True coelom appeared first in the course of evolution in

Options

A. Echinodermata
B. Annelida
C. Chordata
D. Aschelminthes

Answer

To determine where true coelom first appeared in the course of evolution, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Concept of Coelom**:
- A coelom is a fluid-filled body cavity that is completely lined by mesoderm. It is important for the organization of the body and the development of complex organs.

2. **Differentiate Between Types of Coelom**:
- True coelom: A body cavity that is fully lined by mesoderm.
- Pseudocoelom: A body cavity that is not completely lined by mesoderm (found in organisms like roundworms).

3. **Identify the Phyla**:
- We need to look at various phyla to see where true coelom first appeared. The options given are:
- A. Kainu Dormita
- B. Anilida
- C. Cordata
- D. Helmets (Nematoda)

4. **Analyze Each Option**:
- **Kainu Dormita**: While it has a true coelom, it does not represent the first appearance in evolution.
- **Anilida**: This phylum includes segmented worms like earthworms, which are known to have true coelom and are the first to exhibit this feature in evolution.
- **Cordata**: Members of this phylum also possess a true coelom, but they evolved after Anilida.
- **Helmets (Nematoda)**: These organisms have a pseudocoelom, not a true coelom.

5. **Conclusion**:
- Based on the analysis, the first appearance of true coelom in evolution is in the phylum **Anilida**.

### Final Answer:
The correct answer is **B. Anilida**.