The pharynx is the area just behind the mouth. During the embryonic stage of the chordates these slits connect the outside to the pharynx. Early chordates used them as devices for filter feeding. As time progressed the slits became modified to function in gas exchange. Post anal Tail .
Most chordates have a tail extending beyond the anus.
In many aquatic species the tail is equipped with skeletal and muscular tissue for movement. These invertebrate chordates contain all 4 phylum characteristics in their adult body. The lancelet Brachiostoma is the characteristic organism of this group. The animal wiggles into the sand , leaving its anterior end exposed. It pulls in water through its mouth and traps the food on the mucus of its pharyngeal gill slits.