In coelomates, what surrounds the coelom on all sides?

• A. The mesoderm surrounds the coelom on all sides
• B. The endoderm surrounds the coelom on all sides
• C. The ectoderm surrounds the coelom on all sides
• D. The notochord surrounds the coelom on all sides

Answer: (A)

In coelomates, the coelomic cavity is formed and enclosed entirely by mesodermal tissue. This means the coelom is surrounded on all sides by mesoderm, which also gives rise to the body wall and the membranes lining the cavity (like the peritoneum). The other germ layers have different roles: endoderm lines the gut lumen inside the coelom, ectoderm forms the outer covering and nervous system, and the notochord (when present) is a midline mesodermal structure that doesn’t encircle the entire coelom. Hence, mesoderm being all around the coelom best explains why this choice is correct.