The body of an adult tunicate is quite simple, being essentially a sack with two siphons through which water enters and exits.
Water is filtered inside the sack-shaped body.
Urochordates have a sparse fossil record. A Precambrian fossil known as Yarnemia has been referred to the Urochordata, but this assignment is doubtful. Complete body fossils of tunicates are rare, but tunicates in some families generate microscopic spicules that may be preserved as microfossils. Such spicules have occasionally been described from Jurassic and later rocks. Shown below is a spicule from a living tunicate from Moorea, French Polynesia, photographed with UCMP's Environmental Scanning Electron Microscope .