Editing Cnidaria (section)

===Cnidocytes===
These "nettle cells" function as [[harpoon]]s, since their [[Wikt:payload|payload]]s remain connected to the bodies of the cells by threads. Three types of [[cnidocyte]]s are known:<ref name="Hinde2001" /><ref name="Ruppert" />

[[File:Hydra nematocyst firing 01.png|thumb|right|200px| Firing sequence of the cnida in a hydra's nematocyst<ref name="Ruppert" /><br>{{Color box|blue|border=silver}} Operculum (lid)<br>{{Color box|red|border=silver}} "Finger" that turns inside out<br>/ / / Barbs<br>{{Color box|yellow|border=silver}} Venom<br>{{Color box|silver|border=silver}} Victim's skin<br>{{Color box|#ffcad0|border=silver}} Victim's tissues]]
*[[Nematocyst]]s inject [[venom]] into prey, and usually have barbs to keep them embedded in the victims. Most species have nematocysts.<ref name="Hinde2001" />
*[[Spirocyst]]s do not penetrate the victim or inject venom, but entangle it by means of small sticky hairs on the thread.
*[[Ptychocyst]]s are not used for prey capture&nbsp;— instead the threads of discharged ptychocysts are used for building protective tubes in which their owners live. Ptychocysts are found only in the [[order (biology)|order]] [[Ceriantharia]], [[Tube-dwelling anemone|tube anemones]].<ref name="Ruppert" />

The main components of a cnidocyte are:<ref name="Hinde2001" /><ref name="Ruppert" />
[[File:Hydra nematocyst 01.png|thumb|200px| A [[hydra (genus)|hydra]]'s nematocyst, before firing.<br>{{Color box|#00b900|border=silver}} "trigger" cilium<ref name="Ruppert" />]]

*A [[cilium]] (fine hair) which projects above the surface and acts as a trigger. Spirocysts do not have cilia.
*A tough capsule, the [[cnida]], which houses the thread, its payload and a mixture of chemicals that may include venom or [[adhesive]]s or both. ("cnida" is derived from the Greek word κνίδη, which means "nettle"<ref>{{Cite book| title=Shorter Oxford English Dictionary| contribution=Cnida|author1=Trumble, W. |author2=Brown, L.| publisher=Oxford University Press| year=2002}}</ref>)
*A tube-like extension of the wall of the cnida that points into the cnida, like the finger of a rubber glove pushed inwards. When a cnidocyte fires, the finger pops out. If the cell is a venomous nematocyte, the "finger"'s tip reveals a set of barbs that anchor it in the prey.
*The thread, which is an extension of the "finger" and coils round it until the cnidocyte fires. The thread is usually hollow and delivers chemicals from the cnida to the target.
*An [[Operculum (animal)|operculum]] (lid) over the end of the cnida. The lid may be a single hinged flap or three flaps arranged like slices of pie.
*The cell body, which produces all the other parts.

It is difficult to study the firing mechanisms of cnidocytes as these structures are small but very complex. At least four hypotheses have been proposed:<ref name="Hinde2001" />
*Rapid contraction of fibers round the cnida may increase its internal pressure.
*The thread may be like a coiled spring that extends rapidly when released.
*In the case of ''[[Chironex]]'' (the "sea wasp"), chemical changes in the cnida's contents may cause them to expand rapidly by [[polymerization]].
*Chemical changes in the liquid in the cnida make it a much more [[concentration|concentrated]] solution, so that [[osmotic pressure]] forces water in very rapidly to dilute it. This mechanism has been observed in nematocysts of the class [[Hydrozoa]], sometimes producing pressures as high as 140 [[Atmosphere (unit)|atmospheres]], similar to that of [[Scuba set|scuba]] air tanks, and fully extending the thread in as little as 2&nbsp;milliseconds (0.002&nbsp;second).<ref name="Ruppert" />

Cnidocytes can only fire once, and about 25% of a hydra's nematocysts are lost from its tentacles when capturing a [[brine shrimp]]. Used cnidocytes have to be replaced, which takes about 48&nbsp;hours. To minimise wasteful firing, two types of stimulus are generally required to trigger cnidocytes: nearby [[sensory neuron|sensory]] cells detect chemicals in the water, and their cilia respond to contact. This combination prevents them from firing at distant or non-living objects. Groups of cnidocytes are usually connected by nerves and, if one fires, the rest of the group requires a weaker minimum stimulus than the cells that fire first.<ref name="Hinde2001" /><ref name="Ruppert" />