Cnidocytes are specialized cells located in the epidermis of all cnidarians. These cells are unique to cnidarians, no other organism possesses them. Cnidocytes are most concentrated within the epidermis of the tentacles.
Cnidocytes contain organelles called cnidea. There are several types of cnidea which include nematocysts, spirocysts, and ptychocysts. The most notable of these is the nematocysts. Nematocysts consist of a capsule containing a coiled thread and barbs known as stylets. Nematocysts, when discharged, deliver a stinging venom that serves to paralyze prey and enable the cnidarian to ingest its victim. Spirocysts are cnidea found in some corals and sea anemones that consist of sticky threads and help the animal capture prey and adhere to surfaces. Ptychocysts are found in members of a group of cnidarians known as the Ceriantaria. These organisms are bottom dwellers adapted to soft substrates into which they bury their base. They eject ptychocysts into the substrate which help them establish a secure hold.
In hydras and jellyfish, the cnidocytes cells have a stiff bristle that projects out from the surface of the epidermis. This bristle is called a cnidocyl (it is not present in corals and sea anemones, which instead possess a similar structure called a ciliary cone). The cnidocyl serves as a trigger to release the nematocyst.