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INTRODUCTION TO THE UROCHORDATA




Tunicates. Photo by Crissy Huffard, UCMP.
 

The Urochordata, sometimes known as the Tunicata, are commonly known as "sea
squirts." 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. However, many tunicates have a larva that is
free-swimming and exhibits all chordate characteristics: it has a notochord, a
dorsal nerve cord, pharyngeal slits, and a post-anal tail. This "tadpole larva"
will swim for some time; in many tunicates, it eventually attaches to a hard
substrate, it loses its tail and ability to move, and its nervous system largely
disintegrates. Some tunicates are entirely pelagic; known as salps, they
typically have barrel-shaped bodies and may be extremely abundant in the open
ocean.






 

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. Few paleontologists
are familiar with them; tunicate spicules may be mistaken for sponge spicules.
Shown below is a spicule from a living tunicate from Moorea, French Polynesia,
photographed using an Environmental Scanning Electron Microscope. Click on the
image to view the full-sized version.