All vertebrates are chordates but all chordates are not vertebrates. Explain.

(N/A) $\rightarrow$ The members of subphylum $Vertebrata$ possess a notochord during the embryonic period.
$\rightarrow$ The notochord is replaced by a cartilaginous or bony vertebral column in the adult.
$\rightarrow$ Since all vertebrates possess a vertebral column,they are classified as chordates. However,other chordates,such as those in subphyla $Urochordata$ and $Cephalochordata$,possess a notochord but lack a vertebral column. Thus,all vertebrates are chordates,but all chordates are not vertebrates.