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A hectocotylus is one of the arm
Cephalopod arm
A cephalopod arm is distinct from a tentacle, though the terms are often used interchangeably.Generally, cephalopod arms have suckers along most of their length, as opposed to tentacles, which have suckers only near their ends. Octopuses have eight arms and no tentacles, while squid and cuttlefish...

s of the male of most kinds of cephalopod
A cephalopod is any member of the molluscan class Cephalopoda . These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles modified from the primitive molluscan foot...

s that is modified in various ways to effect the fertilization of the female's eggs. It is a specialized, extended muscular hydrostat
Muscular hydrostat
A muscular hydrostat is a biological structure found in animals. It is used to manipulate items or to move its host about and consists mainly of muscles with no skeletal support...

 used to store spermatophore
A spermatophore or sperm ampulla is a capsule or mass created by males of various animal species, containing spermatozoa and transferred in entirety to the female's ovipore during copulation...

s, the male gametophore. Males generally form a new hectocotylus in each new season.

The shape of the tip of the hectocotylus has been much used in octopod systematics. In many species it is considerably elaborated. However, in the males of some species, such as the Seven-arm Octopus
Seven-arm Octopus
The Seven-arm Octopus is the largest known species of octopus based on scientific records, with a total estimated length of 4 m and weight of 75 kg...

 (Haliphron atlanticus), the hectocotylus develops in an inconspicuous sac in front of the right eye that gives the male the appearance of having only seven arms.

The term is also used to specifically refer to the greatly modified arm of Argonauta
Argonaut (animal)
The argonauts are a group of pelagic octopuses. They are also called paper nautiluses, referring to the paper-thin eggcase that females secrete...

 and allied genera
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

. In argonauts, the male transfers the spermatophores to the female by putting its hectocotylus into a cavity in the mantle
Mantle (mollusc)
The mantle is a significant part of the anatomy of molluscs: it is the dorsal body wall which covers the visceral mass and usually protrudes in the form of flaps well beyond the visceral mass itself.In many, but by no means all, species of molluscs, the epidermis of the mantle secretes...

 of the female. This mantle cavity is known as the pallial cavity. This is the only contact the male and female have with each other during copulation, and it can be at a distance. During copulation, the hectocotylus breaks off from the male. The funnel-mantle locking apparatus on the hectocotylus keeps it lodged in the pallial cavity of the female.

The name hectocotylus was devised by Georges Cuvier
Georges Cuvier
Georges Chrétien Léopold Dagobert Cuvier or Jean Léopold Nicolas Frédéric Cuvier , known as Georges Cuvier, was a French naturalist and zoologist...

, who first found one embedded in the mantle of a female argonaut; supposing it to be a parasitic worm
The term worm refers to an obsolete taxon used by Carolus Linnaeus and Jean-Baptiste Lamarck for all non-arthropod invertebrate animals, and stems from the Old English word wyrm. Currently it is used to describe many different distantly-related animals that typically have a long cylindrical...

, Cuvier gave it a generic name. The hectocotyl arm was first described in the biological works of Aristotle
Aristotle was a Greek philosopher and polymath, a student of Plato and teacher of Alexander the Great. His writings cover many subjects, including physics, metaphysics, poetry, theater, music, logic, rhetoric, linguistics, politics, government, ethics, biology, and zoology...

, and it was widely disbelieved until its rediscovery in the nineteenth century.

Rare examples of double and bilateral hectocotylization have been recorded in incirrate octopuses.