With more than 200 species found worldwide, jellyfish, or sea jellies, are one of the world’s most common sea creatures. They range in size from just under an inch to giants with tentacles measuring over 100 feet.
Sea jellies consist of a gelatinous bell and trailing tentacles. They lack a centralized nervous system, heart, bones, and gills. They do have a simple nervous system, sometimes referred to as a “nerve net,” and their body walls are thin enough to allow oxygen to pass directly from the water to their internal organs.
Though they may look rather harmless, jellyfish have special defensive cells, called nematocysts. Used only once and then replaced, nematocysts help them capture prey and defend themselves.