Pharyngeal gill slits are absent in which of the following animals?

$A$ rod-like structure formed on the dorsal side during embryonic development in some animals, which is mesodermally derived:

Chordates are distinguished from non-chordates by the presence of

Vertebrates possess which of the following characteristics?

Give three major differences between chordates and non-chordates and draw a schematic sketch of a chordate showing those features.

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: