The body cavity that is not completely lined by mesoderm-derived tissue is called what?

• A. Coelom
• B. Endoskeleton
• C. Exoskeleton
• D. Pseudocoelom

Answer: (D)

In this concept, what defines a body cavity is whether it is completely lined by mesoderm-derived tissue. A true coelom is a body cavity that the mesoderm fully surrounds, forming a complete lining. When the cavity is not entirely lined by mesoderm—sitting between the endoderm and the mesoderm—the structure is called a pseudocoelom. The term “pseudo” means false, reflecting that the lining isn’t complete. This is seen in organisms like nematodes, where the gut is lined by endoderm, the outer body wall by mesoderm, and the cavity in between is the pseudocoelom. The other terms refer to skeletal supports (endoskeleton, exoskeleton) or to the true coelom concept, not to the specific cavity that isn’t fully mesoderm-lined.