The beak in birds is toothed in

$A$ fish differs from a whale in having

Assertion : Acraniata is a group of organisms which do not have a distinct cranium.
Reason : It includes small marine forms without a head.

Assertion $A$: $Amphioxus$ is not a vertebrate.
Reason $R$: $Amphioxus$ is not a chordate.
Which option is correct for Assertion $A$ and Reason $R$?

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.

$A$ common characteristic of all vertebrates is $..........$