Subphylum Urochordata or Tunicata

Urochordata or Tunicata (Greek oura, a tail; Latin chorda, cord) are exclusively marine animals, commonly known as sea squirts. Most are sessile, though some are pelagic or free-swimming, and they may exist as solitary or colonial forms. The unsegmented adult body is covered by a protective covering called a test or tunic, and the sac-like adult form gives the group its common name.

The coelom is absent in urochordates, though an atrial cavity surrounds the pharynx. Critically, the notochord is present only in the tail region of the free-swimming larval stage — the origin of the name Urochordata ("tail-chordates"). The alimentary canal is complete, and circulation is of the open type, with a ventral, tubular heart. Respiration takes place through gill slits and clefts in the pharynx. The dorsal, tubular nerve cord is likewise present only in the larval stage; in the sessile adult it is reduced to a single dorsal ganglion. …