Digestive system is first completed in which phylum

Options

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

Explanation

To determine which phylum first exhibits a complete digestive system, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Digestive System**:
- A complete digestive system has two openings: a mouth and an anus. This allows for a more efficient processing of food compared to an incomplete digestive system, which has only one opening.

2. **Identify the Phyla in Question**:
- The options provided are Arthropoda, Annelida, Platyhelminthes, and Nematoda (also known as Excalaminthus).

3. **Examine Each Phylum**:
- **Platyhelminthes**: This phylum includes flatworms, which have an incomplete digestive system (one opening).
- **Annelida**: This phylum includes segmented worms, which do have a complete digestive system.
- **Arthropoda**: This phylum includes insects, arachnids, and crustaceans, which also have a complete digestive system.
- **Nematoda (Excalaminthus)**: This phylum includes roundworms, which are known for having a complete digestive system.

4. **Determine the First Phylum with a Complete Digestive System**:
- Among the listed phyla, Nematoda (roundworms) is recognized as the first phylum to exhibit a complete digestive system in the evolutionary sequence of animal development.

5. **Conclusion**:
- Therefore, the phylum in which the digestive system is first completed is **Nematoda (Excalaminthus)**.

### Final Answer:
The digestive system is first completed in the phylum **Nematoda (Excalaminthus)**.

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