In the two other subphyla of the chordates, Cephalochordata and Urochordata, the notochord is either maintained in the adult (Cephalochordata such as Lancelets) or present in the larval form and lost during metamorphosis into the adult form (Urochordata such as Tunicates).    From Wordnik.com.  [Haeckel had a point - The Panda's Thumb] Reference

The case in Cephalochordata is even more bizarre since there is no real organ that spreads oxygen through the body.    From Wordnik.com.  [Blogs de Ciência] Reference