In which of the following is the $rotatory$ $organ$ (wheel organ) found?

In vertebrates,the notochord is replaced by which structure during the embryonic stage?

Which of the following are referred to as non-vertebrate chordates?

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:

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

To which phylum do the classes Pisces,Amphibia,Aves,Reptilia,and Mammalia belong?