Which of the following is a living fossil?

Options

A. Euglena
B. Balanoglossus
C. sycon
D. Limulus

Answer

**Step-by-Step Solution:**

1. **Understanding Living Fossils**:
- A living fossil is defined as a present-day organism that has remained relatively unchanged over a long geological time scale. This means that when we compare them to their fossilized ancestors, they look very similar.
- Living fossils have existed for millions of years, spanning different geological eras (like the Cenozoic and Mesozoic eras).

2. **Criteria for Living Fossils**:
- They have undergone very little or no evolutionary change over time.
- Most of their relatives or closely related species have gone extinct, meaning they are often the only surviving member of their lineage.

3. **Analyzing the Options**:
- **Euglena**: A unicellular organism that can be both autotrophic and heterotrophic. It belongs to the kingdom Protista but has undergone changes over time.
- **Balenoglossus**: A primitive chordate that has also undergone several changes and does not fit the criteria of a living fossil.
- **Cyclone**: Belongs to the phylum Porifera (sponges) and has evolved over time.
- **Limulus (Horseshoe Crab)**: This organism has remained relatively unchanged for millions of years and is often referred to as a living fossil.

4. **Conclusion**:
- Among the options provided, Limulus (horseshoe crab) is the correct answer as it meets the criteria of a living fossil.

**Final Answer**: Limulus (Horseshoe Crab) is a living fossil.

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