Nematocysts

Nematocysts are small capsules that contain a toxin which is injected into prey or predators.

Nematocysts are found in jellyfish, coral, and other animals in the phylum Cnidaria (also called Coelenterata).

Nematocysts can be dangerous to humans, and swimmers should always avoid contact with jellyfish or coral. Although some of them just itch or give a mild sting, others, like the Portuguese man-of-war cause excruciating pain and occasionally have been fatal.

Like other kinds of venom, people vary in their reaction, so a jellyfish that only causes a mild sting for one person may be extremely dangerous to another person.

In one of the Sherlock Holmes stories, The Lion's Mane, the plot turns on a fatal sting from another kind of jellyfish, Cyanea capillata.

Worst of all are the nematocysts of the cubomedusae (also called "box jellyfish" or "sea wasps") found on Australia's Barrier Reef. These are believed to be the most venomous sea creature known. Most people who are stung by them do survive the sting, but experience excruciating pain and require hospitalization; however, they have been known to kill a human in less than a minute. They have been responsible for at least 63 known deaths since Australia began keeping records in 1883.