ming above the bottom. While there are many extant body plans that are not primarily involved in locomotion (sponges are a good example), most phyla are mobile either as adults or in early growth stages and, clearly, the need to move has affected the body design of many phyla. The integration of skeletons and locomotion also was important in resultant body plans.

Thus we have at least one answer as to why the phyla have the shapes and designs that they do: these were responses to the challenge of attaining movement, first on sea bottoms. We see clear manifestation of this in body plans incorporating streamlining and in the evolution of anterior and posterior regions on the body. But there are other body plans that initially seem to be related to movement but that actually are not. One of these is segmentation, where the body shows a repetitive grouping of smaller units, such as that seen in annelid worms and arthropods. Segmentation does allow a certain kind of movement such as in worms. And, with appendages attached, segmentation leads to other body plans. But is locomotion the primary reason for segmentation? Chapter 3 will return to segmentation and present a new hypothesis for its origin and use.

OXYGEN, ENERGY, AND ANIMAL LIFE

Why do organisms bother with oxygen at all? Today there are huge areas of Earth, most underwater, that have little oxygen, so a body plan that helps an animal to live in low-oxygen environments would be very useful. But no animals use this kind of body plan. Why not? Aerobic respiration, the chemical reactions of metabolism in the presence of oxygen, yields up to 10 times more energy than does anaerobic respiration, a kind of respiration used by many bacteria. Fermentation is an example of how to get energy without oxygen. Complex life requires vast amounts of energy to meet its needs. For example, the formation of large molecules from smaller molecules in the synthesis of nucleic acids, lipids, and proteins involves a large input of free energy, just as the acquisition of energy requires the formation of specific cells or molecules, such as the chlorophyll molecule in plants—which takes energy. There is an old adage: it takes money to make money. This same idea can be analogized to energy.



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