When the Paramecium is attacked or feels threatened, it unleashes its defense system - a battery of harpoon-like spears called trichocysts. This defense system can be triggered by allowing a drop of vinegar to seep between the cover slip and the microscope slide. When the vinegar hits the Paramecium it immediately deploys its harpoons in the hope that the attacker will back away. Often, as in the cases displayed in these images, the Paramecium will release everything at its disposal, including undigested food, digested waste and waste water through its several water expelling vesicles and anal pore. This release is readily apparent in the image at the right. The long trichocysts too are plainly visible. Most remain attached to the Paramecium, but some are actually thrown off a short distance.

Under very high magnification these trichocysts appear as long spears, with sharp pointed ends, much like whalers' harpoons.