Which characteristic differentiates trematodes from nematodes?

The distinguishing characteristic that differentiates trematodes from nematodes is that trematodes have suckers. Trematodes, commonly known as flukes, are flatworms that possess specialized structures called suckers, which they use to adhere to their hosts and facilitate feeding. This adaptation is essential for their parasitic lifestyle, allowing them to anchor themselves within the tissues of the host, such as the liver or intestines.

In contrast, nematodes, also known as roundworms, do not have suckers. Instead, they typically have a more cylindrical shape and may have other adaptations for their own parasitic or free-living lifestyles, such as a tough outer cuticle. This fundamental difference in structure highlights the variation between these two groups of worms in terms of morphology and ecology.

The other options do not accurately represent the differences between trematodes and nematodes; for instance, trematodes are not segmented, which is a characteristic of certain other types of worms, and they do not possess a complete digestive system like nematodes do. Instead, trematodes have an incomplete digestive system with a mouth but no true anus. Additionally, while some trematodes can be quite long, the size comparison can vary widely