In museum study, an earthworm changes body shape by using body-wall muscles against incompressible coelomic fluid. The specimen or feature is best linked with:

The coelomic fluid of annelids acts as a hydrostatic skeleton. Circular and longitudinal muscles work against it to produce burrowing and crawling movements. This MCQ belongs to the Annelida topic in Animal Diversity-I and is written to test concept understanding rather than memorized wording.