trichocyst

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An organelle in certain protozoans, especially ciliates, consisting of filamentous fibers that are discharged suddenly.

from The Century Dictionary.

• noun A hair-cell; one of the minute rod-like or hair-like bodies developed in the subcuticular layer of many infusorians: so named by G. J. Allman in 1855. They represent or resemble the cnidæ or threadcells of cœlenterates.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Zoöl.) A lasso cell.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A threadlike organ in certain protozoans that can be discharged suddenly in order to grasp or sting