The tunicates are marine, filter-feeding animals. Most of the body of the adult is occupied by a very large pharynx with numerous gill slits that act as a sieve for food. Water taken into the incurrent siphon enters the pharynx and passes out through the gill slits, leaving food particles trapped in the pharynx.
Water leaves the atrium, a sac surrounding the pharynx, by way of the excurrent siphon.
Thus the gill slits in tunicates serve a feeding function, not a respiratory function. The sea squirt larva is a free-swimming animal resembling a tadpole. The head, which will become the entire body of the adult, contains a rudimentary brain and sense organs, a small pharynx and digestive tract, and a ventral heart. Incurrent and excurrent openings are located at the top of the head. The tail is a muscular appendage that functions as a swimming organ.