Belonging to the phylum Chordata, a chordate refers to an animal that at some point during their development has a notochord, a dorsal, hollow nerve cord, pharyngeal slits or clefts, and a muscular, post-anal tail. They are bilaterally symmetrical animals that belong to the clade of animals known as Deuterostomia. Two distinct species of chordates are tunicates and lancelets.
Tunicates belong to the phylum Chordata and the subphylum Urochordata. They are sessile marine chordates that lack a backbone. They are very closely related to craniates (hagfish, lampreys, and jawed vertebrates). They belong to the deepest-branching lineage of chordates, meaning they represent an early stage in chordate evolution. It was suggested by William Garstang that ancestral tunicates accelerated their sexual maturity, becoming more mature while still in their larval stage. This process of how these tunicates and the chordates that evolved from them retained the notochord and other features as adults is known as paedomorphosis. However, Garstang's idea is not longer believed by scientists. They not think that the degenerate adult stage of tunicates is a derived trait that evolved only after the tunicate lineage branched off from other chordates. Studies suggest that the tunicate larva does not develop the posterior regions of its body axis, but rather the anterior region is elongated and contains a heart and digestive system.
Lancelets belong to the phylum Chordata and the subphylum Cephalochordata. They are small blade-shaped marine chordates that lack a backbone. Lancelets have only a slightly swollen tip on the anterior end of their dorsal nerve cord instead of a full-fledged brain. However, the same Hox genes that organize major regions of the forebrain midbrain, and hindbrian of vertebrates are expressed in a corresponding pattern the the small cluster of cells in the lancelet's nerve cord. This information suggests that the vertebrate brain is an elaboration of an ancestral structure similar to the lancelet's simple nerve cord tip. This and other features of lancelets has assisted scientists in the study of how vertebrates have evolvedand adapat