Which of the following is a living fossil

Options

A. Limulus
B. Trilobite
C. Nereis
D. Thyone

Answer

### Step-by-Step Solution:

1. **Understanding the Term "Living Fossil":**
- A living fossil refers to organisms that have remained unchanged for millions of years and still exist today. They are represented in the fossil record and show little to no evolutionary change over a long period.

2. **Analyzing the Options:**
- The question provides several options to identify which one is a living fossil. We need to evaluate each option based on their historical and biological significance.

3. **Evaluating Each Option:**
- **Limulus (King Crab):**
- Limulus is found in the phylum Arthropoda. It has been around for approximately 150 million years with no significant changes in its structure. Therefore, it qualifies as a living fossil.
- **Trilobite:**
- Trilobites are also part of Arthropoda but are extinct. They do not qualify as living fossils since they are no longer found on Earth.
- **Nereus:**
- Nereus is a polychaete worm from the phylum Annelida. Although it is still alive, it does not fit the criteria of a living fossil as it has undergone changes over time.
- **Thione (Sea Cucumber):**
- Thione belongs to the phylum Echinodermata. While it is a marine organism, it is not classified as a living fossil due to changes in its evolutionary lineage.

4. **Conclusion:**
- Based on the evaluations, the only organism that qualifies as a living fossil is **Limulus**.

### Final Answer:
**Limulus** is the living fossil among the options provided.

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