http://media-2.web.britannica.com/eb-media/68/8968-004-A94DB3DF.jpg (Picture of a Trichocyst)
A trichocyst is a structure in the cortex

Cortex (anatomy)

In anatomy and zoology the cortex is the outermost layer of an organ. Organs with well-defined cortical layers include kidneys, adrenal glands, ovaries, the thymus, and portions of the brain, including the cerebral cortex, the most well-known of all cortices.The cerebellar cortex is the thin gray...

 of certain ciliate

Ciliate

The ciliates are a group of protozoans characterized by the presence of hair-like organelles called cilia, which are identical in structure to flagella but typically shorter and present in much larger numbers with a different undulating pattern than flagella...

 and flagellate

Flagellate

Flagellates are organisms with one or more whip-like organelles called flagella. Some cells in animals may be flagellate, for instance the spermatozoa of most phyla. Flowering plants do not produce flagellate cells, but ferns, mosses, green algae, some gymnosperms and other closely related plants...

 protozoans consisting of a cavity and long, thin threads that can be ejected in response to certain stimuli

Stimulus (physiology)

In physiology, a stimulus is a detectable change in the internal or external environment. The ability of an organism or organ to respond to external stimuli is called sensitivity....

. Trichocysts may be widely distributed over an organism or restricted to certain areas.
They are characteristics of the Phylum

Phylum

In biology, a phylum The term was coined by Georges Cuvier from Greek φῦλον phylon, "race, stock," related to φυλή phyle, "tribe, clan." is a taxonomic rank below kingdom and above class. "Phylum" is equivalent to the botanical term division....

 Ciliophora.

In ciliates and dinoflagellates, the trichocyst is an organelle that releases long filamentous proteins that capture prey

Predation

In ecology, predation describes a biological interaction where a predator feeds on its prey . Predators may or may not kill their prey prior to feeding on them, but the act of predation always results in the death of its prey and the eventual absorption of the prey's tissue through consumption...

 in "nets" to slow them down when the cell is disturbed. The protein is nontoxic and shaped like a long, striated, fibrous shaft.

It may also be used to anchor the protozoa while it is feeding or capture food to bring into itself. In Paramecium

Paramecium

Paramecium is a group of unicellular ciliate protozoa, which are commonly studied as a representative of the ciliate group, and range from about 0.05 to 0.35 mm in length. Simple cilia cover the body, which allow the cell to move with a synchronous motion at speeds of approximately 12 body...

, it is almost certain that the anchorage function is the correct one.

The trichocyst is shaped like a hollow cavity filled with long, thin threads of protein. Trichocysts may be found in only one area, like around the tentacles or mouth, or it may be found all over the organism. Different types of trichocysts seem to have different functions. The Paramecium filamentous trichocysts have filaments that are similar to harpoons: a long filament with a cross-striated shaft and a tip. Trichocysts for toxicysts tend to be around the mouth, where the toxin can be shot out to paralyze or kill other microorganisms.

A trichocyst can be found in tetrahymena and along cila pathways of several metabolic systems.