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The Thalicea comprise a class of marine animals within the subphylum Urochordata. Unlike their bottom-dwelling relatives the ascidians, thaliaceans are free-floating for their entire lifespan. There are three orders of Thaliacea: Pyrosomida, Doliolida, and Salpia. Pyrosomes are colonies of tiny zooids arranged around a central chamber called the cloaca. As the water exhaled by the zooids exits the cloaca through a common opening, the water movement slowly propels the pyrosome through the sea. Doliolids and salps alternate between solitary and colonial life stages. Salp colonies can be several meters in length. Doliolids and salps rely on muscular action to propel themselves through surrounding sea water. All of the thaliceans are filter feeders. They have a barrel-shaped body through which they pump water, propelling them through the sea and from which they extract food.
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