The aschelminthes- are commonly called roundworms because

Options

A. they have a round body
B. their body is circular in cross-section
C. They have bilaterally-symmetric body
D. Their body have a round visceral hump

Explanation

### Step-by-Step Solution:

1. **Understanding Aschelminthes**: Aschelminthes, commonly referred to as roundworms, belong to a group of invertebrates that exhibit certain distinct characteristics.

2. **Identifying the Shape**: The primary reason for their common name "roundworms" is related to their body shape. When we look at the cross-section of these organisms, we find that their body appears circular.

3. **Cross-Section Analysis**: To understand this better, we can visualize the transverse section (cross-section) of Aschelminthes. When cut horizontally, the body shape is circular, which is a key feature that distinguishes them from other worm-like organisms.

4. **Evaluating Options**:
- **Option 1**: "They have a round body" - This is not entirely accurate as it does not specify the cross-section.
- **Option 2**: "Their body is circular in cross-section" - This is the correct answer, as it directly relates to the reason they are called roundworms.
- **Option 3**: "They have bilateral symmetry" - While they do exhibit bilateral symmetry, this characteristic does not explain why they are called roundworms.
- **Option 4**: "Their body has a round visceral hump" - This is incorrect as their visceral hump is not round.

5. **Conclusion**: The correct answer is that Aschelminthes are called roundworms because their body is circular in cross-section.