question: What in roundworms is a partial body cavity filled with fluid?
distractor3: spicule
distractor1: abdomen
distractor2: cocklebur
correct_answer: pseudocoelom
support: Ancestors of roundworms also evolved a pseudocoelom. This is a partial body cavity that is filled with fluid. It allows room for internal organs to develop. The fluid also cushions the internal organs. The pressure of the fluid within the cavity provides stiffness. It gives the body internal support, forming a hydrostatic skeleton. It explains why roundworms are round and flatworms are flat. Later, a true coelom evolved. This is a fluid-filled body cavity, completely enclosed by mesoderm. It lies between the digestive cavity and body wall (see Figure below ). Invertebrates with a true coelom include mollusks and annelids.