Question

Annelida have closed circulatory system, while Anthropoda and Mollusca have open blood circulatory system. Although, closed circulatory system is considered more advanced than open blood circulatory system, then why is Anthropoda and Mollusca grouped after Annelida?

Answer 1

Arthropods and Molluscus have more complex body design as compared to Annelids. Their body is distinguished into head, thorax and abdomen (incase of arthropods) or head, muscular foot and visceral hump ( incase of molluscus). They posses more advanced organs or organ systems like gills, sensory organs, excretory organs. Based on these they are considered more advanced organisms than annelids.