sepia officinalis 2.jpg|70px]]
                |2={{clade
                     |1={{clade
                       |1= ''Sepiolida'' (bobtail squid) [[File:Heteroteuthis hawaiiensis1.jpg|90px]]
                       |2= ''Idiosepida'' [[File:Tropical Pygmy Squid (Idiosepius pygmaeus) (25377304317).jpg|70px]]
                     }}
                    |2={{clade
                        |1=''[[Biology:Myopsida|Myopsida]]'' [[File:アオリイカ-Sepioteuthis lessoniana.jpg|70px]]
                        |2={{clade
                            |1=[[Biology:Bathyteuthida|Bathyteuthida]] [[File:Bathyteuthisabyssicola.jpg|90px]]
                            |2=[[Biology:Oegopsida|Oegopsida]]
                             }}
                             }}
                         }}
                     }}
                 }}
             }}
         }}
     }}
 }}
}}

The internal phylogeny of the cephalopods is difficult to constrain; many molecular techniques have been adopted, but the results produced are conflicting.<ref name=Strugnell2007>{{cite journal |doi=10.1093/mollus/eym038 |title=Molecular phylogeny of coleoid cephalopods (Mollusca: Cephalopoda) inferred from three mitochondrial and six nuclear loci: a comparison of alignment, implied alignment and analysis methods |year=2007 |last1=Strugnell |first1=J. |last2=Nishiguchi |first2=M. K. |journal=Journal of Molluscan Studies |volume=73 |issue=4 |pages=399–410 |doi-access=free}}</ref><ref name=Strugnell2005>{{cite journal |last1=Strugnell |first1=J. |last2=Norman |first2=M. |last3=Jackson |first3=J. |last4=Drummond |first4=A. |last5=Cooper |first5=A. |title=Molecular phylogeny of coleoid cephalopods (Mollusca: Cephalopoda) using a multigene approach; the effect of data partitioning on resolving phylogenies in a Bayesian framework |journal=Molecular Phylogenetics and Evolution |volume=37 |issue=2 |pages=426–441 |year=2005 |pmid=15935706 |doi=10.1016/j.ympev.2005.03.020}}</ref> ''Nautilus'' tends to be considered an outgroup, with ''[[Biology:Vampyroteuthis|Vampyroteuthis]]'' forming an outgroup to other squid; however in one analysis the nautiloids, octopus and teuthids plot as a [[Biology:Polytomy|polytomy]].<ref name=Strugnell2007/> Some molecular phylogenies do not recover the mineralized coleoids (''Spirula'', ''Sepia'', and ''Metasepia'') as a clade; however, others do recover this more parsimonious-seeming clade, with ''Spirula'' as a sister group to ''Sepia'' and ''Metasepia'' in a clade that had probably diverged before the end of the Triassic.<ref>{{cite journal |last4=Cooper |first3=A. J. |first4=A. |journal=Cladistics |pages=89–96 |volume=22 |last3=Drummond |first2=J. |year=2006 |title=Divergence time estimates for major cephalopod groups: evidence from multiple genes |last1=Strugnell |first1=J. |last2=Jackson |issue=1 |doi=10.1111/j.1096-0031.2006.00086.x |pmid=34892892 |s2cid=84743000 }}</ref><ref>{{cite journal |doi=10.1093/oxfordjournals.molbev.a026419 |pmid=10958852 |year=2000 |last1=Carlini |first1=D. B. |last2=Reece |first2=K. S. |last3=Graves |first3=J. E. |title=Actin gene family evolution and the phylogeny of coleoid cephalopods (Mollusca: Cephalopoda) |volume=17 |issue=9 |pages=1353–1370 |journal=Molecular Biology and Evolution |doi-access=free }}</ref>

