In enterocoelomates the coelom forms through

• A. Out-pocketing of embryonic gut into mesoderm space
• B. Separation of mesoderm cells
• C. Splitting of the ectoderm
• D. Direct mesodermal differentiation

Answer: (A)

Enterocoely is when the coelom forms as outpocketings of the primitive gut (the archenteron) that push into the surrounding mesoderm and then pinch off to become the coelomic cavities. This pattern, seen in enterocoelomates such as echinoderms and chordates, means the gut forms first and the mesoderm forms coelomic sacs by these gut-derived outgrowths. The other ideas describe different routes—splitting of solid mesoderm (schizocoely), splitting of ectoderm, or direct mesodermal differentiation—which are not how enterocoelomates develop their coelom.