select the incorrect statement about the phylum in which Obelia is placed.

Options

A. They are mostly marine and radially symmetrical animals .
B. The have presence of Cnidoblast .
C. Some of the members shows metagenesis .
D. They have a central gastro - vascular cavity with a single opening , mouth on osculum.

Answer

To solve the question regarding the incorrect statement about the phylum in which Obelia is placed, we will analyze each option provided.

### Step-by-Step Solution:

1. **Identify the Phylum of Obelia**:
- Obelia belongs to the phylum Cnidaria, which includes organisms like jellyfish, corals, and sea anemones.

2. **Review the Characteristics of Cnidarians**:
- Cnidarians are primarily marine animals, although some species can be found in freshwater.
- They exhibit radial symmetry.
- They possess specialized cells called cnidocytes, which contain nematocysts (not "nidoblasts" as mentioned in the transcript, but this is likely a misnomer).

3. **Evaluate Each Statement**:
- **Statement 1**: "They are mostly marine and their body is radially symmetrical."
- This statement is true. Cnidarians, including Obelia, are mostly marine and exhibit radial symmetry.

- **Statement 2**: "They have the presence of Nidoblast."
- This statement is misleading. The correct term is "cnidocytes," which contain nematocysts. However, if we consider the intent, it refers to the presence of stinging cells, which is true for Obelia.

- **Statement 3**: "Some of the members show metagenesis."
- This statement is true. Obelia exhibits metagenesis, which is the alternation between asexual and sexual reproduction.

- **Statement 4**: "They have a central gastrovascular cavity with a single opening mouth on osculum."
- This statement is incorrect. While Cnidarians do have a gastrovascular cavity, they do not have a separate anus; they have a single opening that serves both functions (mouth and anus), but the term "osculum" is typically used for the opening in sponges, not Cnidarians.

4. **Conclusion**:
- The incorrect statement about the phylum in which Obelia is placed is **Statement 4**.

### Final Answer:
The incorrect statement is: **"They have a central gastrovascular cavity with a single opening mouth on osculum."**

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