Nuchal organ

The nuchal organ is a ciliated pit^[1] or groove present at the posterior end of the prostomium of annelid worms,^[1] some cephalopods,^[2] and other invertebrates.^[3]

Annelids only possess one nuchal organ, although the nature of the grooving may make it appear to be a pair of organs.^[4]

It may be involved in light detection,^[2] and may have a role in food detection and mating.^[5]

References

^ ^a ^b ^c Hessling, R.; Purschke, Günter (23 October 2000). "Immunohistochemical (cLSM) and ultrastructural analysis of the central nervous system and sense organs in Aeolosoma hemprichi (Annelida, Aeolosomatidae)". Zoomorphology. 120 (2): 65–78. doi:10.1007/s004350000022. S2CID 38957268.
^ ^a ^b Parry, Matthew (October 2000). "A description of the nuchal organ, a possible photoreceptor, in Euprymna scolopes and other cephalopods". Journal of Zoology. 252 (2): 163–177. doi:10.1111/j.1469-7998.2000.tb00612.x.
^ Peñalva-Arana, Carolina D.; Manca, Marina (1 August 2007). "An SEM study of the nuchal organ in Daphnia himalaya (nov. sp.) embryos and neonates collected from the Khumbu region (Nepalese Himalayas)" (PDF). Journal of Limnology. 66 (2): 153. doi:10.4081/jlimnol.2007.153.
^ Purschke, G.; Wolfrath, F.; Westheide, W. (21 March 1997). "Ultrastructure of the nuchal organ and cerebral organ in Onchnesoma squamatum (Sipuncula, Phascolionidae)". Zoomorphology. 117 (1): 23–31. doi:10.1007/s004350050026. S2CID 39830837.
^ Schlötzer-Schrehardt, U. (December 1986). "Ultrastructural investigation of the nuchal organs of Pygospio elegans (Polychaeta). I. Larval nuchal organs". Helgoländer Meeresuntersuchungen. 40 (4): 397–417. Bibcode:1986HM.....40..397S. doi:10.1007/BF01983820.

Cephalopod anatomy

Shell

Types

External	Ammonoid shell Argonautid eggcase Nautiloid shell Orthocone
Internal	Belemnoid guard Cirrate shell Cuttlebone Gladius (pen) Spirula shell Stylet

Features

External	Aperture Apex Callus Lirae Periostracum Sculpture Spire Sutures Umbilicus
Internal	Body chamber Camerae Nacre Phragmocone Septa Siphuncle Whorls

Mantle &
funnel

External anatomy	Dermal structures Fins (wings) Flotation devices ("secondary fins") Funnel–mantle locking apparatus Tail
Internal anatomy	Branchial hearts Ctenidia (gills) Hepatopancreas (digestive gland) Ink sac and ink Needham's sac Nephridia ("kidneys") Nidamental glands Osphradium Pericardial glands

Head &
limbs

Brachial crown	Arms Hectocotylus Tentacles Carpus Dactylus Manus Suckers and hooks
Buccal region	Aptychus Beak Odontophore Radula Spadix and antispadix
Occipital region	Nuchal crest (occipital crest) Nuchal folds (occipital folds) Occipital membrane Olfactory organ Nuchal organ
Other parts	Eyes Statocysts