The body of the ______ is circular in cross-section, hence, the name roundworms. They may be free living, aquatic and terrestrial or parasitic in plants and animals.

The correct answer is Aschelminthes. Key Points

• ​Roundworms are the popular name for Aschelminthes.
• They are distinguished by having a pseudocoelom.
• They either live freely or are parasites.
• They can be found on plants, animals, humans, and even in freshwater and soil.
• In the human intestine, Ascaris is discovered to exist as an endoparasite.
• The body is triploblastic and bilaterally symmetric.
• The body is long, slender, and not segmented; it tapers out at the end.
• There is no evidence of metameric segmentation.
• Males are often smaller than females since they are dioecious.
• Their bodies are organised on an organ-system level.​

Additional Information

• Platyhelminthes:
  • The flatworms, also known as platyhelminths, are a phylum of soft-bodied, relatively uncomplicated bilaterian invertebrates.
  • They are bilaterally symmetrical, triploblastic, and acoelomate.
  • They could be parasites or free-living beings.
• Ctenophores:
  • Sea waters are home to a phylum of marine invertebrates known as comb jellies, or Ctenophora.
  • They have two embryonic layers, the ectoderm and endoderm, making them diploblastic.
  • They lack a bodily cavity since they are acoelomates.
  • Ctenophores are symmetrical in all directions.
• Mollusca:
  • After Arthropoda, Mollusca is the second-largest phylum of invertebrate animals.
  • Organ system level organization can be seen in them.
  • A cavity exists in their body.
  • The skull, visceral mass, muscular foot, and mantle are the different parts of the body.
  • Tentacles and complex eyes make up the head.
  • The body has a calcareous shell covering it.