Earthworms are annelid, or segmented, worms that have ventured onto land. Earthworms are so familiar to us that it is surprising to learn what a rare thing a terrestrial annelid is. Of the 15,000 or so species in the phylum Annelida, about 10,000 are polychaete worms inhabiting marine environments, like the lugworms discussed in Chapter 5. Another 4,000 or so are polychaete and oligochaete worms and leeches inhabiting fresh water. Less than a thousand species of oligo-chaetes (the earthworms) and a few species of leeches have moved out of water and onto land. So, despite their almost banal familiarity, there actually is something rare and wonderful about an earthworm.
But are they really earth-worms, that is, truly terrestrial annelids? Well, it depends upon how you look at it. One could argue, for example, that anything that does not live in water is terrestrial, and by that criterion earthworms are unequivocally terrestrial. Just as walking catfish are terrestrial, and diving spiders and whales are aquatic . . . There is another way to make the distinction, however, and that is to ask whether their physiology equips them for life on land. Now, it might just be my prejudice as a physiologist, but I think this definition offers a stronger criterion than a judgement based simply on where an animal lives. I believe this because of what physiology is: doing work to maintain internal orderliness in a physical environment that is always pushing an organism toward disorder. Thus, the nature of the physical environment an animal is meant to inhabit should leave its mark vividly on that animal's physiology. Take, for example, the problems animals face in keeping the proper balance of water and salts within their bodies. It is almost absurdly obvious that animals living in water and animals that live on land will deal with this problem differently. It will be no surprise to find that the organs of water and salt balance of, say, a crayfish differ significantly from those of, say, a cockroach.
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