Phylum chordata, also known as chordates, includes thousands of fossils and more than 45,000 living species that can be found in any type of environment on earth. Chordates consist of three groups: vertebrates, tunicates, and lancelets. The most familiar group is the vertebrates, which humans are a part of, and also includes birds, reptiles, fishes, amphibians, lampreys, and mammals. Chordata is named based on its structure of having a notochord, which is present in all embryos and may be absent or reduced in adults. The notochord is a semi-flexible rod that extends most of the length of the body, stiffens the body, and acts as a support during movement and locomotion.
Chordata have a bilateral symmetry, a segmented body, and segmented muscles based on their arrangement in myomeres (blocks of segmented muscle) down the length of the body. The notochord acts like an anchor to support the animal’s large body muscles. Chordates have four appendages in the shapes of legs, arms, wings or fins. Their internal skeleton has muscles attached that allow for a greater range of movement and the development of the body size. Chordates that lack bones have muscles that work against the notochord for movement.