Urochordata is a subphylum of chordate animals, which includes tunicates. These sea creatures are characterized by a barrel-shaped body and a feeding apparatus called a pharynx. They are filter feeders and play an important role in marine ecosystems. Apart from Urochordata, this subphylum has other synonyms such as Tunicata, Ascidiacea or Sea Squirt. The term Tunicate derives from their external covering, which resembles a tunic. Meanwhile, Ascidiacea comes from the word ascidium, which means a bag-like structure. Lastly, Sea Squirt is a common name for tunicates due to their ability to squirt water out of their body when threatened.