Editing Octopus (section)

===External characteristics===
The octopus has an elongated body that is [[Symmetry in biology|bilaterally symmetrical]] along its dorso-ventral (back to belly) axis; the head and [[Foot (mollusc)|foot]] are on the ventral side, but act as the anterior (front). The heads contains both the mouth and the brain.<ref name=Ruppert/>{{rp|343–344}} The mouth has a sharp [[chitin]]ous [[Cephalopod beak|beak]] and is surrounded by and underneath the foot, which evolved into flexible, prehensile [[cephalopod limb]]s, known as "arms", which are attached to each other near their base by a webbed structure.<ref name=Ruppert>{{cite book |title=Invertebrate Zoology |last1=Ruppert |first1=Edward E. |last2=Fox |first2=Richard S. |last3=Barnes |first3=Robert D. |year=2004 |publisher=Cengage Learning |isbn=978-0-03-025982-1 }}</ref>{{rp|343–344}}<ref name=Courage>{{cite book |last=Courage |first=K. H. |year=2013 |title=Octopus! The Most Mysterious Creature in the Sea |url={{google books |plainurl=y |id=eSMRlaceRIEC}} |publisher=[[Penguin Group]] |isbn=978-0-698-13767-7}}</ref>{{rp|40–41}}<ref name=Mather/>{{rp|13–15}} The arms can be described based on side and sequence position (such as L1, R1, L2, R2) and divide into four pairs.<ref name=Wells>{{cite book |last=Wells |first=M. J. |year=1978 |url={{google books |plainurl=y |id=AM_tCAAAQBAJ}} |title=Octopus, Physiology and Behaviour of an Advanced Invertebrate |publisher=[[Springer Science+Business Media]] |isbn=978-94-017-2470-8}}</ref>{{rp|12}} The two rear appendages are generally used to walk on the sea floor, while the other six are used to forage for food.<ref>{{cite journal |title=Does ''Octopus vulgaris'' have preferred arms? |last1=Byrne |first1=Ruth A. |last2=Kuba |first2=Michael J. |last3=Meisel |first3=Daniela V. |last4=Griebel |first4=Ulrike |last5=Mather |first5=Jennifer A. |journal=[[Journal of Comparative Psychology]] |volume=120 |number=3 |date=August 2006 |pages=198–204 |doi=10.1037/0735-7036.120.3.198 |pmid=16893257 }}</ref> The bulbous and hollow [[Mantle (mollusc)|mantle]] is fused to the back of the head and contains most of the vital organs.<ref name=Mather>{{cite book |last1=Mather |first1=J. A. |last2=Anderson |first2=R. C. |last3=Wood |first3=J. B. |year=2010 |title=Octopus: The Ocean's Intelligent Invertebrate |url={{google books |plainurl=y |id=m-Mv7awvtIQC}} |publisher=[[Timber Press]] |isbn=978-1-60469-067-5}}
</ref>{{rp|13–15}}<ref name=Courage/>{{rp|40–41}} The mantle also has a cavity with muscular walls and a pair of gills; it is connected to the exterior by a funnel or [[Siphon (mollusc)|siphon]].<ref name=Ruppert/>{{rp|343–344}}<ref>{{Cite journal |last1=Semmens |title=Understanding octopus growth: patterns, variability and physiology |date=2004 |doi=10.1071/MF03155 |volume=55 |issue=4 |journal=Marine and Freshwater Research |page=367 |bibcode=2004MFRes..55..367S |s2cid=84208773 }}</ref>

[[File:Schematic lateral aspect of octopod features.jpg|thumb|400px|left|alt=Schematic of external anatomy|Diagram of octopus from side, with gills, funnel, eye, [[ocellus]] (eyespot), web, arms, suckers, [[hectocotylus]] and [[hectocotylus|ligula]] labelled.]]

