"All vertebrates are chordates but all chordates are not vertebrates". Justify the statement.

(N/A) The characteristic features of the phylum $Chordata$ include the presence of a notochord and paired pharyngeal gill slits.
In the sub-phylum $Vertebrata$, the notochord present in the embryo is replaced by a cartilaginous or bony vertebral column in the adult.
Since all vertebrates possess a vertebral column (which is a modified notochord), they are classified as chordates.
However, some chordates, such as those in the sub-phyla $Urochordata$ and $Cephalochordata$, possess a notochord throughout their life or only in the larval stage but do not develop a vertebral column.
Therefore, it is correct to state that all vertebrates are chordates, but all chordates are not vertebrates.