Aequorea victoria, the crystal jelly, a bioluminescent hydrozoan jellyfish species found off the west coast of North America
Other cnidarians, such as the jellyfish Cyanea capillata (the "Lion's Mane" made famous by Sherlock Holmes) or the hydrozoan Physalia physalis (Portuguese man o' war, "Bluebottle") can cause extremely painful and sometimes fatal stings.
Gonionemus is a genus of hydrozoan that uses adhesive discs near the middle of each tentacle to attach to eelgrass, sea lettuce or various types of algae instead of swimming.