True coelom evolved first in phylum

Options

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

Answer

To determine which phylum first evolved true coelom, we need to analyze the characteristics of the given phyla: Annelida, Platyhelminthes, Aschelminthes, and Arthropoda.

### Step-by-Step Solution:

1. **Understanding Coelom**:
- A true coelom is a body cavity that is completely lined by mesoderm. Animals with a true coelom are called coelomates.

2. **Identifying the Phyla**:
- **Platyhelminthes**: These are flatworms and are classified as acoelomates, meaning they do not have a body cavity.
- **Aschelminthes**: This group includes pseudocoelomates, which have a body cavity that is not entirely lined by mesoderm (it has mesoderm only on one side).
- **Annelida**: This phylum includes segmented worms and is characterized by having a true coelom, which is completely lined by mesoderm.
- **Arthropoda**: This phylum includes insects, arachnids, and crustaceans, which also possess a true coelom.

3. **Comparing Annelida and Arthropoda**:
- Both Annelida and Arthropoda are coelomates, but we need to determine which one evolved the true coelom first.
- Annelida is considered to be more primitive compared to Arthropoda, and it is generally accepted that true coelom evolved first in Annelida.

4. **Conclusion**:
- Based on the analysis, the phylum in which true coelom first evolved is **Annelida**.

### Final Answer:
True coelom evolved first in the phylum **Annelida**.