Mollusca embryonic development is best described as which pattern?

• A. Protostome
• B. Deuterostome
• C. Ectodermic
• D. Endodermic

Answer: (A)

Embryonic development in molluscs follows the protostome pattern. In protostomes, the first opening that forms during early development, the blastopore, becomes the mouth, and the anus forms later from another opening. Mollusks show spiral, determinate cleavage, meaning the fate of early cells is fixed early and development is mosaic. The body cavity (coelom) forms by schizocoely, where blocks of mesoderm split to create the coelom. These features place molluscs squarely in the protostome group rather than the deuterostome pattern. Ectodermic and endodermic describe tissue layers rather than a developmental mode, so they don’t describe how molluscan embryos develop.