Organs and Anatomy
Jellyfish have a very simple anatomy with no real organs and no central nervous system.
Jellyfish do not have any internal organs or bones. Their anatomy is basically structured as a sac within a sac and most of their tissue is made of water. Special proteins give it a gelatinous texture. Prey trapped on their tentacles is passed to their oral arms and transported up to the stomach, which is a simple digesting sac on the underside of the bell. There the food is digested and transported to the rest of the body via tiny radial canals. Since jellyfish are translucent, you can actually observe the food moving through their bodies over several hours. Oxygen is absorbed by diffusion from the surrounding water.
Jellyfish have no brain or central nervous system. Instead, they have a nerve net that coordinates their muscle contractions and allows them to react to their surroundings. They are able to sense up from down, light from dark, and when they are being touched. Box jellyfish exhibit more advanced behaviors and are able to swim around obstacles using light-sensing organs.