The three subphylums in Chordata are Vertebrata, Urochordata and Cephalochordata. Vertebrata is the largest group, including such familiar animals as fish, birds, reptiles, amphibians and mammals. Urochordata contains about 4,600 to 5,100 species total, and Cephalochordata contains only 28.
The three subphyla do not appear very closely related on the surface. Cephalochordates resemble tiny, worm-like fish, but urochordates include species which superficially resemble plants, such as the sea squirt. All chordates have the same basic features, although they may become highly modified during growth and have changed during the course of evolution.
Chordates have notochords as embryos, even if as adults they do not. They have nerve chords running down their backs. Tails are present when they are embryos, whether or not the adult form retains the tail. They also have pharangeal slits, according to biology department at Clermont College. In fish, these slits function as gills, but in land animals, they have been converted into parts of the inner ear and jaw. The most primitive chordates use the slits for filter feeding.