2. A dorsal nerve cord: a bundle of nerve fibers that runs along the back of the animal and serves as the main communication center.
3. Pharyngeal slits: openings in the pharynx (throat) that allow water to enter and exit the body. In some animals, these slits develop into gills, which are used for breathing.
4. A post-anal tail: a tail that extends beyond the anus. This tail is used for locomotion in some animals, such as fish.