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. Others are colonial with tiny individual zooids in a common tunic. Ciona intestinalis is a widely distributed, shallow water, solitary sea squirt. Because it is common, easy to dissect, achieves large size, and exemplifies the simplest, least specialized ascidian body plan, it is frequently used in invertebrate zoology laboratories as a representative sea squirt.