What primary factor differentiates nematodes from other types of helminths?

Nematodes are classified as roundworms and are distinguished by their cylindrical body shape, which is a key characteristic that sets them apart from other types of helminths, such as flatworms (trematodes and cestodes) that tend to have a more flattened morphology. The cylindrical structure of nematodes allows for a distinct mode of movement and a different biological function compared to flatworms.

The fact that nematodes lack segmentation further emphasizes this distinction, as segmentation is typical in other groups of helminths. Furthermore, while size can vary among helminths and levels of infectiousness depend on specific species, these factors are not defining characteristics that differentiate nematodes from other helminths as consistently as their unique body shape. Thus, the cylindrical body shape is a primary and distinguishing factor for nematodes within the broader classification of helminths.