The following are the statements about non-chordates:
$A$. Pharynx is perforated by gill slits.
$B$. Notochord is absent.
$C$. Central nervous system is dorsal.
$D$. Heart is dorsal if present.
$E$. Post anal tail is absent.
Choose the most appropriate answer from the options given below:

Which of the following is $NOT$ an exoskeleton of vertebrates?

Which of the following phyla possess a post-anal tail at some stage of their life?

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:

Which of the following statements is/are correct for the phylum Chordata?
$(a)$ In Cephalochordata,the notochord extends from head to tail region and is persistent throughout their life.
$(b)$ In Vertebrata,the notochord is present only during the embryonic period.
$(c)$ The central nervous system is dorsal and hollow.
$(d)$ Chordata is divided into $3$ subphyla: Hemichordata,Urochordata,and Cephalochordata.

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