Which of the following has a complete digestive tract (mouth to anus)?

Options

A. roundworms only
B. flatworms only
C. Both (a) and (b)
D. neither (a) nor (b)

Answer

To determine which organism has a complete digestive tract (from mouth to anus), we need to analyze the options provided: roundworm, flatworm, both A and B, or neither A nor B.

### Step-by-Step Solution:

1. **Understanding Digestive Tracts**:
- A complete digestive tract has two openings: a mouth for ingestion and an anus for excretion. This allows for a more efficient digestion process as food moves in one direction.

2. **Analyzing Roundworms**:
- Roundworms belong to the phylum Nematoda. They are characterized by their elongated, cylindrical bodies.
- Roundworms have a complete digestive system. They possess a mouth where food enters and an anus where waste is expelled. This means they have a digestive tract that runs from mouth to anus.

3. **Analyzing Flatworms**:
- Flatworms belong to the phylum Platyhelminthes. They are typically flat and have a more primitive body structure.
- Flatworms do not have a complete digestive system; instead, they have an incomplete digestive system with only one opening that serves as both mouth and anus. This means they cannot process food in a linear fashion as roundworms do.

4. **Evaluating the Options**:
- Option A (Roundworm only): Correct, as roundworms have a complete digestive tract.
- Option B (Flatworm only): Incorrect, as flatworms do not have a complete digestive tract.
- Option C (Both A and B): Incorrect, since only roundworms have a complete digestive tract.
- Option D (Neither A nor B): Incorrect, as roundworms do have a complete digestive tract.

5. **Conclusion**:
- The correct answer is **A: Roundworm only**, as it is the only organism listed that has a complete digestive tract.