Question

Annelida and Mollusca resemble many embryonic features because both have:

• A. Spiral cleavage and mesoderm formation
• B. Identical conspicuous segmentation in body, muscles and nervous system
• C. Meroblastic cleavage and ectoderm formation
• D. Special types of mouth parts

Answer 1

The correct option is A Spiral cleavage and mesoderm formation

Members of the molluscs, annelids, platyhelminths, and nemerteans have been shown to exhibit spiral cleavage.
Spiral cleavage is a holoblastic cleavage characterized by the blastomeres' arrangement or each upper-tier over the cell junctions of the next lower so that the blastomeres spiral around the pole to pole axis of the embryo.
Both molluscs and annelids probably evolved from free-living flatworms. Both flatworms and molluscs are triploblastic, bilaterally symmetric. But molluscs have developed a true coelom, an internal body cavity enclosed by a mesodermal membrane.
Therefore, the correct answer is option A.