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A hectocotylus (plural: ''hectocotyli'') is one of the
arm In human anatomy, the arm refers to the upper limb in common usage, although academically the term specifically means the upper arm between the glenohumeral joint (shoulder joint) and the elbow joint. The distal part of the upper limb between th ...
s of male
cephalopod A cephalopod is any member of the molluscan class Cephalopoda ( Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head ...
s that is specialized to store and transfer
spermatophore A spermatophore or sperm ampulla is a capsule or mass containing spermatozoa created by males of various animal species, especially salamanders and arthropods, and transferred in entirety to the female's ovipore during reproduction. Spermatophore ...
s to the female. Structurally, hectocotyli are
muscular hydrostat A muscular hydrostat is a biological structure found in animals. It is used to manipulate items (including food) or to move its host about and consists mainly of muscles with no skeletal support. It performs its hydraulic movement without fluid in ...
s. Depending on the species, the male may use it merely as a conduit to the female, analogously to a
penis A penis (plural ''penises'' or ''penes'' () is the primary sexual organ that male animals use to inseminate females (or hermaphrodites) during copulation. Such organs occur in many animals, both vertebrate and invertebrate, but males d ...
in other animals, or he may wrench it off and present it to the female. The hectocotyl arm was first described in Aristotle's biological works. Although
Aristotle Aristotle (; grc-gre, Ἀριστοτέλης ''Aristotélēs'', ; 384–322 BC) was a Greek philosopher and polymath during the Classical period in Ancient Greece. Taught by Plato, he was the founder of the Peripatetic school of ...
knew of its use in mating, he was doubtful that a tentacle could deliver sperm. The name ''hectocotylus'' was devised by
Georges Cuvier Jean Léopold Nicolas Frédéric, Baron Cuvier (; 23 August 1769 – 13 May 1832), known as Georges Cuvier, was a French naturalist and zoologist, sometimes referred to as the "founding father of paleontology". Cuvier was a major figure in na ...
, who first found one embedded in the mantle of a female argonaut. Supposing it to be a
parasitic worm Parasitic worms, also known as helminths, are large macroparasites; adults can generally be seen with the naked eye. Many are intestinal worms that are soil-transmitted and infect the gastrointestinal tract. Other parasitic worms such as sc ...
, in 1829 Cuvier gave it a generic name, combining the Greek word for "hundred" and Latin word for "hollow thing".


Anatomy

Generalized anatomy of squid and octopod hectocotyli:


Variability

Hectocotyli are shaped in many distinctive ways, and vary considerably between species. The shape of the tip of the hectocotylus has been much used in
octopus An octopus ( : octopuses or octopodes, see below for variants) is a soft-bodied, eight- limbed mollusc of the order Octopoda (, ). The order consists of some 300 species and is grouped within the class Cephalopoda with squids, cuttlefish, ...
systematics Biological systematics is the study of the diversification of living forms, both past and present, and the relationships among living things through time. Relationships are visualized as evolutionary trees (synonyms: cladograms, phylogenetic t ...
. * Many coleoids lack hectocotyli altogether. * Among
Decapodiformes Decapodiformes is a superorder of Cephalopoda comprising all cephalopod species with ten limbs, specifically eight short arms and two long tentacles. It is hypothesized that the ancestral coleoid had five identical pairs of limbs, and that one ...
(ten-limbed cephalopods), generally either one or both of arms IV are hectocotylized. * In incirrate octopuses it is one of arm pair III.Young, R.E., M. Vecchione & K.M. Mangold (1999)
Cephalopoda Glossary
Tree of Life Web Project The Tree of Life Web Project is an Internet project providing information about the diversity and phylogeny of life on Earth. This collaborative peer reviewed project began in 1995, and is written by biologists from around the world. The site ...
.
Rare examples of double and bilateral hectocotylization have also been recorded in incirrate octopuses.Palacio, F.J. 1973. ''The Nautilus'' 87: 99–102. * In male Seven-arm Octopuses (''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. * In
argonauts The Argonauts (; Ancient Greek: ) were a band of heroes in Greek mythology, who in the years before the Trojan War (around 1300 BC) accompanied Jason to Colchis in his quest to find the Golden Fleece. Their name comes from their ship, ''Argo ...
, the male transfers the spermatophores to the female by putting its hectocotylus into a cavity in the
mantle A mantle is a piece of clothing, a type of cloak. Several other meanings are derived from that. Mantle may refer to: *Mantle (clothing), a cloak-like garment worn mainly by women as fashionable outerwear **Mantle (vesture), an Eastern Orthodox ve ...
of the female, called the ''pallial cavity''. This is the only contact the male and female have with each other during
copulation Sexual intercourse (or coitus or copulation) is a sexual activity typically involving the insertion and thrusting of the penis into the vagina for sexual pleasure or reproduction.Sexual intercourse most commonly means penile–vaginal penetra ...
, and it can be at a distance. During copulation, the hectocotylus breaks off from the male. The
funnel–mantle locking apparatus The funnel–mantle locking apparatus is a structure found in many cephalopods that connects the mantle and hyponome (funnel) and restricts their movement relative to each other.Young, R.E., M. Vecchione & K.M. Mangold (1999)Cephalopoda Glossary ...
on the hectocotylus keeps it lodged in the pallial cavity of the female.


References

{{Cephalopod anatomy Cephalopod zootomy Sex organs Animal male reproductive system