detritus
Most tunicates live a sessile existence in shallow ocean waters and are suspension feeders. The primary foods of tunicates are plankton and detritus. Seawater enters the tunicates body through its incurrent siphon. Suspended material is filtered out of this water by a mucus net pharyngeal slits and is passed into the intestine through the action of cilia. The anus empties into the excurrent siphon, which expels wastes and water. Lancelets possess a notochord, dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail in the adult stage Figure 15.35 . The notochord extends into the head, which gives the subphylum its name Cephalochordata . Extinct fossils of this subphylum date to the middle of the Cambrian period 540488 mya . The living forms, the lancelets, are named for their blade-like shape. Lancelets are only a few centimeters long and are usually found buried in sand at the bottom of warm temperate and tropical seas. Like tunicates, they are suspension feeders.