Which of the following is incorrect for $Urochordata$?

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

Which one of the following features are true for chordates?
$(a)$ Heart is ventral.
$(b)$ Pharyngeal gill slits present.
$(c)$ Central nervous system is ventral,solid and single.
$(d)$ Post-anal tail is present.
$(e)$ Notochord is present.
Choose the most appropriate answer from the options given below:

What is present in the larval stage of Urochordata?

In vertebrates,the notochord is replaced by which structure during the embryonic stage?

Which of the following is not a characteristic of chordates?