One example of animals having a single opening to the outside that serves both as mouth as well as anus is

Options

A. Octopus
B. Asterias
C. Ascidia
D. Fasciola

Answer

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks for an example of an animal that has a single opening that functions as both the mouth and the anus. This is a characteristic of certain organisms, particularly in the phylum Cnidaria and some other non-chordates.

2. **Analyzing the Options**:
- **Option 1: Octopus**: The octopus is a mollusk and has a distinct mouth and anus. Therefore, it does not meet the criteria of having a single opening. This option is incorrect.
- **Option 2: Asterias (Starfish)**: Asterias, or starfish, belong to the phylum Echinodermata. They also have separate openings for the mouth and anus, thus this option is incorrect as well.
- **Option 3: Aceria**: Aceria is a genus that belongs to the phylum Chordata, which also has separate mouth and anus openings. Therefore, this option is incorrect.
- **Option 4: Fasciola**: Fasciola, commonly known as liver fluke, belongs to the phylum Platyhelminthes (flatworms) and has a single opening that serves as both mouth and anus.

3. **Conclusion**: Based on the analysis of the options, the correct answer is **Fasciola**. It is an example of an animal that has a single opening serving both functions.

### Final Answer:
**Fasciola**

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