name the phylum in which organ system lvel of body organisation appeared first time:

Answer

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks for the phylum where the organ system level of body organization first appeared. This indicates we need to identify a specific group of organisms that exhibit this level of complexity.

2. **Identifying the Phylum**: The phylum that is known for the first appearance of organ system level of organization is Aesculminthus.

3. **Common Name and Example**: Organisms in this phylum are commonly referred to as roundworms. A well-known example of an organism from this phylum is Ascaris.

4. **Features of Aesculminthus**:
- **Unisexuality**: Organisms in this phylum are unisexual, meaning they have distinct male and female individuals.
- **Organ System Level of Organization**: This phylum is characterized by having an organ system level of body organization.
- **Internal Fertilization**: Fertilization occurs inside the body of the organism.
- **Bilateral Symmetry**: The body of these organisms can be divided into two symmetrical halves.
- **Habitat**: They can be free-living or parasitic, living on other animals or humans.

5. **Conclusion**: Therefore, the answer to the question is the phylum Aesculminthus.

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