Molecular estimates for clade divergence vary. One 'statistically robust' estimate has ''Nautilus'' diverging from ''Octopus'' at {{Ma|415|error=24}}.<ref>{{cite journal |doi=10.1007/s00239-005-0160-x |pmid=16501879 |year=2006 |last1=Bergmann |first1=S. |last2=Lieb |first2=B. |last3=Ruth |first3=P. |last4=Markl |first4=J. |s2cid=4389953 |title=The hemocyanin from a living fossil, the cephalopod ''Nautilus pompilius'': protein structure, gene organization, and evolution |volume=62 |issue=3 |pages=362–374 |journal=Journal of Molecular Evolution |bibcode=2006JMolE..62..362B}}</ref>

===Taxonomy===
[[File:Nautilus pompilius 3.jpg|thumb|230px|Chambered nautilus (''Nautilus pompilius'')]]
[[File:Sepia officinalis (aquarium).jpg|thumb|230px|Common cuttlefish (''Sepia officinalis'')]]
[[Image:Sepiola atlantica.jpg|thumb|230px|Atlantic bobtail (''Sepiola atlantica'')]]
[[Image:Loligo vulgaris.jpg|thumb|230px|[[Biology:European squid|European squid]] (''Loligo vulgaris'')]]
[[Image:Octopus vulgaris2.jpg|thumb|230px|[[Biology:Common octopus|Common octopus]] (''Octopus vulgaris'')]]
The classification presented here, for recent cephalopods, follows largely from [http://www.mnh.si.edu/cephs/newclass.pdf Current Classification of Recent Cephalopoda] (May 2001), for fossil cephalopods takes from Arkell et al. 1957, Teichert and Moore 1964, Teichert 1988, and others. The three subclasses are traditional, corresponding to the three orders of cephalopods recognized by Bather.<ref>
{{cite journal
 | last=Bather |first= F.A. | year=1888b
 | title=Professor Blake and Shell-Growth in Cephalopoda
 | journal=Annals and Magazine of Natural History
 | volume=1|issue= 6 |series =6 | pages=421–426
 | url=https://www.biodiversitylibrary.org/page/26195416 | doi=10.1080/00222938809460761
}}
</ref>

'''Class Cephalopoda''' († indicates extinct groups)
* Subclass [[Biology:Nautiloid|Nautiloid]]ea: Fundamental ectocochliate cephalopods that provided the source for the Ammonoidea and Coleoidea.
** Order † [[Biology:Plectronocerida|Plectronocerida]]: the ancestral cephalopods from the [[Earth:Cambrian|Cambrian]] Period
** Order † [[Biology:Ellesmerocerida|Ellesmerocerida]] ({{Ma|500|470|Ma}})
** Order † [[Biology:Endocerida|Endocerida]] ({{Ma|485|430|Ma}})
** Order † [[Biology:Actinocerida|Actinocerida]] ({{Ma|480|312|Ma}})
** Order † [[Biology:Discosorida|Discosorida]] ({{Ma|482|392|Ma}})
** Order † [[Biology:Pseudorthocerida|Pseudorthocerida]] ({{Ma|432|272|Ma}})
** Order † [[Biology:Tarphycerida|Tarphycerida]] ({{Ma|485|386|Ma}})
** Order † [[Biology:Oncocerida|Oncocerida]] ({{Ma|478.5|324.0|Ma}})
** Order [[Biology:Nautilida|Nautilida]] (extant; 410.5 Ma to present)
** Order † [[Biology:Orthocerida|Orthocerida]] ({{Ma|482.5|211.5|Ma}})
** Order † [[Biology:Ascocerida|Ascocerida]] ({{Ma|478|412|Ma}})
** Order † [[Biology:Bactritida|Bactritida]] ({{Ma|418.1|260.5|Ma}})
* Subclass † [[Biology:Ammonoidea|Ammonoidea]]: Ammonites ({{Ma|479|66|Ma}})
** Order † [[Biology:Goniatitida|Goniatitida]] ({{Ma|388.5|252.0|Ma}})
** Order † [[Biology:Ceratitida|Ceratitida]] ({{Ma|254|200|Ma}})
** Order † [[Biology:Ammonitida|Ammonitida]] ({{Ma|215|66|Ma}})
* Subclass [[Biology:Coleoidea|Coleoidea]] (410.0 Ma-Rec)
** Cohort † [[Biology:Belemnoidea|Belemnoidea]]: Belemnites and kin
*** Genus † ''[[Biology:Jeletzkya|Jeletzkya]]''<!-- Palaeontology43(5): 919-926 -->
*** Order † [[Biology:Aulacocerida|Aulacocerida]] ({{Ma|265|183|Ma}})
*** Order † [[Biology:Phragmoteuthida|Phragmoteuthida]] ({{Ma|189.6|183.0|Ma}})
*** Order † [[Biology:Hematitida|Hematitida]] ({{Ma|339.4|318.1|Ma}})
*** Order † [[Biology:Belemnitida|Belemnitida]] ({{Ma|339.4|66|Ma}})
*** Genus † ''Belemnoteuthis'' ({{Ma|189.6|183.0|Ma}})
** Cohort [[Biology:Neocoleoidea|Neocoleoidea]]
*** Superorder [[Decapodiformes]] (also known as Decabrachia or Decembranchiata)
**** Order [[Biology:Spirulida|Spirulida]]: Ram's horn squid
**** Order [[Biology:Sepiida|Sepiida]]: cuttlefish
**** Order Sepiolida: pygmy, bobtail and bottletail squid
**** Order [[Biology:Idiosepiidae|Idiosepida]]
**** Order [[Biology:Oegopsida|Oegopsida]]: neritic squid
**** Order [[Biology:Myopsida|Myopsida]]: coastal squid
**** Order [[Biology:Bathyteuthida|Bathyteuthida]]
*** Superorder [[Biology:Octopodiformes|Octopodiformes]] (also known as Vampyropoda)
**** Family † Trachyteuthididae
**** Order [[Biology:Vampyromorphida|Vampyromorphida]]: Vampire squid
**** Order Octopoda: octopus
*** Superorder † [[Biology:Palaeoteuthomorpha|Palaeoteuthomorpha]]
**** Order † Boletzkyida

Other classifications differ, primarily in how the various [[Decapodiformes|decapod]] orders are related, and whether they should be orders or families.

====Suprafamilial classification of the Treatise====
This is the older classification that combines those found in parts K and L of the ''Treatise on Invertebrate Paleontology'', which forms the basis for and is retained in large part by classifications that have come later.

Nautiloids in general (Teichert and Moore, 1964) sequence as given.

: Subclass † [[Biology:Endocerida|Endoceratoidea]]. Not used by Flower, e.g. Flower and Kummel 1950, interjocerids included in the Endocerida.
:: Order † [[Biology:Endocerida|Endocerida]]
:: Order † [[Biology:Intejocerida|Intejocerida]]

: Subclass † [[Biology:Actinocerida|Actinoceratoidea]] Not used by Flower, ibid
:: Order † [[Biology:Actinocerida|Actinocerida]]

: Subclass [[Biology:Nautiloid|Nautiloid]]ea Nautiloidea in the restricted sense.
:: Order † [[Biology:Ellesmerocerida|Ellesmerocerida]] Plectronocerida subsequently split off as separate order.
:: Order † [[Biology:Orthocerida|Orthocerida]] Includes orthocerids and pseudorthocerids
:: Order † [[Biology:Ascocerida|Ascocerida]]
:: Order † [[Biology:Oncocerida|Oncocerida]]
:: Order † [[Biology:Discosorida|Discosorida]]
:: Order † [[Biology:Tarphycerida|Tarphycerida]]
:: Order † [[Biology:Tarphycerida|Barrandeocerida]] A polyphyletic group now included in the Tarphycerida
:: Order [[Biology:Nautilida|Nautilida]]

: Subclass † [[Biology:Bactritida|Bactritoidea]]
:: Order † [[Biology:Bactritida|Bactritida]]

