Metamerism first evolved in the phylum

Options

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

Answer

To determine in which phylum metamerism first evolved, we need to analyze the characteristics of the given options.

1. **Understanding Metamerism**: Metamerism refers to the segmentation of the body into repeated units or segments. This is a significant evolutionary trait that allows for greater complexity in body structure and function.

2. **Evaluating the Options**:
- **Aschelminthes**: This phylum includes organisms that are primarily unsegmented and parasitic. Since they lack segmentation, they cannot be the first to evolve metamerism.
- **Arthropoda**: While arthropods are segmented and are the first terrestrial organisms, they are not the first to exhibit metamerism. They evolved segmentation later in their evolutionary history.
- **Annelida**: This phylum is known for its segmented body plan. Annelids, such as earthworms, are characterized by their metameric segmentation, making them the first organisms to develop this trait.
- **Lophotrochozoa**: This group includes various organisms, but flatworms (which are part of this group) are also unsegmented, similar to Aschelminthes.

3. **Conclusion**: Based on the analysis, the phylum in which metamerism first evolved is **Annelida**.

**Final Answer**: Metamerism first evolved in the phylum **Annelida**.

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