The notochord is an organ derived from which embryonic layer?

What is present in the larval stage of Urochordata?

What is the location and function of the nerve cord in chordates?

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

Differentiate between: Notochord and Nerve cord.

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: