What group of parasites is generally flat dorsoventrally, has suckers, and has larval forms that hatch in water?

• A. Cestodes
• B. Trematodes
• C. Nematodes
• D. Acanthocephalids

Answer: (B)

Trematodes, or flukes, are classically described as flat, leaflike worms that attach to hosts with suckers—an oral sucker and a ventral sucker (acetabulum) for gripping tissues. A key feature of their life cycles is that the early larval stage develops in water: eggs released into the environment hatch into free-swimming miracidia in water, which then infect an intermediate host (usually a snail) before producing other larval forms that continue the cycle in aquatic settings. This combination of dorsoventral flattening, presence of suckers for attachment, and eggs/larvae that hatch in water is distinctive for trematodes. Other groups don’t fit this same pattern: nematodes are cylindrical, acanthocephalids have a different body plan with a retractable proboscis, and cestodes are flat as well but lack true suckers and have different larval life-cycle patterns that don’t involve free-living aquatic hatchings in the same way.