The gut is U-shaped and both openings are anterior, with the anus dorsal to the mouth. The gut is dominated by an enormous pharynx whose wall is perforated by numerous tiny gill slits. The pharynx is surrounded by a water-filled atrium into which the gill slits open and which itself opens to the sea. It is a respiratory organ and filter feeding device. Water and food particles enter the pharynx and the water passes through the gill slits to the atrium and then out the siphon.
Food remains in the gut and passes posteriorly to be digested.
The blood vascular system includes a heart, vessels, and hemocoelic spaces. The pattern of blood flow resembles that of the cephalochordates and early vertebrates except that the heart reverses direction periodically and the blood thus flows in both directions through the system. Ascidians have no structure recognizable as a kidney. Ascidians are simultaneous hermaphrodites and the gonoducts open into the atrium. Some ascidians are solitary and may be relatively large.