A distinct anus is absent in echinodderms

Options

A. Asteroidea
B. Ophiuroidea
C. Echinoidea
D. Holothuroidea.

Answer

### Step-by-Step Text Solution:

1. **Understanding Echinoderms**: Echinoderms are a group of marine animals that include sea stars, brittle stars, sea urchins, and sea cucumbers. They are characterized by their radial symmetry and a calcareous endoskeleton.

2. **Identifying the Groups**: The question provides four groups of echinoderms: Asteroidea (sea stars), Ophiuroidea (brittle stars), Echinoidea (sea urchins), and Holothuroidea (sea cucumbers).

3. **Analyzing the Digestive System**:
- **Asteroidea**: Sea stars have a complete digestive system with a distinct mouth and anus.
- **Echinoidea**: Sea urchins also possess a complete digestive system with separate openings for ingestion and excretion.
- **Holothuroidea**: Sea cucumbers have a well-defined mouth and anus as well.

4. **Focusing on Ophiuroidea**:
- Members of the Ophiuroidea class, commonly known as brittle stars, have a unique digestive system. They possess a "blind gut," meaning they have only one opening that serves both as a mouth and an anus. This characteristic indicates that they do not have a distinct anus.

5. **Conclusion**: Based on the analysis, the correct answer to the question "A distinct anus is absent in echinoderms" is **Ophiuroidea**. This is because brittle stars have a single opening that performs both functions of ingestion and excretion.

### Final Answer:
The correct option is **Ophiuroidea**.

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