The skin consists of a thin [[epidermis]] with mucous cells and sensory cells and a fibrous [[dermis]] made of [[collagen]] and containing various cells that allow colour change.<ref name=Ruppert/>{{rp|362}} Most of the body is made of soft tissue, allowing it to squeeze through tiny gaps; even the larger species can pass through a gap little more than {{convert|2.5|cm|in|0|abbr=on}} in diameter.<ref name=Courage/>{{rp|40–41}} Lacking skeletal support, the arms work as [[muscular hydrostat]]s and feature longitudinal, transverse, and circular muscles around a central axial nerve. They can squash and stretch, coil at any place in any direction or stiffen.<ref name=Crowfootcrawling>{{cite web |url=http://www.asnailsodyssey.com/LEARNABOUT/OCTOPUS/octoLoco.php# |title=Octopuses and Relatives: Locomotion, Crawling |last=Carefoot |first=Thomas |work=A Snail's Odyssey |access-date=19 April 2017 |archive-url=https://web.archive.org/web/20130522005231/http://www.asnailsodyssey.com/LEARNABOUT/OCTOPUS/octoLoco.php |archive-date=22 May 2013 |url-status=dead }}</ref><ref>{{cite journal |last1=Zelman |first1=I. |last2=Titon |first2=M. |last3=Yekutieli |first3=Y. |last4=Hanassy |first4=S. |last5=Hochner |first5=B. |last6=Flash |first6=T.|year=2013|title=Kinematic decomposition and classification of octopus arm movements |journal=Frontiers in Computational Neuroscience |volume=7 |page=60 |doi=10.3389/fncom.2013.00060 |pmid=23745113 |pmc=3662989 |doi-access=free }}</ref>

The interior surfaces of the arms are covered with circular, adhesive suckers. The suckers allow the octopus to secure itself in place or to handle objects. Each sucker is typically circular and bowl-like and has two distinct parts: an outer disc-shaped [[Cephalopod limb|infundibulum]] and a inner cup-like [[acetabulum (morphology)|acetabulum]], both of which are thick muscles covered in [[connective tissue]]. A [[chitinous]] cuticle lines the outer surface. When a sucker attaches to a surface, the orifice between the two structures is sealed and the infundibulum flattens. Muscle contractions allow for attachment and detachment.<ref>{{cite journal |last1=Tramacere |first1=F. |last2=Beccai |first2=L. |last3=Kuba |first3=M. |last4=Gozzi |first4=A. |last5=Bifone |first5=A. |last6=Mazzolai |first6=B. |year=2013|title=The morphology and adhesion mechanism of ''Octopus vulgaris'' suckers |journal=PLOS ONE |volume=8 |issue=6 |page=e65074 |doi=10.1371/journal.pone.0065074 |pmid=23750233 |pmc=3672162|bibcode=2013PLoSO...865074T |doi-access=free }}</ref><ref name=kier>{{cite journal |last1=Kier |first1=W. M. |last2=Smith |first2=A. M. |title=The structure and adhesive mechanism of octopus suckers |year=2002 |journal=Integrative and Comparative Biology |volume=42 |issue=6 |pages=1146–1153 |pmid=21680399 |doi=10.1093/icb/42.6.1146|citeseerx=10.1.1.512.2605 |s2cid=15997762 }}</ref><ref name=Crowfootcrawling/> Each of the eight arms senses and responds to light, allowing the octopus to control its limbs even if its head is obscured.<ref>{{Cite journal |last1=Katz |first1=Itamar |last2=Shomrat |first2=Tal |last3=Nesher |first3=Nir |date=1 January 2021 |title=Feel the light – sight independent negative phototactic response in octopus' arms |url=https://jeb.biologists.org/content/early/2021/02/01/jeb.237529 |journal=Journal of Experimental Biology |volume=224 |issue=5 |doi=10.1242/jeb.237529 |issn=0022-0949 |pmid=33536305 |doi-access=free|bibcode=2021JExpB.224B7529K }}</ref>

[[File:Dumbo-hires (cropped).jpg| thumb |A finned ''[[Grimpoteuthis]]'' species with its atypical octopus body plan|alt=A stubby round sea-creature with short ear-like fins]]

The cranium has two [[cartilage|cartilaginous]] capsules each containing one large eye, which resembles those of fish. The [[cornea]] is formed from a [[translucent]] epidermal layer; the slit-shaped [[pupil]] forms a hole in the [[iris (anatomy)|iris]] just behind the cornea. The lens hangs behind the pupil; photoreceptive [[Retina|retinal cells]] line the back. The pupil can expand and contract; a retinal pigment screens incident light in bright conditions.<ref name=Ruppert/>{{rp|360–361}}

Some species differ in form from the typical body shape. [[Basal (phylogenetics)|Basal]] species, the [[Cirrina]], have gelatinous bodies with two [[Cephalopod fin|fins]] located above the eyes, an [[cirrate shell|internal shell]] and mostly webbed arms that are lined with fleshy papillae or [[cirrus (biology)|cirri]] underneath.<ref name="marinebio">{{cite web |title=Finned Deep-sea Octopuses, Grimpoteuthis spp |date=18 May 2017 |publisher=MarineBio |url=https://www.marinebio.org/species/finned-deep-sea-octopuses/grimpoteuthis-spp/ |access-date=14 May 2021}}</ref><ref name="Corporation2004"/>