Heart pumps blood in closed blood vessels in closed circulatory system, present in annelids and chordates.

Introduction:
The circulatory system is responsible for transporting blood, nutrients, oxygen, and hormones throughout the body. There are two main types of circulatory systems: open circulatory system and closed circulatory system. In this question, we are asked to identify the circulatory system where blood is pumped by the heart and circulated through a closed network of blood vessels.

Explanation:
The correct answer is option 'B', which is a closed circulatory system. Let's understand why it is the correct answer by exploring the characteristics and functions of a closed circulatory system.

Closed circulatory system:
A closed circulatory system is a type of circulatory system in which blood is pumped by the heart and is always circulated through a closed network of blood vessels. In this system, blood flows within a network of vessels and does not directly come in contact with the body cavities. Let's delve deeper into the characteristics of a closed circulatory system:

1. Heart: The heart is the central organ of the circulatory system. It pumps blood throughout the body to ensure proper circulation.

2. Blood vessels: The closed circulatory system consists of three types of blood vessels: arteries, veins, and capillaries.

- Arteries: Arteries carry oxygenated blood away from the heart to various parts of the body. They have thick, muscular walls to withstand the high pressure created by the pumping action of the heart.

- Veins: Veins carry deoxygenated blood back to the heart. They have thinner walls and contain valves to prevent the backward flow of blood.

- Capillaries: Capillaries are the smallest blood vessels that connect arteries and veins. They allow for the exchange of nutrients, oxygen, and waste products between the blood and surrounding tissues.

3. Circulation: In a closed circulatory system, blood circulates within a closed network of blood vessels. It follows a systematic pathway: the heart pumps oxygenated blood to the body tissues through arteries, which then branch into smaller arterioles and eventually into capillaries. In capillaries, exchange of nutrients and waste products occurs. Deoxygenated blood is then collected by venules and further flows into veins, which return it back to the heart for oxygenation.

4. Efficiency: The closed circulatory system is highly efficient in delivering oxygen and nutrients to the body's cells and removing waste products. The blood flow can be regulated, and nutrients and oxygen can be selectively delivered to specific tissues as required.

Conclusion:
The closed circulatory system is characterized by blood being pumped by the heart and circulated through a closed network of blood vessels. It ensures efficient transport of oxygen, nutrients, and waste products throughout the body. Understanding the different types of circulatory systems is vital to comprehend the functioning of various organisms and their adaptation to their environments.