Myelin sheath is not found around nerve fibres of

Options

A. Fishes
B. Egg laying mammals
C. Frog
D. Cyclostomates.

Answer

### Step-by-Step Solution:

1. **Understanding Myelin Sheath**: The myelin sheath is a protective covering that surrounds nerve fibers (axons) in many vertebrates. It is composed of fatty substances and helps in the rapid transmission of electrical signals along the nerve fibers.

2. **Identifying the Options**: The question provides four options:
- A) Fishes
- B) Egg-laying mammals
- C) Frog
- D) Cyclostomates

3. **Analyzing Each Option**:
- **Fishes**: Most fish have myelinated nerve fibers, especially the jawed fishes (Gnathostomes).
- **Egg-laying mammals**: These mammals, such as monotremes, have myelinated nerve fibers.
- **Frog**: Frogs, being amphibians, also possess myelinated nerve fibers.
- **Cyclostomates**: This group includes jawless vertebrates like lampreys and hagfishes, which are considered the most primitive vertebrates. Their nerve fibers are predominantly unmyelinated.

4. **Conclusion**: Since cyclostomates (lampreys and hagfishes) do not have myelin sheath around their nerve fibers, the correct answer to the question is option D) Cyclostomates.

### Final Answer:
**Myelin sheath is not found around nerve fibers of Cyclostomates.**