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All cephalopods possess flexible limbs extending from their heads and surrounding their beaks. These appendages, which function as muscular hydrostats, have been variously termed arms, legs or tentacles.

Description

In the scientific literature, a cephalopod arm is often treated as distinct from a tentacle, though the terms are sometimes used interchangeably, often with the latter acting as an umbrella term for cephalopod limbs. Generally, arms have suckers along most of their length, as opposed to tentacles, which have suckers only near their ends. Barring a few exceptions, octopuses have eight arms and no tentacles, while squid and cuttlefish have eight arms (or two "legs" and six "arms") and two tentacles. The limbs of nautiluses, which number around 90 and lack suckers altogether, are called cirri.

The tentacles of Decapodiformes are thought to be derived from the fourth arm pair of the ancestral coleoid, but the term arms IV is used to refer to the subsequent, ventral arm pair in modern animals (which is evolutionarily the fifth arm pair).

The males of most cephalopods develop a specialised arm for sperm delivery, the hectocotylus.

Anatomically, cephalopod limbs function using a crosshatch of helical collagen fibres in opposition to internal muscular hydrostatic pressure.

Suckers

Cephalopod limbs bear numerous suckers along their ventral surface as in octopus, squid and cuttlefish arms and in clusters at the ends of the tentacles (if present), as in squid and cuttlefish. Each sucker is usually circular and bowl-like and has two distinct parts: an outer shallow cavity called an infundibulum and a central hollow cavity called an acetabulum. Both of these structures are thick muscles, and are covered with a chitinous cuticle to make a protective surface. Suckers are used for grasping substratum, catching prey and for locomotion. When a sucker attaches itself to an object, the infundibulum mainly provides adhesion while the central acetabulum is free. Sequential muscle contraction of the infundibulum and acetabulum causes attachment and detachment.

Abnormalities

Many octopus arm anomalies have been recorded, including a 6-armed octopus (nicknamed Henry the Hexapus), a 7-armed octopus, a 10-armed Octopus briareus, one with a forked arm tip, octopuses with double or bilateral hectocotylization, and specimens with up to 96 arm branches.

Branched arms and other limb abnormalities have also been recorded in cuttlefish, squid, and bobtail squid.

Variability

Cephalopod limbs and the suckers they bear are shaped in many distinctive ways, and vary considerably between species. Some examples are shown below.

Arms

For hectocotylized arms see hectocotylus variability.

Shape of arm	Species	Family
	Todarodes pacificus	Ommastrephidae

Tentacular clubs

Shape of tentacular club	Species	Family
	Abraliopsis morisi	Enoploteuthidae
	Ancistroteuthis lichtensteini	Onychoteuthidae
	Architeuthis sp.	Architeuthidae
	Austrorossia mastigophora	Sepiolidae
	Berryteuthis magister	Gonatidae
	Idioteuthis cordiformis	Mastigoteuthidae
	Iridoteuthis iris	Sepiolidae
	Mastigoteuthis glaukopis	Mastigoteuthidae
	Onykia ingens	Onychoteuthidae
	Semirossia tenera	Sepiolidae
	Spirula spirula	Spirulidae
	Todarodes pacificus	Ommastrephidae

Suckers

Shape of sucker	Species	Family
	Idioteuthis cordiformis	Mastigoteuthidae
	Idioteuthis latipinna	Mastigoteuthidae
	Magnapinna talismani	Magnapinnidae
	Mastigoteuthis agassizii	Mastigoteuthidae
	Mastigoteuthis atlantica	Mastigoteuthidae
	Mastigoteuthis dentata	Mastigoteuthidae
	Mastigoteuthis grimaldii	Mastigoteuthidae
	Mastigoteuthis magna	Mastigoteuthidae

References

Notes

1. A study has determined that the octopus has two legs and six arms, often commonly referred to as "tentacles". Another study found that there is a functional difference in the way the appendages are used for task division. "These findings give evidence for limb-specialization in an animal whose 8 arms were believed to be equipotential." The two rear appendages are generally used to walk on the sea floor, while the other six are used to forage for food.

Citations

1. Thomas, David (12 August 2008). "Octopuses have two legs and six arms". The Telegraph. Retrieved 30 July 2018. To most of us it has always seemed obvious that an octopus has eight arms. Octopuses have two legs and six arms Claire Little, a marine expert from the Weymouth Sea Life Centre in Dorset, said: 'We've found that octopuses effectively have six arms and two legs.'
2. Ruth A., Byrne; Kuba, Michael J.; Meisel, Daniela V.; Griebel, Ulrike; Mather, Jennifer A. (August 2006). "Does Octopus vulgaris have preferred arms?". Journal of Comparative Psychology. 120 (3): 198–204. doi:10.1037/0735-7036.120.3.198. PMID 16893257.
3. Lloyd, John; Mitchinson, John (2010). QI: The Second Book of General Ignorance. London: Faber and Faber. p. 3. ISBN 978-0571273751. As result, marine biologists tend to refer to them as animals with two legs and six arms.
4. ^ Young, R.E., M. Vecchione & K.M. Mangold 1999. Cephalopoda Glossary. Tree of Life web project.
5. ^ Norman, M. 2000. Cephalopods: A World Guide. ConchBooks, Hackenheim. p. 15. "There is some confusion around the terms arms versus tentacles. The numerous limbs of nautiluses are called tentacles. The ring of eight limbs around the mouth in cuttlefish, squids and octopuses are called arms. Cuttlefish and squid also have a pair of specialised limbs attached between the bases of the third and fourth arm pairs . These are known as feeding tentacles and are used to shoot out and grab prey."
6. Fukuda, Y. 1987. Histology of the long digital tentacles. In: W.B. Saunders & N.H. Landman (eds.) Nautilus: The Biology and Paleobiology of a Living Fossil. Springer Netherlands. pp. 249–256. doi:10.1007/978-90-481-3299-7_17
7. Kier, W.M. 1987. "The functional morphology of the tentacle musculature of Nautilus pompilius" (PDF). Archived from the original (PDF) on 2010-06-17. Retrieved 2010-06-11. In: W.B. Saunders & N.H. Landman (eds.) Nautilus: The Biology and Paleobiology of a Living Fossil. Springer Netherlands. pp. 257–269. doi:10.1007/978-90-481-3299-7_18
8. Inside Nature's Giants, episode 5
9. von Byern J, Klepal W (2005). "Adhesive mechanisms in cephalopods: a review". Biofouling. 22 (5–6): 329–38. doi:10.1080/08927010600967840. PMID 17110356.
10. Walla G (2007). "A study of the Comparative Morphology of Cephalopod Armature". tonmo.com. Deep Intuition, LLC. Retrieved 2013-06-08.
11. Kier WM, Smith AM (2002). "The structure and adhesive mechanism of octopus suckers". Integr Comp Biol. 42 (6): 1146–1153. doi:10.1093/icb/42.6.1146. PMID 21680399.
12. Octopuses & Relatives. "Learn about octopuses & relatives: locomotion". asnailsodyssey.com. Archived from the original on 2013-05-22. Retrieved 2013-06-08.
13. Kumph H.E. (1960). "Arm abnormality in octopus". Nature. 185 (4709): 334–335. Bibcode:1960Natur.185..334K. doi:10.1038/185334a0. S2CID 4226765.
14. Toll R.B., Binger L.C. (1991). "Arm anomalies: cases of supernumerary development and bilateral agenesis of arm pairs in Octopoda (Mollusca, Cephalopoda)". Zoomorphology. 110 (6): 313–316. doi:10.1007/BF01668021. S2CID 34858474.
15. Gleadall I.G. (1989). "An octopus with only seven arms: anatomical details". Journal of Molluscan Studies. 55 (4): 479–487. doi:10.1093/mollus/55.4.479.
16. Minor birth defect resulting in 10-armed juvenile, all arms fully present and functional. CephBase.
17. Minor birth defect showing bifurcated arm tip. Both tips were fully functional. CephBase.
18. Robson G.C. (1929). "On a case of bilateral hectocotylization in Octopus rugosus". Journal of Zoology. 99 (1): 95–97. doi:10.1111/j.1469-7998.1929.tb07690.x.
19. Palacio, F.J. 1973. "On the double hectocotylization of octopods". Melbourne, Fla., etc., American Malacologists, inc., etc. 1973. The Nautilus 87: 99–102.
20. Okada Y.K. (1965). "On Japanese octopuses with branched arms, with special reference to their captures from 1884 to 1964". Proceedings of the Japan Academy. 41 (7): 618–623. doi:10.2183/pjab1945.41.618.
21. Okada Y.K. (1965). "Rules of arm-branching in Japanese octopuses with branched arms". Proceedings of the Japan Academy. 41 (7): 624–629. doi:10.2183/pjab1945.41.624.
22. Monster octopi with scores of extra tentacles. Pink Tentacle, July 18, 2008.
23. Okada Y.K. (1937). "An occurrence of branched arms in the decapod cephalopod, Sepia esculenta Hoyle". Annotated Zoology of Japan. 17: 93–94.
24. Bradbury H.E., Aldrich F.A. (1971). "The occurrence of morphological abnormalities in the oegopsid squid Illex illecebrosus (Lesueur, 1821)". Canadian Journal of Zoology. 49 (3): 377–379. doi:10.1139/z71-055. PMID 5103494.
25. Voss G.L. (1957). "Observations on abnormal growth of the arms and tentacles in the squid genus Rossia". The Quarterly Journal of the Florida Academy of Sciences. 20 (2): 129–132.

v t e Cephalopod anatomy
Shell	Types External Ammonoid shell Argonautid eggcase Nautiloid shell Orthocone Internal Belemnoid guard Cirrate shell Cuttlebone Gladius (pen) Spirula shell Stylet Features External Aperture Apex Callus Lirae Periostracum Sculpture Spire Sutures Umbilicus Internal Body chamber Camerae Nacre Phragmocone Septa Siphuncle Whorls
Mantle & funnel	External anatomy Dermal structures Fins (wings) Flotation devices ("secondary fins") Funnel–mantle locking apparatus Tail Internal anatomy Branchial hearts Ctenidia (gills) Hepatopancreas (digestive gland) Ink sac and ink Needham's sac Nephridia ("kidneys") Nidamental glands Osphradium Pericardial glands
Head & limbs	Brachial crown Arms Hectocotylus Tentacles Carpus Dactylus Manus Suckers and hooks Buccal region Aptychus Beak Odontophore Radula Spadix and antispadix Occipital region Nuchal crest (occipital crest) Nuchal folds (occipital folds) Occipital membrane Olfactory organ Nuchal organ Other parts Eyes Statocysts
General	Chromatophores Photophores Nervous system Squid giant axon Squid giant synapse
Developmental stages: Spawn → Paralarva (Doratopsis stage) → Juvenile → Subadult → Adult • Egg fossils • Protoconch (embryonic shell)

• Cephalopod zootomy