Chordate
Chordates, members of the animal phylum Chordata, which includes all mammals, birds, amphibians, reptiles, and fish. There are about 45,000 species of chordates. They are classified in four subphyla, the largest of which contains the vertebrates. Chordates share common characteristics. The first characteristic is a single dorsal nerve cord, the brain and the spional nerve cord. The second is a notochord, which in vertebrates takes the form of the backbone. The third is the presence of gill slits at some stage during the development of the animal. All chordates have bilateral symmetry.