<span>a. Fish have a single central heart; earthworms have several hearts.
</span> Multi cellular organisms are made up of billions of cells each specialized to a unique role and these cells are organized into structural and functional units such as tissues and organs. For each cell and organ to function optimally, it requires oxygen and nutrients and needs carbon dioxide and metabolic waste taken away in a continuous, never ending process. <span>A mechanism is needed to accomplish the important function of transporting the relevant materials to and from the cell. The circulatory system fits this bill splendidly. It is a specialized organ system which allows blood and lymph to flow through the entire body carrying nutrients such as glucose, amino acids and electrolytes as well as oxygen to each cell, at the same time carting away carbon dioxide and all sorts of metabolic waste over to excretory organs to be expelled from the body. </span>
This could mean that the pink ball had a little bit more air in it than the blue ball. They could also have been dropped at the same time, but they were dropped on a hill, and the pink one landed first at the top of the hill, and the blue one landed second at the bottom of the hill.
Unlike a bar magnet<span>, however, </span>Earth's magnetic field<span> changes over time because it is generated by a geodynamo (in </span>Earth's<span> case, the motion of molten iron alloys in its outer core).</span>
