A learner makes a note: older zoology texts group rotifers, nematodes, and related forms together because of a shared body cavity type. The missing scientific idea is:

Aschelminthes is an older grouping often used for pseudocoelomate animals with a complete digestive tract. Modern classifications may separate these phyla based on phylogeny. This MCQ belongs to the Aschelminthes topic in Animal Diversity-I and is written to test concept understanding rather than memorized wording.