Which of the following is present during some stage of the life cycle in animals belonging to the phylum $Chordata$?

Post-anal tail is present in

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

Given below are two statements: one is labelled as Assertion $(A)$ and the other is labelled as Reason $(R)$.
Assertion $(A)$: All vertebrates are chordates but all chordates are not vertebrates.
Reason $(R)$: Notochord is replaced by vertebral column in the adult vertebrates.
In the light of the above statements,choose the most appropriate answer from the options given below:

Which of the following organisms possesses a notochord that extends from head to tail throughout its entire life?

Which of the following is $NOT$ found in vertebrate animals?