What is true about all sponges without exception ?

Options

A. They are all marine
B. They have flagellated collar cells
C. They have a mixed skeleton consisting of spicules and spongin fibres
D. They reproduce only asexually by budding

Explanation

To answer the question "What is true about all sponges without exception?" we will analyze the options provided and determine which statement holds true for all sponges.

### Step-by-Step Solution:

1. **Understanding Sponges**: Sponges are simple, multicellular organisms belonging to the phylum Porifera. They are primarily aquatic and can be found in both marine and freshwater environments.

2. **Evaluating the Options**:
- **Option 1**: "They all are marine."
- This statement is **false** because there are freshwater sponges, such as **Spongilla**.

- **Option 2**: "They are flagellate collar cells."
- This statement is **true**. All sponges possess collar cells (choanocytes) that have flagella. These cells are essential for the sponge's feeding and water circulation.

- **Option 3**: "They have a mixed skeleton consisting of spicules and sponges."
- This statement is **false**. While sponges can have spicules made of silica or calcium carbonate, not all sponges have a mixed skeleton. The composition can vary significantly among different sponge species.

- **Option 4**: "They reproduce asexually."
- This statement is **false**. Sponges can reproduce both asexually (through budding) and sexually (through gametes).

3. **Conclusion**: The only statement that is true for all sponges without exception is that they have flagellate collar cells (choanocytes).

### Final Answer:
The correct statement about all sponges without exception is: **They are flagellate collar cells (choanocytes).**

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