Chordates and non-chordates

Chordates are defined by three features: a notochord, a dorsal hollow nerve cord and paired pharyngeal gill slits. They are also bilateral, triploblastic, coelomate, organ-system level, with a post-anal tail and closed circulation.

Table 4.1 in brief. Chordates: notochord present; central nervous system dorsal, hollow and single; pharynx pierced by gill slits; heart ventral; post-anal tail present. Non-chordates: no notochord; nerve cord ventral, solid and double; no gill slits; heart dorsal where there is one; no post-anal tail.

Chordata has three subphyla: Urochordata (Tunicata), Cephalochordata and Vertebrata. The first two are called protochordates and are all marine.

In Urochordata the notochord is found only in the larval tail. In Cephalochordata it runs from head to tail and lasts through life. Examples: Ascidia, Salpa, Doliolum (urochordates); Branchiostoma, also called Amphioxus or lancelet (cephalochordate).

Vertebrates have a notochord only in the embryo; in the adult it is replaced by a vertebral column of cartilage or bone. So every vertebrate is a chordate, but not every chordate is a vertebrate.

On top of the chordate features, vertebrates have a muscular ventral heart with two, three or four chambers, kidneys that excrete and osmoregulate, and paired appendages in the form of fins or limbs.

Vertebrata splits into Agnatha (no jaws: class Cyclostomata) and Gnathostomata (jaws). Gnathostomata has two super classes: Pisces with fins (Chondrichthyes, Osteichthyes) and Tetrapoda with limbs (Amphibia, Reptilia, Aves, Mammalia).