Animals with blood flowing in blood vessels are said to have a closed circulatory system.
In a closed circulatory system blood is completely enclosed within:
What do you call a circulatory system where blood flows through tubes called blood vessels?
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?