Which group of animals are considered protochordates (invertebrate chordates)?

Draw a labeled diagram showing the fundamental characteristics of Phylum Chordata.

Assertion : Acraniata is a group of organisms which do not have a distinct cranium.
Reason : It includes small marine forms without a head.

Assertion $(A)$: $Ascidia$ and $Salpa$ are placed under sub-phylum $Urochordata$.
Reason $(R)$: In $Ascidia$ and $Salpa$,notochord is present only in larval tail.
In the light of the above statements,choose the most appropriate answer from the options given below:

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:

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