Moon jellyfish (Ch. Leone / CC BY-ND)

The jellyfish is the free-swimming life stage of members of the phylum cnidaria. Its body consists of a pulsating gelatinous bell and tentacles suspended from its edge. The tentacles are covered with stinging cells (cnidocytes), which are used to capture prey. Cyanea arctica can reach 2 meters (6.6 feet) across the body. The sea wasp (Chironex fleckeri) and some other species of box jellyfish are infamous for their extremely painful and occasionally fatal stings. The sting of the Portuguese man o' war or bluebottle jellyfish (Physalia physalis) can cause pain, fever, vomiting, and unconsciousness in severe cases.

skull and crossbones
Some species venomous.

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This supplemental online resource accompanies the book Scuba Diving Hand Signals by Lars Behnke, available on Amazon.