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(invertebrate zoology)
A minute structure in the cortex of certain protozoans that releases filamentous or fibrillar threads when discharged.



a type of extrusome; a cytoplasmic organelle of protozoans that is discharged when the protozoan is mechanically or chemically irritated.

Filamentous trichocysts are protective organelles typical of Infusoria; they are elongated (2–6 microns) albuminous bodies of paracrystalline structure. They generally have a dense tip and are situated in the cytoplasm perpendicular to the surface, as in par-amecia. When discharged, they extend into a filament 20–60 microns long and ending in a sharp point.

There are other types of trichocysts in infusorians, as follows. Mucoid trichocysts, or protrichocysts, are without a sharp point; they swell into a jellylike substance with a houndstooth pattern when discharged. Rhabdocysts are rod-shaped and are completely extruded to the surface of the protozoan. Toxicysts are poisonous organelles of predatory infusorians; they are used for attack and are generally located in a circumoral position. They consist of a long capsule containing a telescoping tubule that extends when discharged. Haptocysts are minute extrusomes located at the tips of antennae; they consist of an ampulla and a short tubule. Cnidocysts, or stinging capsules, are oval or spherical in shape. They contain a wound-up tubular filament that can unwind.

Mastigophorans have mucoid trichocysts, filamentous trichocysts, cnidocysts, and taeniobolocysts. The last is in the form of an albuminous rolled-up ribbon that unrolls when discharged. Cnidosporidians have cnidocysts (polar capsules), which are characteristic of spores. When cnidocysts are discharged, they help attach spores to the host’s body.