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 statement step by step.

### Step 1: Identify the Phylum
Obelia belongs to the phylum Cnidaria. This phylum includes various marine organisms such as jellyfish, corals, and sea anemones.

### Step 2: Evaluate the Statements
1. **Statement 1**: "They are mostly marine and radially symmetrical animals."
- **Analysis**: This statement is correct. Cnidarians are predominantly marine and exhibit radial symmetry.

2. **Statement 2**: "They have a presence of nidoblast cells."
- **Analysis**: This statement is also correct. Cnidarians are characterized by the presence of specialized cells called cnidocytes (or nidoblasts), which contain stinging structures known as nematocysts.

3. **Statement 3**: "Some of the members show metagenesis."
- **Analysis**: This statement is correct as well. Metagenesis refers to the alternation of generations, which is observed in some Cnidarians, such as Obelia, where both asexual and sexual phases occur.

4. **Statement 4**: "They have a central gastrovascular cavity with a single opening mouth or osculum."
- **Analysis**: This statement is incorrect. Cnidarians have a gastrovascular cavity that serves as both a mouth and an anus, meaning they have a single opening that functions for both ingestion and egestion. However, the term "osculum" is typically used to refer to the opening in sponges, not Cnidarians.

### Conclusion
Based on the analysis, the incorrect statement is **Statement 4**.

### Final Answer
The incorrect statement about the phylum in which Obelia is placed is: "They have a central gastrovascular cavity with a single opening mouth or osculum."

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