In ciliates and dinoflagellates, the trichocyst is an organelle that releases long filamentous proteins that capture predators 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, 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.
Overall, information about the trichocyst is incomplete, but scientists are working hard to uncover its mysteries.
Web Resources On Trichocyst
Britannica entry on trichocyst
Image of trichocyst structure
Book Resources On Trichocyst
Factors affecting the feeding response of oyster larvae, with special reference to trichocysts from the dinoflagellate, prorocentrum by R. Ukeles
Dinoflagellates by David L. Spector