Some examples of coelenterates include jellyfish, anemones and coral. Historically, scientists used the term “coelenterate,” which refers to the cavity-like bodies of the animals, to identify the phylum now called cnidaria. All coelenterates are aquatic, and while most are marine, a few lineages inhabit fresh water.
All coelenterates possess small structures, technically complex secretions produced by the Golgi apparatus, called nematocysts or cnidas. These eject a small tubule when appropriately stimulated, such as when contacted by predator or prey. Some nematocysts deliver venom, while others ensnare the instigating animal in long, sticky threads. Jellyfish tentacles, for example, deliver a paralyzing neurotoxin when touched by fish. Once it has incapacitated its prey, the jellyfish can devour the meal. While all coelenterates are carnivorous, relatively few hunt prey. Most coelenterates, especially corals and anemones, are sessile organisms that wait for prey to pass by its tentacles.
Some coelenterates are dangerous to humans. For example, fire coral causes an itchy skin rash in those who touch it. Portuguese man-o-wars are colonial organisms called siphonophores, made up of four different types of coelenterate; touching their tentacles elicits a painful, but rarely life-threatening sting. One particularly dangerous species, the box jellyfish, produces and excruciatingly painful and often fatal venom, which has killed more people than sharks have, over the last 50 years, according to the Huffington Post.