Segmentation in the body is first observed in which of the following:
(a) Platyhelminthes (b) Aschelminthes (c) Annelida (d) Arthropoda

Explanation

To determine where segmentation in the body is first observed, we can analyze the given options step by step:

1. **Understanding Segmentation**: Segmentation refers to the division of the body into repetitive segments or metameres. This is an important characteristic in the classification of animals.

2. **Evaluating the Options**:
- **(a) Platyhelminthes**: This group includes flatworms, which do not exhibit segmentation. Their bodies are not divided into segments.
- **(b) Aschelminthes**: This group, also known as pseudocoelomates, includes roundworms. They also do not show true segmentation in their body structure.
- **(c) Annelida**: Annelids, or segmented worms, are characterized by metameric segmentation. Their bodies are divided into segments both externally and internally, with each segment known as a metamer.
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