Concept:Arthropods and most molluscs have an open circulatory system where blood is not always confined to blood vessels.
Explanation:The heart pumps blood into large spaces called sinuses or the hemocoel.
This blood, known as hemolymph, directly bathes the organs and tissues.
Hemolymph then returns to the heart through small openings called ostia.
In a closed circulatory system, blood remains enclosed in vessels, as seen in mammals and birds.
Single and double circulatory systems describe how many times blood passes through the heart per complete circuit.
Arthropods and the molluscs in question do not use these vessel-based systems.
Therefore, the correct answer is the open circulatory system.
Answer:A. open circulatory system