The final phylum of animals is Chordata. All its members, called chordates, have bilateral symmetry, as well as a head, a body cavity, a digestive system, and body segmentation.

In addition, chordates have several unique structures. One such structure is the notochord. This is a stiff-yet-flexible rod of tissue extending the length of the animal that provides internal support. A second structure is a hollow nerve cord (also called a spinal cord) that extends the length of the animal just above the notochord. A third characteristic is the presence of gill slits: paired openings from the back of the organism’s mouth to the outside.

Not all these characteristics exist in the adult form of the chordates; some exist only in the embryonic form. However, all these characteristics exist at some time in the chordate’s life cycle. Most biologists agree that chordates evolved from the echinoderms. The special features of the chordates, such as the notochord, spinal cord, and gill slits, are adaptations to the environment.

The chordates are divided into two major groups: the chordates without backbones (the invertebrates) and the chordates with backbones (the vertebrates). The most primitive chordates are invertebrates. They include the tunicates and lancelets, both of which lack a backbone but have all the chordate traits. Both of these groups somewhat resemble tadpoles and are found clinging to rocks in marine environments. The vertebrate chordates are discussed in Chapter 22.