Vertebrates are members of of a larger group, the chordates .
During some part of their developmental sequence, chordates have a notocord , a flexible, but stiff, supportive rod which runs the length of the body. But, the larval form has many characteristics of chordates, including a notocord. Another characteristic of chordates found in the urochordata include a dorsal, hollow nerve cord formed by invagination of the embryonic ectoderm. The pharynx of urochordates is large and its walls have many gill slits, which let water which enters through the mouth escape without passing through the digestive tract. In many chordates, such as fish, these gill slits have been modified to act as oxygen gas absorbers.