The phylum Chordata contains all animals that possess, at some point during their lives, a hollow nerve cord and a notochord, a flexible rod between the nerve cord and the digestive track. The phylum Chordata is an extremely diverse phylum, and the one most recognizable to us. The phylum contains about 43,700 species, most of them concentrated in the subphylum Vertebrata, making it the third-largest phylum in the animal kingdom.Animal groups in the chordate phylum include fish, amphibians, reptiles, birds and mammals. Animals with backbones are the most highly evolved groups of animals, the most complex and complicated. They have interior skeletons and have bilateral symmetry. They have more advanced brains.