The type of circulatory system found in arthropods and some molluscs is

Analyze the question: It asks which circulatory system is typical of arthropods (insects, crabs, spiders, etc.) and some molluscs (snails, clams).

Explain the concept:

• Open circulatory system: The heart pumps blood (called haemolymph) into body cavities called the haemocoel. The blood bathes the tissues and organs directly, then returns to the heart through small openings called ostia. There are few or no true blood vessels.
• Closed circulatory system: Blood stays inside vessels (arteries, veins, capillaries) all the time. This is found in vertebrates, earthworms, and cephalopod molluscs (like octopus and squid).
• Single circulation: Blood passes through the heart once per complete circuit (e.g., fish).
• Double circulation: Blood passes through the heart twice per circuit (e.g., mammals, birds).

Note that "single" and "double" describe the path of blood through the heart, while "open" and "closed" describe whether blood is enclosed in vessels.

Step-by-step solution:

1. Arthropods (e.g., grasshopper, cockroach) have a tubular heart and blood that flows freely in the haemocoel, so they have an open system.
2. Most molluscs (snails, mussels) also have an open system. Only cephalopods are an exception with a closed system, and that is why the question says "some molluscs."
3. Single and double circulation do not apply here, since they are classifications for closed systems in vertebrates.
4. Therefore, the answer is open circulatory system.

Answer: A. open circulatory system

Exam tip: Remember "Arthropods = A: Open" or think "Insects have no real veins, so their blood is open to the body." Also remember the exception: octopus and squid are molluscs with closed circulation, a favourite JAMB trick.