This class consists of planktonic free living tunicates that have a trunk seldom more than 0.5 cm long and a permanent tail four or five times that length. Development is direct, without the intervention of a larva. Generally, individuals are protandrous, with the testis (with a duct to the exterior) maturing first. Eggs, externally fertilised, are released later by rupture of the adult trunk before the animal degenerates and dies. The mucous house, which encloses the trunk, is discarded as the animal grows and generates a new house. The discarded house, with its load of entangled organic material rejected from the feeding stream, is a source of food for other organisms. The pharyngeal perforations are reduced to two tubes (spiracles) from the floor of the pharynx to the ventral surface of the trunk. The endostyle is relatively short.
Only about 60 species are known worldwide and, being planktonic, they are spread around the world in ocean currents and are carried into coastal waters with other oceanic species usually in spring and summer.
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