Jellyfish are animals that belong to Phylum Cnidaria, included in the class Scyphozoa. The name is also sometimes used for the medusae of Hydrozoa. Almost all jellyfish live in the seas, only a few live in fresh water.
The jellyfish have two major body forms in their life. First is the polyp stage, characterized by either a non-moving stalk that catches food drifting by or a similar form that is free-floating. The second form is called the medusa stage, characterized by a round body with food catching tentacles hanging down. This is the form most people are familiar with.
Most jellyfish are passive drifters that feed on small fish and zooplankton that become caught in their tentacles. Jellyfish have an incomplete digestive system, meaning that the same orifice is used for both food intake and waste expulsion.
During the polyp stage, jellyfish do not have males or females, only asexual reproduction occurs. This happens in two ways: (1) budding, to produce other polyps; and (2) strobilating, to produce medusae. Jellyfish also reproduce sexually later in life after they differentiate into males and females. This occurs only when they have reached the medusa stage of their life cycle.
Jellyfish have stinging cells called nematocysts on their tentacles. When prey comes in contact with a tentacle, hundreds to thousands of nematocysts fire into the prey's direction. These stinging cells are latch onto the prey and the tentacles bring the prey into their large "mouth" for digestion.