Figure 5.18 a: In negative feedback control, the effector alters the state of the system in a direction opposite to its departure from some "desired" state. b: A negative feedback system keeps the state of the system within narrowly circumscribed limits.

ries with it costs in terms of energy and infrastructure. Judging from its widespread existence among animals, though, homeostasis seems to have been a good investment. Indeed, some paleontologists assert that the appearance of homeostasis among the first animals was a key feature in their origin and the success of their descendants.

Does this mean that external physiology, mediated by structures animals build, is a phenomenon limited to animals too primitive to have developed internal physiology? I have to admit that the answer might be yes, but the fact that I am writing this book means that

I think there is a substantial possibility that the answer is no, that in fact external physiology should be a widespread phenomenon. It is hard to see how it could not be, actually—even if there is a lot of physiology going on inside animals, the environmental gradients that drive external physiology will always be there, and judging from the examples I have cited so far, they should be capable of doing enormous quantities of physiological work: just contemplate the magnitude of the "engineering project" represented by a coral reef. Perhaps, then, external physiology is not a primitive condition that is swept aside by an inherently superior internal physiology. Perhaps it is always there, not always obviously visible, but operating continuously as long as the external sources of energy are there to drive it.

. . . Then the world seemed none too bad, And I myself a sterling lad; And down in lovely muck I've lain, Happy till I woke again.

chapter six

If you are disgusted by things that crawl around in mud, you're in good company. Karl Linné (Carolus Linnaeus), the Swedish naturalist who gave us the practice of classifying animal species, felt anything that crawled on or in the ground was a loathsome, vile creature. Aristotle also believed this and even had a theory for explaining why: animals that lived in the ground were lowly because they were furthest from the elements of air (pneuma) and heat (calor) that raise the "higher" animals (like us) to their exalted positions in nature. So our prejudice against things that crawl in mud runs long and deep, and it remains a prejudice strongly felt today. Worms are "icky." Things that crawl out of the ground are stock devices in horror movies. Our prejudice even extends to our fellow humans: many regard mining, ditch digging, grave digging, and the other, shall we say, plutonic professions as distasteful or, at the least, unrefined.

In the next two chapters, I wish to take a closer look at things that make their homes in the ground and the structures they build there. This chapter will be devoted to aquatic animals that live in so-called anoxic muds, the smelly black ooze that lurks just below the surface of mud flats and swamps. The next chapter will move onto land to examine the burrows constructed by earthworms. The functional significance of the burrows these creatures dig is best understood against the backdrop of some grand evolutionary events. In the case of animals living in marine sediments, there are two: the emergence, roughly 650 million years ago, of the Metazoa from the simpler organisms that preceded them;and the origin of photosynthesis among bacte ria, roughly 2.2-2.5 billion years ago. In the case of earthworms, the major evolutionary event was the origin of terrestriality, the ability to leave water and live on land.

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