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 determine which of the following is not a characteristic feature of nematodes, let's analyze the options based on the known characteristics of nematodes (roundworms).

### Step-by-Step Solution:

1. **Understanding Nematodes**: Nematodes, also known as roundworms, belong to the phylum Nematoda. They are characterized by their elongated, tubular shape.

2. **Identifying Key Features**:
- **Tubular Shape**: Nematodes have a cylindrical body structure.
- **Tissue Level Organization**: They exhibit tissue-level organization, meaning their body is organized into tissues.
- **Pseudocoelom**: Nematodes possess a pseudocoelom, which is a body cavity that is not completely lined by mesoderm.
- **Complete Alimentary Canal**: They have a complete digestive system, which includes a distinct mouth and anus.
- **Bilateral Symmetry**: Nematodes exhibit bilateral symmetry, meaning their body can be divided into two symmetrical halves.
- **Dioecious Nature**: Most nematodes are dioecious, meaning they have separate male and female sexes, although some can be hermaphroditic.
- **Habitat**: Nematodes can be found in various environments, including soil and as parasites in plants and animals.

3. **Analyzing the Options**:
- **Option 1: Sexes are separated** - This is true for most nematodes.
- **Option 2: Alimentary canal is complete** - This is also true as nematodes have a complete digestive system.
- **Option 3: Bilateral symmetry** - This is a characteristic feature of nematodes.
- **Option 4: They are always endoparasitic** - This statement is incorrect. While some nematodes are endoparasitic (living inside hosts), many nematodes live freely in soil and are not parasitic.

4. **Conclusion**: The statement "they are always endoparasitic" is not a characteristic feature of nematodes. Therefore, this is the correct answer to the question.

### Final Answer:
The option that is not a characteristic feature of nematodes is: **They are always endoparasitic.**