Among the following a complete digestive system seen in

Options

A. coelenterates
B. platyhelminths
C. ctenophores
D. pseudocoelomates

Answer

To determine which of the following animals have a complete digestive system, we need to understand what a complete digestive system entails. A complete digestive system is characterized by the presence of both a mouth and an anus, along with a coelom (body cavity) and an alimentary canal that allows for the complete digestion of food.

### Step-by-Step Solution:

1. **Define Complete Digestive System**:
- A complete digestive system includes a mouth and an anus, allowing for a one-way flow of food through the digestive tract. It also typically has a coelom, which is a fluid-filled body cavity.

2. **Identify Types of Digestive Systems**:
- **Incomplete Digestive System**: This type has only one opening (mouth) that serves for both ingestion and excretion. Examples include organisms from the phylum Cnidaria and Platyhelminthes, which have a single opening for their digestive processes.
- **Complete Digestive System**: This type has two openings (mouth and anus) and allows for more efficient digestion and absorption of nutrients. Examples include organisms from the phylum Nematoda (roundworms).

3. **Analyze the Options**:
- **Platyhelminthes**: These organisms have an incomplete digestive system with a single opening.
- **Cnidarians**: They also possess an incomplete digestive system with only one opening.
- **Nematodes (Pseudo coelomates)**: These organisms have a complete digestive system with distinct mouth and anus, along with a coelom.

4. **Conclusion**:
- Based on the analysis, the organism that has a complete digestive system is the nematodes (option D).

### Final Answer:
The correct answer is **option D (Nematodes)**, as they possess a complete digestive system with both a mouth and an anus.

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