The group includes both colonial and solitary animals. They are distinguished from other sea squirts by the presence of longitudinal vessels in the pharyngeal basket. This provides the etymology of their name: in ancient greek, φλέψς, φλεβός means "blood vessel". Another characteristic of phlebobranchians is the gonads being surrounded by a loop of gut. The posterior part of the abdomen is absent, and many species also lack the epicardial cavity that surrounds the heart and other internal organs in many other sea squirts.
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