Paleozoic Ammonoidea (Miller, Furnish and Schindewolf, 1957)
:: Suborder † Anarcestina
:: Suborder † [[Biology:Clymeniida|Clymeniina]]
:: Suborder † [[Biology:Goniatitida|Goniatitina]]
:: Suborder † [[Biology:Prolecanitida|Prolecanitina]]

Mesozoic Ammonoidea (Arkel et al., 1957)
:: Suborder † [[Biology:Ceratitida|Ceratitina]]
:: Suborder † Phylloceratina
:: Suborder † Lytoceratina
:: Suborder † [[Biology:Ammonitida|Ammonitina]]

Subsequent revisions include the establishment of three Upper Cambrian orders, the Plectronocerida, Protactinocerida, and Yanhecerida; separation of the pseudorthocerids as the Pseudorthocerida, and elevating orthoceratid as the Subclass Orthoceratoidea.

==== Shevyrev classification ====
Shevyrev (2005) suggested a division into eight subclasses, mostly comprising the more diverse and numerous fossil forms,<ref>
{{cite journal
 | last=Shevyrev
 | first=A. A.
 | year=2005
 | title=The Cephalopod Macrosystem: A Historical Review, the Present State of Knowledge, and Unsolved Problems: 1. Major Features and Overall Classification of Cephalopod Mollusks
 | journal=Paleontological Journal
 | volume=39 | issue=6 | pages=606–614}} Translated from ''Paleontologicheskii Zhurnal'' No. 6, 2005, 33–42.
</ref><ref>
{{cite journal
 | last=Shevyrev
 | first=A. A.
 | s2cid=84616115
 | year=2006
 | title=The cephalopod macrosystem; a historical review, the present state of knowledge, and unsolved problems; 2, Classification of nautiloid cephalopods
 | journal=Paleontological Journal
 | volume=40
 | issue=1
 | pages=46–54
 | doi=10.1134/S0031030106010059
}}
</ref> although this classification has been criticized as arbitrary, lacking evidence, and based on misinterpretations of other papers.<ref>{{cite web |last=Kroger |first=B. |title=Peer review in the Russian 'Paleontological Journal' |url=http://www.tiefes-leben.de/2009/03/peer-review-in-the-russian-paleontological-journal |url-status=dead |archive-url=https://web.archive.org/web/20090831224201/http://www.tiefes-leben.de/2009/03/peer-review-in-the-russian-paleontological-journal/ |archive-date=2009-08-31}}</ref>
[[File:Gyronaedyceras eryx.jpg|thumb|''Gyronaedyceras eryx'', a nautiloid from the [[Earth:Devonian|Devonian]] of Wisconsin]]
[[File:Ammonites 180308.jpg|thumb|Various species of ammonites]]
[[File:Ostenoteuthis siroi.JPG|thumb|Holotype of ''Ostenoteuthis siroi'' from family Ostenoteuthidae]]
[[File:Fossil-Belemnoidea-complete.jpg|thumb|A fossilised belemnite]]
'''Class Cephalopoda'''
* Subclass † Ellesmeroceratoidea
**Order † [[Biology:Plectronocerida|Plectronocerida]] ({{Ma|501|490|Ma}})
**Order † [[Biology:Protactinocerida|Protactinocerida]]
**Order † [[Biology:Yanhecerida|Yanhecerida]]
**Order † [[Biology:Ellesmerocerida|Ellesmerocerida]] ({{Ma|500|470|Ma}})
* Subclass † [[Biology:Endocerida|Endoceratoidea]] ({{Ma|485|430|Ma}})
**Order † [[Biology:Endocerida|Endocerida]] ({{Ma|485|430|Ma}})
**Order † [[Biology:Intejocerida|Intejocerida]] ({{Ma|485|480|Ma}})
* Subclass † [[Biology:Actinocerida|Actinoceratoidea]]
** Order † [[Biology:Actinocerida|Actinocerida]] ({{Ma|480|312|Ma}})
* Subclass [[Biology:Nautiloid|Nautiloid]]ea (490.0 Ma- Rec)
** Order † [[Biology:Basslerocerida|Basslerocerida]] ({{Ma|490|480|Ma}})
** Order † [[Biology:Tarphycerida|Tarphycerida]] ({{Ma|485|386|Ma}})
** Order † [[Biology:Lituitida|Lituitida]] ({{Ma|485|480|Ma}})
** Order † [[Biology:Discosorida|Discosorida]] ({{Ma|482|392|Ma}})
** Order † [[Biology:Oncocerida|Oncocerida]] ({{Ma|478.5|324.0|Ma}})
** Order [[Biology:Nautilida|Nautilida]] (410.5 Ma-Rec)
* Subclass † [[Biology:Orthoceratoidea|Orthoceratoidea]] ({{Ma|482.5|211.5|Ma}})
** Order † [[Biology:Orthocerida|Orthocerida]] ({{Ma|482.5|211.5|Ma}})
** Order † [[Biology:Ascocerida|Ascocerida]] ({{Ma|478|412|Ma}})
** Order † [[Biology:Dissidocerida|Dissidocerida]] ({{Ma|479.0|457.5|Ma}})
** Order † [[Biology:Bajkaloceras|Bajkalocerida]]
* Subclass † Bactritoidea ({{Ma|422|252|Ma}})
* Subclass † [[Biology:Ammonoidea|Ammonoidea]] ({{Ma|410|66|Ma}})
* Subclass [[Biology:Coleoidea|Coleoidea]] (410.0 Ma-rec)<ref>
{{cite journal
 | last=Bather |first=F.A.
 | year=1888a
 | title=Shell-growth in Cephalopoda (Siphonopoda)
 | journal=Annals and Magazine of Natural History
 | volume=1 |issue=4
 |series =6 | pages=298–310
 | url=https://www.biodiversitylibrary.org/page/26195293
 | doi=10.1080/00222938809460727
}}
</ref>

===Cladistic classification===
[[Image:Vampylarge.JPG|thumb|250px|Pyritized fossil of ''Vampyronassa rhodanica'', a vampyromorphid from the Lower Callovian ({{Ma|Callovian}})]]
Another recent system divides all cephalopods into two [[Biology:Clade|clade]]s. One includes nautilus and most fossil nautiloids. The other clade (Neocephalopoda or Angusteradulata) is closer to modern coleoids, and includes belemnoids, ammonoids, and many orthocerid families. There are also [[Biology:Stem group|stem group]] cephalopods of the traditional [[Biology:Ellesmerocerida|Ellesmerocerida]] that belong to neither clade.<ref>
{{cite journal
 | last1=Berthold |first1=Thomas | last2= Engeser |first2=Theo | year=1987
 | title=Phylogenetic analysis and systematization of the Cephalopoda (Mollusca)
 | journal=Verhandlungen Naturwissenschaftlichen Vereins in Hamburg
 | volume=29 | pages= 187–220
}}
</ref><ref>
{{cite web |url=http://userpage.fu-berlin.de/~palaeont/fossilnautiloidea/fossnautcontent.htm |title=Fossil Nautiloidea Page |last=Engeser |first=Theo |year=1997 |archive-url=https://web.archive.org/web/20060925110442/http://userpage.fu-berlin.de/~palaeont/fossilnautiloidea/fossnautcontent.htm |archive-date=2006-09-25}}
</ref>

The coleoids, despite some doubts,<ref name="mollusca11"/>{{Rp|289}} appear from molecular data to be monophyletic.<ref name=Lindgren2004>{{cite journal |doi=10.1111/j.1096-0031.2004.00032.x |title=A combined approach to the phylogeny of Cephalopoda (Mollusca) |year=2004 |last1=Lindgren |first1=A. R. |last2=Giribet |first2=G. |last3=Nishiguchi |first3=M. K. |journal=Cladistics |volume=20 |issue=5 |pages=454–486 |pmid=34892953 |citeseerx=10.1.1.693.2026 |s2cid=85975284}}</ref>

