The animal that possesses a notochord throughout its life is

The common characteristic of all vertebrates without exception is:

All vertebrates are chordates but all chordates are not vertebrates,why?

In chordates,the brain is formed from which of the following structures?

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:

Which one of the following statements is totally wrong about the occurrence of notochord,while the other three are correct?