Flatworms, roundworms, and thorny-headed worms all belong to which group of parasites?

• A. Ciliates
• B. Apicomplexans
• C. Helminths
• D. Arthropoda

Answer: (C)

Flatworms, roundworms, and thorny-headed worms are all parasitic worms, categorized together as helminths. Helminths are multicellular parasites that include various worm groups—flatworms (trematodes and cestodes), roundworms (nematodes), and thorny-headed worms (acanthocephalans). They share the common lifestyle of living as parasites in hosts, even though their bodies and life cycles can differ quite a bit. This grouping is distinct from ciliates and apicomplexans, which are protozoans and are single-celled organisms, not worms; and from arthropods, which are invertebrates like insects and ticks with segmented bodies and exoskeletons. So, the best overall category for these worm parasites is helminths.