Vertebrates are a group of animals with backbones or vertebral columns that protect their nerve cords. As of 2015, there are about 57,000 species of vertebrates alive today, though this number only amounts to about 3 percent of the world's total number of living species, with invertebrates comprising 97 percent.
The vertebral columns or backbones are made of vertebrae and are usually part of a developed endoskeleton that protects the vertebrate's inner organs, while a skull protects its well-developed brain. The endoskeleton of vertebrates contrasts with the hard exoskeleton of insects and arachnids.
Some scientists divide vertebrates into four groups: Osteichthyes, Chondricthyes, Agnatha and Tetrapoda. Osteichthyes includes bony fishes, of which there about 29,000 species. Members of this group have skeletons made out of bone, as opposed to members of the group Chondricthyes, who have skeletons made out of cartilage. Sharks and stingrays are two representatives of the cartilaginous fishes.
The group Agnatha includes lampreys, which are long, slender creatures that lack jaws. Some species of lamprey act as parasites, sucking blood out of their hosts.
Many of the most familiar vertebrates to humans are in the group Tetrapoda. These creatures have four limbs and include birds, mammals, reptiles and amphibians.