Echinoderms are

Options

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

Answer

**Step-by-Step Solution:**

1. **Identify the Phylum**: Echinoderms belong to the phylum Echinodermata, which is a group of invertebrate animals.

2. **Characteristics of Echinoderms**:
- Echinoderms have a calcareous endoskeleton, which means their internal skeleton is made of calcium carbonate.
- They possess a spiny body texture due to the presence of ossicles (small calcareous plates).

3. **Habitat**: Echinoderms are exclusively marine organisms, meaning they are found only in ocean environments and not in freshwater.

4. **Organizational Level**: They exhibit organ system level of organization, which is a higher level of complexity than just cellular or tissue organization.

5. **Symmetry**:
- The larval stage of echinoderms shows bilateral symmetry, which means that the left and right sides of the larva are mirror images.
- In contrast, adult echinoderms exhibit radial symmetry, where body parts are arranged around a central axis.

6. **Tissue Layers**: Echinoderms are triploblastic, meaning they have three germ layers: ectoderm, mesoderm, and endoderm.

7. **Coelom**: They possess a true coelom, which is a fluid-filled body cavity completely lined by mesodermal tissue, categorizing them as coelomates.

8. **Conclusion**: Based on the characteristics and habitat of echinoderms, the correct statement is that they are exclusively marine organisms.

**Final Answer**: Echinoderms are exclusively marine organisms.

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