The only one of the following phyla with no terrestrial forms is :

Options

A. Echinodermata
B. Arthropoda
C. Mollusca
D. Annelida

Explanation

To solve the question "The only one of the following phyla with no terrestrial forms is:", we will analyze each of the given options one by one.

### Step-by-Step Solution:

1. **Understanding the Question**: We need to identify which phylum among the given options does not have any terrestrial (land-dwelling) forms.

2. **Analyzing the Options**:
- **Option 1: Echinodermata**
- Echinoderms, such as starfish and sea urchins, are exclusively marine animals. They live in ocean environments and do not have any forms that live on land.
- Conclusion: Echinodermata has no terrestrial forms.

- **Option 2: Arthropoda**
- Arthropods include insects, arachnids, and crustaceans. They are found in marine, freshwater, and terrestrial ecosystems.
- Conclusion: Arthropoda has terrestrial forms.

- **Option 3: Mollusca**
- Mollusks, such as snails, clams, and octopuses, inhabit a variety of environments including land, freshwater, and marine habitats.
- Conclusion: Mollusca has terrestrial forms.

- **Option 4: Annelida**
- Annelids, which include earthworms and leeches, are found in a wide range of habitats, including terrestrial environments.
- Conclusion: Annelida has terrestrial forms.

3. **Final Conclusion**: After analyzing all the options, we find that the only phylum with no terrestrial forms is **Echinodermata**.

### Answer: Echinodermata

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