hectocotylus

File:Hectocotyle1.jpg's original illustration of an octopus hectocotylus, which he named Hectocotyle octopodis]]

A hectocotylus ({{plural form}}: hectocotyli) is one of the arms of male cephalopods that is specialized to store and transfer spermatophores to the female.{{cite book|author1=Roger T. Hanlon|author2=John B. Messenger|title=Cephalopod Behaviour|url=https://books.google.com/books?id=oppPDwAAQBAJ&q=hectocotylus|date=22 March 2018|publisher=Cambridge University Press|isbn=978-1-108-54674-4}} 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 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 knew of its use in mating, he was doubtful that a tentacle could deliver sperm. The name hectocotylus was devised by Georges Cuvier, who first found one embedded in the mantle of a female argonaut. Supposing it to be a parasitic worm, in 1829 Cuvier gave it a generic name (Hectocotyle),{{cite book|author-link=Armand Marie Leroi|author=Leroi, Armand Marie|title=The Lagoon: How Aristotle Invented Science|date=25 September 2014|publisher=Penguin |isbn=9780698170391|url=https://books.google.com/books?id=-DVBAwAAQBAJ&q=aristotle+hectocotylus&pg=PT80}}{{cite book|author-link=D'Arcy Wentworth Thompson|author=Thompson, D'Arcy Wentworth|year=1913|title=On Aristotle as a biologist, with a prooemion on Herbert Spencer|series=Being the Herbert Spencer Lecture before the University of Oxford, on February 14, 1913|publisher=Oxford University Press|page=19}}{{cite book|author1=Nixon M.|author2-link=John Zachary Young|author2=Young J.Z.|year=2003|url=https://books.google.com/books?id=BRvrtsuKc6MC&q=hectocotylus|title=The brains and lives of Cephalopods|publisher=Oxford University Press|isbn=9780198527619}}{{cite web|url=https://www.gbif.org/species/3246735|title=GBIF:Hectocotylus Cuvier, 1829|access-date=21 November 2016}}

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:

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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.

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). [http://tolweb.org/accessory/Cephalopoda_Glossary?acc_id=587 Cephalopoda Glossary]. Tree of Life Web Project. Rare examples of double and bilateral hectocotylization have also been recorded in incirrate octopuses.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}}Palacio, F.J. 1973. {{cite web|url= https://archive.org/details/nautilus87amer |title=On the double hectocotylization of octopods. |year=1973 |publisher=Melbourne, Fla., etc., American Malacologists, inc., etc. }} 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 on the hectocotylus keeps it lodged in the pallial cavity of the female.