In which type of circulatory system do organisms like arthropods and mollusks transport blood?

Organisms such as arthropods and mollusks possess an open circulatory system, which is characterized by the absence of well-defined blood vessels. In this system, the blood (or hemolymph) is not confined to vessels but instead fills the body cavity, allowing it to directly bathe the organs and tissues. This design facilitates the effective transport of nutrients, gases, and waste products, although it may be less efficient than closed systems found in higher vertebrates.

In an open circulatory system, the heart pumps hemolymph into spaces called sinuses where it surrounds the organs. This arrangement provides a simpler and more adaptable form of circulation, suited to the metabolic needs of these organisms. In contrast, a closed circulatory system, which involves a network of blood vessels, is typical of annelids and vertebrates, where the blood remains contained and circulates more efficiently. Single and double circulatory systems refer to the pathways blood takes in vertebrates and are not applicable to the circulatory systems of arthropods and mollusks.