Sea jellies (or jellyfish, although they are not really fish!) have existed for over 500 million years, placing them among the oldest animals on Earth. Sea jellies have soft, jelly-like bodies that contain >95% water. They have just two layers of tissue; one that lines their external surface (the ‘ectoderm’) and another that lines their gut (the ‘endoderm’). Sandwiched between the ectoderm and endoderm is the ‘mesoglea’. The mesoglea is mainly water but also contains fibres, such as collagen, which gives the animal its distinct jelly-like consistency.