Most jellyfish survive in the ocean by allowing themselves to be carried by the current. Though all jellyfish can contract their muscles enough to swim, even a mild current can overwhelm most of them. Jellyfish have no brain or central nervous system, and they have no blood. Some types of jellyfish lie upside down on the seabed, and others cling to seaweed.
A jellyfish also catches prey with its tentacles and draws the prey into its mouth, located in the center of the underside of the animal's bell, or body. The food is then digested, and the waste is ejected back through the mouth.
The only truly complex part of the jellyfish's life is its reproductive cycle. Some jellyfish start out as tiny polyps that are found at the bottom of the ocean. They split to form stacks of medusae, which eventually swim away or are carried away by the current and grow into adult jellyfish.
Although most jellyfish are relatively small, the lion's mane jellyfish can have a bell that's larger than 6.5 feet across. This animal of the northern seas carries with it a great mass of stinging tentacles that can pose a danger to people, even if the animal is stranded on a beach.