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

Select the correct option for Non-chordates:
$(I)$ Central nervous system is ventral,solid,and double
$(II)$ Gill slits are absent
$(III)$ Heart is ventral
$(IV)$ Post-anal tail is absent

All chordates possess $..........$.

Cephalochordates are ..........

Formation of notochord or the central axial chord in vertebrates is known as

Which of the following is true for $Salpa$?