The common characteristic of all vertebrates without exception is:

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

In both chordates and non-chordates,the intestine develops from which germ layer?

Which of the following features is present in all chordates at some stage of their life?

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

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 $(R):$ The members of subphylum Vertebrata possess a notochord during the embryonic period; the notochord is replaced by a cartilaginous or bony vertebral column in adults.
In the light of the above statements,choose the correct answer from the options given below: