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

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 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.  

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.''

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.''