Which of the following is not a characteristic feature of cnidarians?

Options

A. They exhibit tissue level of organisation
B. They may be aquatic or terrestrial
C. They may be sessile or free-swimming
D. they are diploblastic animals

Explanation

To determine which of the following is not a characteristic feature of cnidarians, we will analyze the provided options based on the known characteristics of the phylum Cnidaria.

### Step-by-Step Solution:

1. **Understand the Characteristics of Cnidarians**:
- Cnidarians are primarily aquatic organisms found in marine and freshwater environments.
- They exhibit a tissue level of organization, which means they have specialized cells grouped together to perform specific functions.
- Cnidarians are diploblastic, meaning they have two germ layers (ectoderm and endoderm).
- They can exist in two forms: sessile (polyp) and free-swimming (medusa).

2. **Analyze the Options**:
- **Option 1**: They exhibit tissue level of organization.
- This is a characteristic feature of cnidarians. Therefore, this option is correct.
- **Option 2**: They may be aquatic or terrestrial.
- Cnidarians are exclusively aquatic; they are not found in terrestrial environments. Therefore, this statement is incorrect and is the answer we are looking for.
- **Option 3**: They may be sessile or free swimming.
- This is true as cnidarians can exist in both forms (polyp and medusa). Hence, this option is correct.
- **Option 4**: They are diploblastic.
- This is also true as cnidarians have two germ layers. Therefore, this option is correct.

3. **Conclusion**:
- The only option that is not a characteristic feature of cnidarians is **Option 2**: "They may be aquatic or terrestrial."

### Final Answer:
**Option 2** is not a characteristic feature of cnidarians.

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