Question

Which among the following have an open circulatory system?
i) Arthropoda
ii) Mollusca
iii) Annelida
iv) Coelenterata

A. i) and ii)
B. iii) and iv)
C. i) and iii)
D. ii) and iv)

Answer 1

Hint: A circulatory system where blood (hemolymph), rather than being sealed tight in arteries and veins, bathes the organs and tissues and may be directly open to the environment at places such as the digestive tract is called an open circulatory system. There is no distinction between blood, lymph, and interstitial fluid here.

Complete Answer:
The open circulatory system is mostly seen in invertebrates and they typically have a relatively high volume of hemolymph and low blood pressure
i) In Arthropods, an Open Circulatory system is found and the blood (hemolymph) is pumped into a cavity called a hemocoel where it surrounds the organs and then returns to the heart through openings called Ostia.
ii) In Molluscs, most of the circulatory systems are open but in some species, a closed circulatory system is also found.
iii) Like humans, the Annelids have a closed circulatory system as well which means that the blood is confined to blood vessels.
iv) Coelenterates lack a circulatory system because it’s not needed, all cells are in direct contact with each other and exchange occurs through them.
Therefore, among these four options, i) Arthropods and ii) Molluscs have an open circulatory system.

Option A: i) and ii) is the correct option.

Additional information: In a Closed Circulatory System, the blood is closed all the time and is distinct from the interstitial fluids, and flows through vessels of different sizes and wall thicknesses. It is mostly found in vertebrates and some invertebrates.

Note: Organisms with an open circulatory system have a hemocoel which is a central body cavity where both digestive and circulatory functions are performed. These systems are less efficient than closed circulatory systems and cannot move oxygen efficiently enough to power large bodies.