Gnathostomata is divided into $2$ superclasses. These are:

To which group does $Salpa$ belong?

Statement-$1$ : Cephalochordata bears notochord all along the body throughout life.
Statement-$2$ : Urochordate bears vertebral column only in tail region throughout the life.
Then which is correct?

The statement that 'all chordates are metamerically segmented' is:

The notochord is an organ derived from which embryonic layer?

What do $a$ and $b$ represent in the given chart respectively?