Echinoderms are

Options

A. Fresh water forms
B. Exclusively marine
C. Both freshwater and marine
D. None of the above

Answer

To answer the question "Echinoderms are:", we will analyze the characteristics of Echinoderms and determine the correct option from the given choices.

### Step-by-Step Solution:

1. **Understanding Echinoderms**:
- Echinoderms are a group of marine animals that include species such as starfish, sea urchins, and sea cucumbers. They are known for their unique body structure and characteristics.

2. **Key Characteristics**:
- **Calcareous Endoskeleton**: Echinoderms possess a skeleton made of calcium carbonate, which provides support and structure.
- **Spiny Body**: Many echinoderms have spines or tubercles on their body surface, giving them a distinctive appearance.
- **Marine Habitat**: Echinoderms are exclusively found in marine environments. They do not inhabit freshwater or terrestrial habitats.

3. **Organ System Level of Organization**:
- Echinoderms exhibit an organ system level of organization, meaning they have specialized systems for circulation, respiration, and digestion. Their digestive system is complete, with a mouth located on the ventral side and an anus on the dorsal side.

4. **Reproductive System**:
- Echinoderms have separate sexes, meaning that individuals are distinctly male or female. This is an important aspect of their biology.

5. **Examples of Echinoderms**:
- Common examples include:
- Starfish (Asteroidea)
- Sea urchins (Echinoidea)
- Sea cucumbers (Holothuroidea)
- Brittle stars (Ophiuroidea)

6. **Conclusion**:
- Based on the characteristics and examples discussed, it is clear that Echinoderms are exclusively marine organisms. Therefore, the correct answer to the question is option B: "exclusively marine."

### Final Answer:
Echinoderms are exclusively marine (Option B).

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