Cardiovascular system
Unlike most animals, box jellyfish use a very different kind of cardiovascular system. This is due to the fact that jellyfish are cold blooded animals. This means that their body temperature will adjust itself in accordance to its general surroundings. This is the opposite of mammals, which are warm blooded creatures. Box jellyfish also lack a heart and blood vessels, with their bodies composed almost entirely of water. To make up for the lack of a closed cardiovascular system, they instead have an open cardiovascular system. As opposed to traveling through blood, gases instead flow through the body through diffusion. They are able to do so because they have very thin skin. Oxygen can also move through the gastrodermis. Humans and other organisms on the other hand, cannot do so because they have thick skin. So while box jellyfish do not have blood, or any cardiovascular elements for that matter, they can still have access to gases like oxygen.