“All vertebrates are chordates but all chordates are not vertebrates”. Justify the statement.

Options

A. Notochord is replaced by vertebral column in adult of some chordates.
B. ventral hollow nerve cord remains throughout life in some chordates.
C. All chordates posses vertebral column.
D. All chordates possess notochord throughout their life.

Answer

To justify the statement “All vertebrates are chordates but all chordates are not vertebrates,” we need to understand the definitions and characteristics of both chordates and vertebrates.

### Step-by-Step Solution:

1. **Definition of Chordates**:
- Chordates belong to the phylum Chordata. They are characterized by the presence of a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail at some stage of their life cycle.

2. **Definition of Vertebrates**:
- Vertebrates are a subphylum within the phylum Chordata, known as Vertebrata. They possess a backbone or vertebral column, which replaces the notochord during development in most adult forms.

3. **Presence of Notochord**:
- In vertebrates, the notochord is present during the embryonic stage but is replaced by the vertebral column as the organism matures. This means that all vertebrates start as chordates because they possess a notochord at some point in their development.

4. **Exceptions in Chordates**:
- Not all chordates develop into vertebrates. For example:
- **Cephalochordates** (like lancelets) retain the notochord throughout their entire life and do not develop a vertebral column.
- **Urochordates** (like tunicates) have a notochord only in their larval stage; it is lost in the adult form.

5. **Conclusion**:
- Since all vertebrates are characterized by having a vertebral column (which is a modified structure of the notochord), they fall under the category of chordates. However, not all chordates have a vertebral column, as seen in cephalochordates and urochordates. Therefore, the statement “All vertebrates are chordates but all chordates are not vertebrates” is justified.

### Final Answer:
- **Justification**: All vertebrates are chordates because they possess a notochord during their embryonic stage, which is replaced by a vertebral column in adults. However, some chordates, like cephalochordates and urochordates, do not develop into vertebrates, thus supporting the statement.