True coelom and metameric segmentation in the body is first observed in which of the following phylum

Options

A. Platyhelminthes
B. Aschelminthes
C. Annelida
D. Arthropoda

Answer

To solve the question regarding which phylum first exhibits true coelom and metameric segmentation in the body, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Terms**:
- **True Coelom**: A true coelom is a fluid-filled body cavity that is completely lined by mesodermal tissue.
- **Metameric Segmentation**: This refers to the division of the body into repeated segments or metameres.

2. **Analyze the Options**:
- **Option 1: Platyhelminthes (Flatworms)**: These organisms are known as acoelomates, meaning they do not have a true coelom. They also do not exhibit metameric segmentation.
- **Option 2: Aschelminthes (Roundworms)**: This group includes nematodes, which also lack a true coelom (they have a pseudocoelom) and do not exhibit metameric segmentation.
- **Option 3: Annelida (Segmented Worms)**: Annelids possess a true coelom and exhibit metameric segmentation. Each segment of their body contains similar sets of organs and structures.
- **Option 4: Arthropoda (Insects, Arachnids, etc.)**: While arthropods do have a body cavity, it is a hemocoel (not a true coelom), and they do not exhibit true metameric segmentation in the same way as annelids.

3. **Identify the Correct Answer**:
- Based on the analysis, the phylum that first exhibits both true coelom and metameric segmentation is **Annelida**.

### Final Answer:
The correct answer is **Option 3: Annelida**.