A hectocotylus (: hectocotyli) is one of the arms of male

cephalopod A cephalopod is any member of the molluscan Taxonomic rank, class Cephalopoda (Greek language, Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral symm ...

s that is specialized to store and transfer spermatophores to the female. Structurally, hectocotyli are muscular hydrostats. Depending on the species, the male may use it merely as a conduit to the female, analogously to a

penis A penis (; : penises or penes) is a sex organ through which male and hermaphrodite animals expel semen during copulation (zoology), copulation, and through which male placental mammals and marsupials also Urination, urinate. The term ''pen ...

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 (; 384–322 BC) was an Ancient Greek philosophy, Ancient Greek philosopher and polymath. His writings cover a broad range of subjects spanning the natural sciences, philosophy, linguistics, economics, politics, psychology, a ...

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 natural history, naturalist and zoology, zoologist, sometimes referred to as the "founding father of paleontology". Cuv ...

, who first found one embedded in the mantle of a female argonaut. Thinking it to be a parasitic worm, in 1829 Cuvier gave it a generic name (''Hectocotyle''), which is a New Latin term combining the Greek words for "hundred" (''hec(a)to(n)'') and for "hollow thing, cup" ('' cotyle'').

Structure

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

systematics 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: phylogenetic trees, phylogenies). Phy ...

. * Many coleoids lack hectocotyli altogether. * Among Decapodiformes (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. 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 male transfers the spermatophores to the female by putting its hectocotylus into a cavity in the mantle of the female, called 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 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 Glossar ...

on the hectocotylus keeps it lodged in the pallial cavity of the female.

Table of hectocotyli

References

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