At present only about 16 species of Priapula have been described, though fossil evidence dates back to the Cambrian. They are up to 15 cm long and are found in colder waters at a variety of depths. Their body cavity has a mesodermal lining, so they can be regarded as coelomate; however the lining is unlike that of other coelomates. The cuticular trunk is moulted periodically. Circular and longitudinal muscles exert the pressure required to evert the head region (see Priapulus bicaudatus right). The larvae move through the sediment using their head region as an anchor, but adults are rarely good burrowers. They are predators feeding on slow-moving animals, especially worms.