Different species of fascinating little swimmers called rotifers each have a set number of cells, no more and no less than their kind calls for. Because the creatures still are close to being microscopic, and are not heavily built into thick layers of tissues, each cell obtains essential oxygen as the Amoeba does, and gets rid of carbon dioxide the same way, but together they have a problem with other accumulating wastes, so rotifers have a simple but elegant system of tubes that conduct unwanted nitrogen-containing fluids to the outside. Getting oxygen is of prime importance to every cell, because burning food, or cellular respiration, cannot take place without it. As animals increase in size, therefore in mass or volume, oxygen in the air or surrounding water is farther and farther away from the innermost cells. Something has to be done about that, and circulation of oxygen-carrying blood is the answer.
Blood also transports wastes away from cells.
But this means that somehow there must be adequate exchange of atmospheric gases. For an earthworm, a moist skin is all the breathing surface it needs, but woe betide any worm that begins to dry out, because essential gases can only pass through moist cellular membranes. Other segmented worms living in the ocean have more elaborate structures, real gills, that provide extensive surface area for the exchange of gases dissolved in the ocean. Insects gulp air by means of accordion-like abdomens, drawing in and exhaling gases through a system of fine tubes called tracheae. They largely forego blood vessels, because the air tubes penetrate every part of their bodies, an excellent system as long as the animals remain small.