Cephalopod Arm

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.Young, R.E., M. Vecchione & K.M. Mangold 1999
Cephalopoda Glossary
Tree of Life web project. 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.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." 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.

Tentacular clubs

Suckers

References

Notes

Citations

{{Cephalopod anatomy

Cephalopod zootomy