Just exactly what is a body? Many of the organisms of the world are composed of only a single cell; does that count as a body? In the animal kingdom, the levels of complexity vary from relatively simple hollow sacks to very complex structures with many internal substructures. What qualifies as a body?

Living things have to perform a variety of tasks in order to survive. For instance, any organism has to keep a balance between the water concentration inside itself and the outside world. If that outside world is salt water, that's relatively simple to do, since the water concentration inside a cell is comparable to ocean water. However, if that outside world is fresh water or air, the problem is much more difficult.

Another kind of problem living things have is gas exchange. An aerobic organism must get oxygen into its cells and get carbon dioxide out. Sometimes these two needs — water balance and gas exchange — are in opposition to each other. Consider a human — a human lives in the air, and so has a constant problem with water loss. That's why humans have waterproof skin — to try to keep water in. But gas exchange has to occur across a moist layer of tissue — it can't occur through waterproof skin. And every living cell in our bodies must have a constant supply of oxygen, and be able constantly get rid of carbon dioxide. That's why we have lungs — they contain many, many thin walled sacs which allow the gas exchange necessary to supply all of the cells in our bodies.

In the same way, our intestines — part of our digestive organ system — provide a vast surface area for the absorption of the nutrients that we extract from our food. Again, we can't do this through our skin, because it has to be immune to the movement of fluids in order to retain the water necessary for our survival.

So the complexities of our bodies are necessary for the performance of those necessary tasks of survival.

However, many, many organisms perform all of those tasks without all of the structural complexities of our bodies. As an extreme example, consider a bacterium. Bacteria perform all of the tasks our bodies perform, but they are composed of a single cell which is much simpler than even the simplest cells in our bodies.

So just what is a body? Like so many words, this is one which is easier to use in a general sense, rather than a specific one. Since there is pretty much a continuum of complexity from those very simple bacteria to the most complex of animals, we really have to accept their single cells as their bodies.

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