1. Jellyfishes are made up of more than 95% water.
Their bodies are soft and lack a skeletal structure or outer shell. They are delicate and easily damanged. Jellyfishes require water to help support their body and if removed from their aquatic surroundings, they collapse and die.
2. Jellyfish are radially symmetrical.
Jellyfish are symmetrical about a central axis that runs through the length of their body, from the top of their bell to the ends of their tentacles. They have a top and a bottom but they lack a left and right side and as a result differ from many other types of animals (such as mammals, reptiles, fish, birds, and arthropods) which exhibit bilateral symmetry.
3. Jellyfishes have no brain, no blood, and no nervous system.
Their senses are primitive and consist of a neural net, eye spots that can sense light from dark, and chemosensory pits that help them identify potential prey.
4. Some Jellyfish are immortal
Turritopsis nutricula, is a hydrozoan whose medusa, or jellyfish, form can revert to the polyp stage after becoming sexually mature. It is the only known case of a metazoan capable of reverting completely to a sexually immature, colonial stage after having reached sexual maturity as a solitary stage. It does this through the cell development process of transdifferentiation. Cell transdifferentiation is when the jellyfish “alters the differentiated state of the cell and transforms it into a new cell”. In this process the medusa of the immortal jellyfish is transformed into the polyps of a new polyp colony.