Which of the following not a characteristic feature of nematodes ?

Options

A. Sexes are separated
B. Alimentary canal is complete
C. Bilateral symmetary
D. They are always endoparasitic

Answer

To solve the question "Which of the following is not a characteristic feature of nematodes?", we will analyze each of the options provided.

### Step-by-Step Solution:

1. **Understand the Characteristics of Nematodes**: Nematodes, also known as roundworms, have several key characteristics that define their biology and morphology.

2. **Evaluate Option 1: "Sexes are separate"**:
- Nematodes are dioecious, meaning they have separate male and female individuals. This statement is true and is a characteristic feature of nematodes.

3. **Evaluate Option 2: "Elementary canal is complete"**:
- Nematodes possess a complete digestive system, which includes a mouth at the anterior end and an anus at the posterior end. This statement is also true and is a characteristic feature of nematodes.

4. **Evaluate Option 3: "Bilateral symmetry"**:
- Nematodes exhibit bilateral symmetry, where the body can be divided into two identical halves along one plane. This statement is true and is a characteristic feature of nematodes.

5. **Evaluate Option 4: "They are always endoparasites"**:
- This statement is false. While some nematodes are endoparasites (living inside a host), many are free-living in soil or aquatic environments, and not all are parasitic. Therefore, this is not a characteristic feature of nematodes.

### Conclusion:
The option that is **not a characteristic feature of nematodes** is **Option 4: "They are always endoparasites."**