Circulatory Systems
When we examine the systems by which blood reaches all the cells of animals, we find two general types, known as open and closed circulatory systems.
Open Circulatory Systems
The essential feature of the open circulatory system is that the blood moves through a body cavity―such as the abdominal cavity―and bathes the cells directly. The open circulatory system is particularly characteristic of insects and other arthropods, although it is also found in some other organisms.
In most insects, the blood does not play a major part in oxygen transport. Oxygen enters the animals body through a separate network of branching tubes that open into the atmosphere on the outside of the animal. (This type of respiratory system will be discussed in more detail in the next chapter.) Blood in an open circulatory system moves somewhat more slowly than it does in the average closed circulatory system. The slower system is adequate for insects because it does not have to supply the cells with oxygen.
Closed Circulatory Systems
In a closed circulatory system, the blood flows through a well-defined system of vessels with many branches. In the majority of closed systems, the blood is responsible for oxygen transport. To supply all the body cells with sufficient oxygen, the blood must move quickly through the blood vessels. A closed circulatory system must therefore have an efficient pumping mechanism, or heart, to set the blood in motion and keep it moving briskly through the body.
All vertebrates possess closed circulatory systems. Simple closed circulatory systems are also found in some invertebrates, including annelid worms. A good example of a simple closed circulatory system can be seen in the earthworm.