==In culture==
[[File:Colossal octopus by Pierre Denys de Montfort.jpg|thumb|upright|right|Pen and wash drawing of an imagined colossal octopus attacking a ship, by the malacologist Pierre de Montfort, 1801|alt=Coloured drawing of a huge octopus rising from the sea and attacking a sailing ship's three masts with its spiraling arms]]

Ancient seafaring people were aware of cephalopods, as evidenced by such artworks as a stone carving found in the archaeological recovery from Bronze Age Minoan Crete at Knossos (1900 – 1100 BC), which has a depiction of a fisherman carrying an octopus.<ref>{{cite web |last=Hogan |first=C. Michael |date=22 December 2007 |url=http://www.themodernantiquarian.com/site/10854/knossos.html#fieldnotes |title=Knossos fieldnotes |website=The Modern Antiquarian}}</ref> The terrifyingly powerful [[Unsolved:Gorgon|Gorgon]] of [[Religion:Greek mythology|Greek mythology]] may have been inspired by the octopus or squid, the octopus's body representing the severed head of [[Unsolved:Medusa|Medusa]], the beak as the protruding tongue and fangs, and its tentacles as the snakes.<ref>{{cite book |url={{google books|plainurl=y|id=OnHO4orvz18C}} |title=Medusa: Solving the Mystery of the Gorgon |last=Wilk |first=Stephen R. |date=2000 |publisher=Oxford University Press |isbn=978-0-19-988773-6}}</ref>

[[File:NROL-39 Patch.jpg|thumb|The NROL-39 mission patch, depicting the National Reconnaissance Office as an octopus with a long reach|alt=A mission badge of an octopus spanning the world against a starry background, labelled "NROL-39" and "Nothing is beyond our reach"]]
The Kraken are legendary sea monsters of giant proportions said to dwell off the coasts of Norway and Greenland, usually portrayed in art as giant cephalopods attacking ships. [[Biography:Carl Linnaeus|Linnaeus]] included it in the first edition of his 1735 ''[[Biology:Systema Naturae|Systema Naturae]]''.<ref>{{cite web |url={{google books |plainurl=y |id=WfQTAAAAQAAJ|page=82}} |title=Caroli Linnaei Systema naturae sistens regna tria naturae |work=google.com}}</ref><ref name=metropolitana>{{cite book |first1=Edward |last1=Smedley |first2=Hugh James |last2=Rose |first3=Henry John |last3=Rose |title=Encyclopaedia Metropolitana, Or, Universal Dictionary of Knowledge: Comprising the Twofold Advantage of a Philosophical and an Alphabetical Arrangement, with Appropriate Engravings |url={{google books |plainurl=y |id=3X1GAQAAIAAJ|page=255}} |year=1845 |publisher=B. Fellowes |pages=255–}}</ref> In a Hawaiian [[Astronomy:Creation myth|creation myth]] that says the present cosmos is the last of a series which arose in stages from the ruins of the previous universe, the octopus is the lone survivor of the previous, alien universe.<ref name=Dixon>{{cite book | volume=9 | last=Dixon | first=Roland Burrage |  title=Oceanic | series=The Mythology of All Races | url={{google books |plainurl=y |id=gLIIAQAAIAAJ&pg=PP2}} | date=1916 | publisher=Marshall Jones Company | pages=2–}}</ref> The [[Unsolved:Akkorokamui|Akkorokamui]] is a gigantic tentacled [[Biology:Monster|monster]] from Ainu folklore.<ref name="Batchelor">{{cite book | last = Batchelor | first = John | title = The Ainu and Their Folklore | url = https://archive.org/details/b29010664 | location = London | publisher = The Religious Tract Society | year = 1901}}</ref>

