The most important distinctive character of $Chordata$ is the presence of

Which of the following animals is included in Urochordata?

Name the subphyla of the phylum Chordata.

$A$ schematic representation of a chordate's embryo is shown in the figure. The structures marked $A, B, C$ and $D$ are respectively:

Non-chordates are characterized by the absence of which of the following features?

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: