Thecosomata are sea snails about the size of a lentil, with shells so thin that they are virtually transparent. Rather than sliding on the seabed, their foot has evolved into a pair of thin ‘wings’ that flap through the water. Sea snails mostly swim upside down because their body and shell is heavier and hangs down from the wings. They are herbivores, feeding on phytoplankton, but they catch their food in webs, like spiders. These webs are made from strands of mucus that can extend out as far as five centimetres (two inches) from the sea butterfly. They are preyed on by a kind of swimming sea slug called a sea angel.