A battle with an octopus plays a significant role in [[Biography:Victor Hugo|Victor Hugo]]'s book ''Travailleurs de la mer'' (''Toilers of the Sea''), relating to his time in exile on Guernsey.<ref>{{cite EB1911 | wstitle=Octopus}}</ref> 
Ian Fleming's 1966 short story collection ''Octopussy and The Living Daylights'', and the 1983 ''James Bond'' film were partly inspired by Hugo's book.<ref>{{cite journal|last1=Cohen-Vrignaud |first1=Gerard |title=On Octopussies, or the Anatomy of Female Power |journal=Differences |date=2012 |volume=23 |issue=2 |pages=32–61 |doi=10.1215/10407391-1533520 }}</ref>

Japanese erotic art, ''shunga'', includes ukiyo-e woodblock prints such as Katsushika Hokusai's 1814 print ''Tako to ama'' (The Dream of the Fisherman's Wife), in which an ama diver is sexually intertwined with a large and a small octopus.<ref name=Helsinki>{{cite book|first1=Sointu |last1=Fritze|first2=Saara|last2= Suojoki|title=Forbidden Images: Erotic Art from Japan's Edo Period|url={{google books |plainurl=y |id=QRtmAAAACAAJ|pate=23}}|year=2000|publisher=Helsingin kaupungin taidemuseo|isbn=978-951-8965-54-4| language=fi | pages=23–28 }}</ref><ref>{{cite book | title=Japanese Erotic Fantasies: Sexual Imagery of the Edo Period | last=Uhlenbeck | first=Chris | author2=Margarita Winkel | author3=Ellis Tinios | author4=Amy Reigle Newland | year=2005 | publisher=Hotei | isbn=978-90-74822-66-4 | page=161}}</ref> The print is a forerunner of tentacle erotica.<ref name=Briel>{{cite book | last=Briel | first=Holger | date=2010 | title=The Roving Eye Meets Traveling Pictures: The Field of Vision and the Global Rise of Adult Manga | editor1-last=Berninger | editor1-first=Mark | editor2-last=Ecke | editor2-first=Jochen | editor3-last=Haberkorn | editor3-first=Gideon | work=Comics As a Nexus of Cultures: Essays on the Interplay of Media, Disciplines |url={{google books |plainurl=y |id=e-aWWTZeergC|page=203}}| publisher=McFarland | isbn=978-0-7864-3987-4 | page=203}}</ref> The biologist P. Z. Myers noted in his science blog, [[Biology:Pharyngula (blog)|Pharyngula]], that octopuses appear in "extraordinary" graphic illustrations involving women, tentacles, and bare breasts.<ref>{{cite web | last1=Myers | first1=P. Z. |  title=Extraordinary Octopus Illustrations | url=http://scienceblogs.com/pharyngula/2011/05/17/extraordinary-octopus-illustra/ | website=[[Biology:Pharyngula|Pharyngula]] | access-date=18 March 2017 | date=17 May 2017}}</ref><ref>{{cite web | last1=Myers | first1=P. Z. | author-link1=PZ Myers | title=Definitely not safe for work | url=http://scienceblogs.com/pharyngula/2006/10/29/definitely-not-safe-for-work/ | website=[[Biology:Pharyngula|Pharyngula]] | access-date=18 March 2017 | date=29 October 2006}}</ref>

Its many arms that emanate from a common center means that the octopus is sometimes used to symbolize a powerful and manipulative organization.<ref>{{cite web | last=Smith | first=S. | url=http://www.imediaethics.org/why-mark-zuckerberg-octopus-cartoon-evokes-nazi-propaganda-german-paper-apologizes/ | title=Why Mark Zuckerberg Octopus Cartoon Evokes 'Nazi Propaganda,' German Paper Apologizes | publisher=iMediaEthics | date=26 February 2010 | access-date=31 May 2017}}</ref>

==See also==
*[[Biology:Cephalopod size|Cephalopod size]]
*Cephalopod eye
*Cephalopod intelligence
*[[Biology:Pain in cephalopods|Pain in cephalopods]]
*Kraken
*List of nautiloids
*List of ammonites

==References==
{{Reflist}}

==Further reading==
*{{cite journal |doi=10.1134/S0031030108110014 |title=Cephalopods in the marine ecosystems of the Paleozoic |year=2008 |last1=Barskov |first1=I. S. |first2=M. S. |last2=Boiko |first3=V. A. |last3=Konovalova |first4=T. B. |last4=Leonova |first5=S. V. |last5= Nikolaeva |s2cid=83608661 |journal=Paleontological Journal |volume=42 |pages=1167–1284 |issue=11 }} A comprehensive overview of Paleozoic cephalopods.
* {{cite book |last1=Campbell |first1=Neil A. |last2=Reece |first2=Jane B. |last3=Mitchell |first3=Lawrence G. |title=Biology, fifth edition |publisher=Addison Wesley Longman, Inc. |location=Menlo Park, California |year=1999 |isbn=978-0-8053-6566-5 }}
* Felley, J., Vecchione, M., Roper, C. F. E., Sweeney, M. & Christensen, T., 2001–2003: ''Current Classification of Recent Cephalopoda''. [http://www.mnh.si.edu/cephs National Museum of Natural History: Department of Systematic Biology: Invertebrate Zoology: Cephalopods]
* {{cite book |title=Octopus, Squid, and Cuttlefish: A Visual, Scientific Guide to the Oceans' Most Advanced Invertebrates |year=2018 |first1=Roger |last1=Hanlon |first2=Mike |last2=Vecchione |first3=Louise |last3=Allcock |publisher=University of Chicago Press |isbn=978-0226459561}}
* N. Joan Abbott, Roddy Williamson, Linda Maddock. ''Cephalopod Neurobiology''. Oxford University Press, 1995. {{ISBN|0-19-854790-0}}
* Marion Nixon & John Z. Young. ''The brains and lives of Cephalopods''. Oxford University Press, 2003. {{ISBN|0-19-852761-6}}
* Hanlon, Roger T. & John B. Messenger. ''[https://books.google.com/books?id=Nxfv6xZZ6WYC&pg=PR47 Cephalopod Behaviour]''. Cambridge University Press, 1996. {{ISBN|0-521-42083-0}}
* Martin Stevens & Sami Merilaita. ''Animal camouflage: mechanisms and function''. Cambridge University Press, 2011. {{ISBN|0-521-19911-5}}
* {{cite journal |last1=Rodhouse |first1=P. G. |last2=Nigmatullin |first2=Ch. M. |title=Role as Consumers |journal=Philosophical Transactions of the Royal Society B: Biological Sciences |year=1996 |volume=351 |issue=1343 |pages=1003–1022 |doi=10.1098/rstb.1996.0090}}
* Classification key to modern cephalopods: ftp://ftp.fao.org/docrep/fao/009/a0150e/a0150e03.pdf{{Dead link|date=November 2019 |bot=InternetArchiveBot |fix-attempted=yes }}

==External links==
{{EB1911 poster|Cephalopoda}}

*[http://www.cco.caltech.edu/~brokawc/Bi11/cephalopods.html Fish vs. Cephalopods]
*[http://www.tonmo.com/ TONMO.COM – The Octopus News Magazine Online – cephalopod articles and discussion]
*[http://www.scientificamerican.com/article.cfm?id=can-squid-fly Scientific American: Can a Squid Fly Out of the Water?]
*[https://www.ibiology.org/ecology/cephalopods/ Roger Hanlon's Seminar: "Rapid Adaptive Camouflage and Signaling in Cephalopods"]
*[https://www.buzzfeed.com/mattcherette/six-terrifying-close-up-photos-of-deep-sea-dwellin Deep Sea Dwelling Bristle Worms]

{{Taxonbar|from=Q128257}}

[[Category:Cephalopods| ]]
[[Category:Marine molluscs]]
[[Category:Extant Cambrian first appearances]]

{{Sourceattribution|Cephalopod}}