Heart

A heart is a means to circulate blood through the body of an animal. Among the lower species such as insects, arachnids, and others, the heart may simply be an expanded area in a blood vessel and may occur a number of times. The earthworm, for example, has 10 such "hearts." These areas contract rhythmically to force the blood through the aorta, or blood vessel.

Not until the evolution of the higher vertebrates does the heart achieve its ultimate form, that of a chambered organ with differentiated purposes. Even the lower chordates, such as amphioxus, possess hearts not more advanced than those in the earthworm. It is simply a pulsating blood vessel that moves blood through the body.