Annelids
Annelids are coelomate animals. The presense of coelom separaes the body wall from the internal organs and gives the advantage of independence of movement of internal structures such as the gut. However, this means there is a need for some form of transport systems in the body. A body system has evolved which connects gut and body wall. The earthworm, for example, has a well developed blood system in which blood circulates around the body through a system of blood vessels.

Two main blood vessels run the length of the body, one dorsal and one ventral. They are connected by the blood vessels in each segment. Near the front of the animal, five pairs of these connecting vessels are contractile and act as pumps. The main blood vessels can also pump blood. The blood contains haemoglobin dissolved in the plasma rather than being carried in red blood cells. Haemoglobin transports oxygen around the body.

Anthropods
Anthropods have an open blood system. The coelom is drastically reduced and its place taken by the haemocoel. This is a network of blood filled spaces called sinuses in which the internal organs are suspended. Gaseous exchange in most arthropods is achieved by the tracheal system and the blood vascular system is not used for transporting respiratory gases. Arthropods blood is colorless and contains no haemoglobin.

Vertebrates
The blood system of all vertebrates possesses a muscular heart, lying in a ventral position near the front of the animal. The heart is responsible for pumping blood rapidly to all parts of the body. Arteries carry blood away from the heart and veins carry blood from the body back to the heart. Oxygen is carried by haemoglobin in red blood cells. The lymphatic system will also be examined briefly where its functions overlap with those of the blood system.