“All vertebrates are chordates but all chordates are not vertebrates”. Justify the statement.
Members of the subphylum Vertebrata possess a notochord during the embryonic period. In adults, this notochord is replaced by a cartilaginous or bony vertebral column. Since the presence of a notochord is a fundamental characteristic of Phylum Chordata, all vertebrates are classified as chordates.
However, Phylum Chordata is divided into three subphyla: Urochordata, Cephalochordata, and Vertebrata. The subphyla Urochordata and Cephalochordata (often called protochordates) are exclusively marine. They possess a notochord but do not develop a vertebral column. Therefore, while all vertebrates are chordates, all chordates are not vertebrates because protochordates are excluded from the subphylum Vertebrata.
Explanation
The answer uses the classification provided in the text to establish that vertebrates are a subset of chordates. It explains that the defining feature of chordates (the notochord) is present in all vertebrates during the embryonic stage, satisfying the first part of the statement. It then identifies the other two subphyla (Urochordata and Cephalochordata) which have a notochord but lack the vertebral column characteristic of vertebrates, thereby justifying the second part of the statement.