To solve the question of how many animals from the given list exhibit bilateral symmetry, an open circulatory system, and a segmented body, we can follow these steps:
### Step-by-Step Solution:
1. **Identify the Characteristics**:
- **Bilateral Symmetry**: This means the body can be divided into two identical halves.
- **Open Circulatory System**: This is a type of circulatory system where blood is not always contained within blood vessels.
- **Segmented Body**: This means the body is divided into repeated segments.
2. **List the Animals**:
The animals provided are:
- Nereis
- Culex
- Limulus
- Bombyx
- Ascaris
- Ancylostoma
- Apis
- Locusta
- Taenia
- Pila
- Dentalium
- Loligo
- Asterias
- Pheretima
3. **Classify Each Animal**:
- **Nereis**: Bilateral symmetry, segmented body (Annelida).
- **Culex**: Bilateral symmetry, segmented body (Arthropoda).
- **Limulus**: Bilateral symmetry, segmented body (Arthropoda).
- **Bombyx**: Bilateral symmetry, segmented body (Arthropoda).
- **Ascaris**: Bilateral symmetry, but not segmented (Nematoda).
- **Ancylostoma**: Bilateral symmetry, but not segmented (Nematoda).
- **Apis**: Bilateral symmetry, segmented body (Arthropoda).
- **Locusta**: Bilateral symmetry, segmented body (Arthropoda).
- **Taenia**: Bilateral symmetry, but not segmented (Platyhelminthes).
- **Pila**: Bilateral symmetry, but not segmented (Mollusca).
- **Dentalium**: Bilateral symmetry, but not segmented (Mollusca).
- **Loligo**: Bilateral symmetry, but not segmented (Mollusca).
- **Asterias**: Radial symmetry in adults (Echinodermata).
- **Pheretima**: Bilateral symmetry, segmented body (Annelida).
4. **Count the Animals with All Three Characteristics**:
From the classification:
- Nereis
- Culex
- Limulus
- Bombyx
- Apis
- Locusta
- Pheretima
These animals exhibit bilateral symmetry, an open circulatory system, and a segmented body.
5. **Final Count**:
The total number of animals that meet all three criteria is **6** (Nereis, Culex, Limulus, Bombyx, Apis, Locusta, Pheretima).
### Conclusion:
The answer to the question is **6** animals.