How many from the following phylum has external fertilisation with indirect development ?
{Porifera, Ctenophora , Echinodermata, Hemichordata }

Options

A. Four
B. Three
C. Two
D. One

Answer

To solve the question of how many of the listed phyla exhibit external fertilization with indirect development, we will analyze each phylum one by one.

### Step-by-Step Solution:

1. **Understanding External Fertilization and Indirect Development**:
- **External Fertilization**: This occurs when the male and female gametes (sperm and egg) unite outside the body of the organism.
- **Indirect Development**: This involves a larval stage where the organism does not resemble the adult form immediately after fertilization. Instead, it goes through several developmental stages before reaching the adult form.

2. **Analyzing Phylum Porifera**:
- In Porifera (sponges), fertilization is typically **internal**. Although they do have a larval stage (which is indirect development), they do not meet the criteria for external fertilization.
- **Conclusion**: Porifera does not qualify.

3. **Analyzing Phylum Ctenophora**:
- Ctenophores (comb jellies) exhibit **external fertilization**. They also undergo **indirect development**, where the fertilized egg develops into a larval stage before becoming an adult.
- **Conclusion**: Ctenophora qualifies.

4. **Analyzing Phylum Echinodermata**:
- Echinoderms (like starfish and sea urchins) also exhibit **external fertilization**. Their development is indirect, involving free-swimming larval stages (e.g., bipinnaria and brachiolaria).
- **Conclusion**: Echinodermata qualifies.

5. **Analyzing Phylum Hemichordata**:
- Hemichordates (like acorn worms) have **external fertilization** and also undergo **indirect development**. They have a larval stage that is different from the adult form.
- **Conclusion**: Hemichordata qualifies.

6. **Final Count**:
- From the analysis, we found that Ctenophora, Echinodermata, and Hemichordata exhibit external fertilization with indirect development.
- Therefore, the total number of phyla that meet the criteria is **3**.

### Final Answer:
The answer is **3**.

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