Assertion (A) is true.
All vertebrates possess the four fundamental chordate characters — a notochord, a dorsal hollow nerve cord, pharyngeal gill slits and a post-anal tail — at least during the embryonic stage, so all vertebrates are chordates.
Chordates also include the protochordates (subphyla Urochordata and Cephalochordata), which never develop a vertebral column, so they are chordates but not vertebrates.
Thus, all vertebrates are chordates but all chordates are not vertebrates.
Reason (R) is also true.
As given in NCERT, the members of the subphylum Vertebrata possess the notochord during the embryonic period, and the notochord is replaced by a cartilaginous or bony vertebral column in the adult.
(R) is the correct explanation of (A) because the notochord (a basic chordate character) present in vertebrates makes them chordates, while the replacement of the notochord by a vertebral column is precisely the feature that marks an animal as a vertebrate — chordates in which no vertebral column forms (Urochordata, Cephalochordata) remain non-vertebrate chordates.
Hence, both (A) and (R) are true and (R) is the correct explanation of (A).
Therefore, the correct option is: Both (A) and (R) are true and (R) is the correct explanation of (A).
