Cnidocytes are prey-capture and defensive cells found on animals of the phylum Cnidaria. Cnidocytes have unique organelles in them called nematocysts that are used to capture prey. Nematocysts are composed of a coiled thread-like structure attached to the bulb-shaped nematocyst capsule body. The externally-oriented side of a cnidocyst cell also has a hair-like trigger on it. When the trigger is activated the cell fires its nematocyst. Nematocyst discharge is one of the fastest processes in biology and takes not more than few microsconds.

Some nematocysts puncture the skin or exoskeleton of an organism and inject a paralyzing toxin. Others stick to the surface of the prey item and yet others coil themselves around cellular projections on the prey. Depending on the species there may be one or more types of nematocyst on the organism.

There are three types of nematocysts:

1. Penetrant: A harpoon-like structure used to penetrate.
2. Glutinant: Sticky surfaces used to stick to prey.
3. Volvent: A lasso-like string that is fired at prey and wraps itself around a cellular projection on the prey.