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Tentacled predators

Probably the most familiar jellies are jellyfish, such as this one—technically called medusae, and belonging to two divisions of the phylum Cnidaria. Jellyfish are predators that use tentacles studded with stinging cells to catch and kill their prey, usually smaller zooplankton or fish. Although jellyfish have a very simple body structure, with no central nervous system, all have evolved efficient ways to catch their food. They swim by jet propulsion, and many are capable of complex migration patterns. In many species, the medusa is a stage that alternates with another form that lives attached to the bottom.(Photo by Larry Madin, Woods Hole Oceanographic Institution)