Identify the phylum in which the notochord is retained in the adult stage.

Options

A. Chordata
B. Craniata
C. Protochordata
D. Echinodermata

Answer

To identify the phylum in which the notochord is retained in the adult stage, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Notochord**: The notochord is a flexible rod-like structure that provides support to the body. It is a characteristic feature of chordates and is crucial for the development of the spinal cord.

2. **Identify the Groups of Animals**: The animals with a notochord are primarily classified under the phylum Chordata. Within this phylum, there are different sub-groups, including vertebrates and non-vertebrates.

3. **Differentiate Between Life Stages**: In many chordates, the notochord is present during the embryonic stage but may be replaced by a vertebral column (spine) in adults. However, some animals retain the notochord throughout their life.

4. **Focus on Specific Groups**:
- **Craniata**: This group includes animals with a skull, such as lampreys, where the notochord is present but may not be retained in the adult stage.
- **Protochordates**: This group includes organisms like amphioxus (lancelets), where the notochord is retained in the adult stage.

5. **Conclusion**: The phylum where the notochord is retained in the adult stage is **Protochordata**. This group includes organisms that exhibit chordate characteristics throughout their life, including the retention of the notochord.

### Final Answer:
The phylum in which the notochord is retained in the adult stage is **Protochordata**.

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