This is a Bell Jellyfish.

Image from Monterey Bay Aquarium

This is a Midwater Jelly.

Image from Monterey Bay Aquarium

Jellyfish have no specialized circulatory system. Oxygen is easily absorbed through their thin bodies. Food is absorbed through the gastrodermis, which lines the gastrovascular cavity. The gastrodermis then passes the absorbed nutrients through the mesoglea then into the epidermis.

This is a great animation of how nutrients are absorbed through the jellyfish.