Notochord occurs in the adult animal of

Options

A. Protochordates
B. Agnatha
C. Chondrichthyes
D. Osteichthyes

Answer

To determine where the notochord occurs in adult animals, we can analyze the given options systematically.

### Step-by-Step Solution:

1. **Understanding Notochord**: The notochord is a flexible, rod-like structure found in the embryos of all chordates and in some adult chordates. It serves as a primary skeletal structure during development.

2. **Analyzing the Options**:
- **A. Protochordates**: This group includes animals like tunicates and lancelets. They retain the notochord throughout their adult life.
- **B. Agnatha**: This class includes jawless fish like lampreys and hagfish. They have a notochord during their larval stage, but it is replaced by a cartilaginous structure in adults.
- **C. Chondrichthyes**: This class includes cartilaginous fish like sharks and rays. They have a notochord during their embryonic stage, but in adults, it is replaced by a cartilaginous skeleton.
- **D. Osteichthyes**: This class includes bony fish. Similar to Chondrichthyes, they have a notochord during development, but it is replaced by a bony skeleton in adults.

3. **Conclusion**: Based on the analysis, the only group that retains the notochord in its adult form is **A. Protochordates**.

### Final Answer:
The notochord occurs in the adult animal of **A. Protochordates**.

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