Which of the following types of organisms have an open circulatory system?

Mollusca

The correct answer is Option (2) – Mollusca

Mollusca is the type of organism that has an open circulatory system.

Explanation:

The open circulatory system is prevalent in organisms belonging to the phyla Mollusca and Arthropoda. In this system, blood circulates freely through cavities because there are no vessels to guide its flow. Unlike closed circulatory systems seen in humans and annelids, organisms with an open circulatory system lack a true heart or blood vessels. The blood in these organisms is a combination of blood and an interstitial fluid known as hemolymph .

In nematodes there is no circulatory system, they transfer the oxygen and other necessary nutrients by diffusion and birds have a closed circulatory system where the blood circulates within vessels.