The anatomy of a vertebrate includes a stiff rod known as a vertebral column that runs the length of the animal's body. Just above the vertebral column is a tube of nervous tissue known as the spinal cord. Below the vertebral column lies the gastrointestinal tract. The vertebral column consists of a series of vertebrae and intervertebral discs that give the animal both flexibility and structural support.
Vertebrates first appeared about 525 million years ago during the Cambrian Period. The earliest known vertebrate is thought to be Myllokunmingia, an animal that is thought to have had a skull and a skeleton made of cartilage. Today there are approximately 50,000 species of vertebrates which account for about 3% of all known species on our planet. The other 97% of species alive today are invertebrates and belong to animal groups such as sponges, cnidarians, flatworms, molluscs, arthropods, insects, segmented worms, and echinoderms as well as many other lesser-known groups of animals.