Spinal bifida is the failure of the nervous system to close. The slits filter out food particles and keep them in the animal's body so they can be put into the digestive tract. In fish, these have become modified as gills, and in humans as our ears and eustachian tubes. In humans, the tail is present during embryonic development, but is subsequently resorbed. This subphylum includes Lancets, so-named for their shape.
Lancets look a lot like the generic chordate described above.
They have a notochord, but no bones. They are about one inch long, and live in the muddy ocean floor. Their larvae show typical chordate characteristics, but the adults have lost many of these organs, and what they do still have have become highly-modified. Subphylum Vertebrata may have come from an Amphioxus -like ancestor, however vertebrates have a definite head and Amphioxus doesn't. They do not have jaws, are eel-shaped, prey on fish, and have larval forms which are different from the adults.