Biology · Animal Kingdom · NEET
All chordates (at some stage) have a notochord. That single feature puts an animal inside phylum Chordata. Vertebrates are just ONE subphylum (Vertebrata) inside Chordata. In vertebrates the embryonic notochord is replaced by a cartilaginous or bony vertebral column. So every vertebrate started with a notochord (making it a chordate), but many chordates never form a vertebral column, so they stay chordates without becoming vertebrates.
A notochord is a soft, mesoderm-derived rod on the dorsal (back) side during embryonic development. A vertebral column (backbone) is a hard, segmented chain of cartilage or bone. In vertebrates, the notochord appears in the embryo and is then replaced by the vertebral column in the adult. Protochordates keep only the notochord for their whole life and never build a vertebral column. NEET loves testing this exact swap.
The protochordates: subphylum Urochordata (examples: Ascidia, Salpa, Doliolum) and subphylum Cephalochordata (example: Branchiostoma / Amphioxus). They are true chordates because they have a notochord, but they never develop a vertebral column, so they are not vertebrates. In Urochordata the notochord is present only in the larval tail; in Cephalochordata it runs from head to tail and stays for life.
No, and this trips up many students. The notochord is a supporting ROD (mesoderm-derived). The spinal cord is part of the nervous system (the dorsal, hollow nerve cord). They are different structures. A chordate has BOTH: a notochord for support and a dorsal hollow nerve cord for the nervous system. Don't confuse notochord with the vertebral column or with the spinal cord.
Because that replacement is exactly WHAT separates vertebrates from other chordates. All chordates have a notochord (so all are in Chordata). The extra step of replacing that notochord with a vertebral column is what upgrades a chordate into a vertebrate. So the fact that the notochord is replaced by a vertebral column in adult vertebrates directly explains why all vertebrates are chordates yet not all chordates are vertebrates. That makes Reason the correct explanation of the Assertion.
No. In older books Hemichordata was placed as a subphylum under Chordata, but now it is a SEPARATE phylum under non-chordates. Hemichordates have only a rudimentary structure called a stomochord in the collar region (similar to a notochord, but not a true notochord). So do not count Hemichordata as a chordate in NEET.
Assertion (A): All vertebrates are chordates but all chordates are not vertebrates. Reason (R): Notochord is replaced by the vertebral column in the adult vertebrates. Choose the most appropriate answer.
Assertion (A): All vertebrates are chordates but all chordates are not vertebrate. Reason (R): The members of subphylum vertebrata possess notochord during the embryonic period, the notochord is replaced by a cartilaginous or bony vertebral column in adults. Choose the correct answer.
Which of the following statements are true for the phylum Chordata? (a) In Urochordata notochord extends from head to tail and present throughout life. (b) In Vertebrates notochord is present during the embryonic period only. (c) Central nervous system is dorsal and hollow. (d) Chordata is divided into 3 subphyla: Hemichordata, Tunicata and Cephalochordata.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Not lost, replaced. Vertebrates form a notochord in the embryo, then replace it with a cartilaginous or bony vertebral column as they grow. So a vertebrate is still a true chordate (it had the notochord); it simply upgraded that rod into a backbone.
Branchiostoma (Amphioxus, a cephalochordate) and Ascidia or Salpa (urochordates). All are protochordates: they have a notochord but never form a vertebral column, so they are chordates but not vertebrates.
Besides the basic chordate characters, vertebrates have a ventral muscular heart with two, three or four chambers, kidneys for excretion and osmoregulation, and paired appendages (fins or limbs). But the defining upgrade is the replacement of the notochord by a vertebral column.
In Cephalochordata (like Amphioxus) the notochord extends from head to tail and stays for life. In Urochordata the notochord is present only in the larval tail. In vertebrates it is present only in the embryo and is then replaced by the vertebral column.
