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 have external fertilization with indirect development, we will analyze each phylum one by one.

### Step 1: Analyze Porifera
- **Fertilization**: In Porifera (sponges), fertilization is internal. Male gametes enter the sponge body through water currents and fertilize the ovum internally.
- **Development**: Since fertilization is internal, we do not consider Porifera for external fertilization with indirect development.

### Step 2: Analyze Ctenophora
- **Fertilization**: In Ctenophora (comb jellies), fertilization is external. The eggs and sperm are released into the water column where fertilization occurs.
- **Development**: The development in Ctenophora is indirect, meaning they go through a larval stage before reaching adulthood.
- **Conclusion**: Ctenophora qualifies as having external fertilization with indirect development.

### Step 3: Analyze Echinodermata
- **Fertilization**: In Echinodermata (like starfish), fertilization is also external. The eggs and sperm are released into the water for fertilization.
- **Development**: However, Echinodermata exhibit retrogressive development. They start with an indirect larval stage but eventually develop into a direct adult form. Since the development is not strictly indirect throughout, we do not count Echinodermata as fitting the criteria.

### Step 4: Analyze Hemichordata
- **Fertilization**: In Hemichordata (like acorn worms), fertilization is external. The eggs and sperm are released into the water.
- **Development**: The development in Hemichordata is indirect, involving a larval stage before reaching adulthood.
- **Conclusion**: Hemichordata qualifies as having external fertilization with indirect development.

### Final Count
After analyzing all four phyla:
- Ctenophora: External fertilization with indirect development (1)
- Hemichordata: External fertilization with indirect development (2)
- Porifera: Internal fertilization (not counted)
- Echinodermata: External fertilization but retrogressive development (not counted)

Thus, the total number of phyla that have external fertilization with indirect development is **2** (Ctenophora and Hemichordata).

### Answer
**2**