What is the location and function of the notochord?

Retrogressive metamorphosis is found in

Name the subphyla of the phylum Chordata.

Differentiate between Invertebrates and Vertebrates.

In chordates,the alimentary canal is located in which position relative to the 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: