Echinoderms, hemichordates and chordates have a

Options

A. different ancestral stock
B. common ancestral stock
C. similar nervour system
D. common external characters.

Answer

To answer the question about Echinoderms, Hemichordates, and Chordates, we need to analyze the options provided and understand the evolutionary relationships among these groups.

### Step-by-Step Solution:

1. **Understanding the Groups**:
- **Echinoderms**: This group includes marine animals like starfish and sea urchins. They are characterized by their radial symmetry and a water vascular system.
- **Hemichordates**: These are marine organisms that share some features with both echinoderms and chordates. They possess a structure called a stomochord, which is somewhat similar to a notochord but is not a true notochord.
- **Chordates**: This group includes animals with a notochord at some stage of their development, such as mammals, birds, and reptiles.

2. **Evaluating the Options**:
- **A. Different Ancestral Stock**: This option suggests that these groups evolved from completely different ancestors, which is not supported by evolutionary biology.
- **B. Common Ancestral Stock**: This option implies that Echinoderms, Hemichordates, and Chordates share a common ancestor, which is supported by evolutionary evidence.
- **C. Similar Nervous System**: While there may be some similarities, the nervous systems of these groups are quite different, especially between echinoderms and chordates.
- **D. Common External Character**: These groups do not share a common external character, as their body plans and structures vary significantly.

3. **Conclusion**:
- Based on the evolutionary relationships and characteristics of these groups, the best answer is **B. Common Ancestral Stock**. Echinoderms are believed to be the ancestors of Hemichordates, and Hemichordates are considered to be the ancestors of Chordates, indicating a shared evolutionary lineage.

### Final Answer:
**B. Common Ancestral Stock**