Almost the entire phylum Chordata is composed of the animals in the subphylum Vertebrata. There are two main hallmarks of this phylum: a skull and a backbone containing vertebrae. Vertebrae are a series of segmented units (exemplifying segmentation in this subphylum) that enclose the nerve cord. The skull encloses the brain, so together, it and the vertebrae enclose the major parts of the nervous system. The skull and vertebrae are part of the endoskeleton, another feature that all members of the subphylum Vertebrata show, but is not unique to the subphylum. The endoskeleton of a vertebrate is made of either hard bone or flexible cartilage, both made of primarily nonliving material secreted by living cells. There are seven classes within the subphylum vertebrata, listed below.

• Class Agnatha: Fish lacking jaws
• Class Chondrichthyes: Cartilaginous fish
• Class Osteichthyes: Bony fish
• Class Amphibia: Amphibians, adapted to terrestrial and aquatic environments
• Class Reptilia: Reptiles, descendants of amphibians adapted to living on land alone
• Class Aves: The birds
• Class Mammalia: The mammals, a diverse class with hair and mammary glands