Gnathostomata is divided into $2$ superclasses. These are:

Which one feature is common to $Amphioxus$,frog,sea horse,and crocodile?

In which of the following is the notochord present during the embryonic stage?

In which of the following is the notochord absent?

Select the correct option for Non-chordates:
$(I)$ Central nervous system is ventral,solid,and double
$(II)$ Gill slits are absent
$(III)$ Heart is ventral
$(IV)$ Post-anal tail is absent

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: