A third chordate trait is pharyngeal slits, which are holes between the pharynx, or throat, and the outside world. They have undergone considerable changes as a result of evolution. These slits are utilized by primitive chordates to filter food particles from the water. As animals grow larger, they need a stronger gill system for breathing. Therefore, the number of filaments in their gills will increase while the size of the slit between each pair of filaments will shrink. After evolution developed limbs instead of gills, these openings were no longer needed for respiration. However, they are still present in many modern-day fish, such as trout and salmon. Pharyngeal slits can be seen in both vertebrate and invertebrate chordates. However, not all chordates have them. Fish are members of the class of vertebrates that do have pharyngeal slits. The other major group of vertebrates without pharyngeal slits are amphibians. Amniotes (reptiles and birds) have a complete set of teeth on the bottom of their mouth that is used to chew their food before swallowing it. Fish and amphibians swallow their food whole; therefore, they cannot chew it first like amniotes can. In conclusion, chordates have pharyngeal slits. Fish are a type of chordate.