Cephalochordates are

Options

A. exclusively fresh water animals
B. exclusively marine animals
C. able to live in estuarine waters
D. Both (1) & (2)

Answer

### Step-by-Step Solution:

1. **Identify the Classification**:
- Cephalochordates are classified as a subphylum within the phylum Chordata.

2. **Characteristics of Chordates**:
- All members of the phylum Chordata possess a notochord at some stage of their life cycle.

3. **Notochord in Cephalochordates**:
- In cephalochordates, the notochord is present throughout their entire life, from the young stage to adulthood.

4. **Orientation of the Notochord**:
- The notochord in cephalochordates extends from the head to the tail, providing structural support.

5. **Habitat**:
- Cephalochordates are exclusively marine animals, meaning they live in oceanic environments.

6. **Example of Cephalochordates**:
- A common example of a cephalochordate is Branchiostoma, also known as Amphioxus or Lenslet.

7. **Conclusion**:
- Based on the characteristics and habitat, cephalochordates can be defined as exclusively marine animals that possess a persistent notochord throughout their life.

### Final Answer:
Cephalochordates are exclusively marine animals that possess a notochord throughout their life.

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