The foi replaced its ancestor. They have adapted to a strange way of feeding, an important part of its digestive system is on the outside of its body. The belly of the foi is sticky; anything small enough to get stuck on it will be dissolved with gastric acid ejected from the skin on the belly. To make this way of feeding possible the foi swims around for some time before it curls up like a ball and only then is the gastric acid released. Its interior body consists of muscles and there is no skeletal structure in its body at all; it uses this muscles to flap its "wings" to swim. The sceletus gurgito used the skin to pick up vibrations in the water to navigate; the foi has inherited this ability as well as gained primitive eyes that can pick up some light to aid it.