Small, yet groundbreaking

I am not proud that it took envy of third-graders to prompt my first look at the earthworm, but I'm glad that something provoked it all the same. The earthworm, it turns out, is not just essential to tilth. It's an infinitely interesting beastie. Look into the hands of those children, and you find the invertebrate that prompted Charles Darwin to say, "It may be doubted whether there are many other animals which have played so important a part in the history of the world, as have these lowly organized creatures."

Standing in that schoolyard, I realized that I had staked my pride as a gardener on the presence of creatures that I knew next to nothing about. So I set out to learn a thing or two about the animals that make earth.

The first surprise was to find that "the earthworm" was in fact more than 4,000 species of creatures worldwide. The terms are not hard and fast. No two texts give the same names. In common with many plant names, the taxonomy, or the way biologists classify this slithering mass, moves as fast as a worm diving down its burrow. Even experts rarely settle on one family, never mind genus, to describe the earthworm. Out of five families commonly discussed in American texts, only several dozen species recur as dominant in California. One day soon I may banter easily using the family names of the Lumbricidae, Acanthodrilinae, Ocnerodrilinae and Sparganophilidae, but for now, I'm sticking with "earthworm."

Where the $10 words did come in handy was in producing three terms for distinct types of worms that occupy three separate layers of earth: epigeic, endogeic and anecic, or surface, shallow and deep. The epigeic worms are the ones given to scooting around in mulch and chewing up fallen leaves and manure, turning them into hummus. They are so exposed, you don't see a whole lot of old ones. They are usually picked off by birds as juvies. They include red worms, the kind that come sealed in plastic cups with compost bins.

The second type, endogeic earthworms, work just beneath the soil surface. According to a collection of excellent papers by Matthew Werner, a retired wormologist from UC Santa Cruz, they take out dead roots and mix and aerate soil. Finally there are the anecic earthworms, which burrow vertically, living in deep holes by day, but surfacing by night to do their work as "detrivores," browsing on fallen leaves and suitably tender, rotting organic matter. These, say most wormologists, do the heavy lifting in soil ecology, processing fallen leaves and turning over as much as 2 inches of soil every year.

The most common worm, the one most people mean when they say "earthworm," turned out to be one of the deep diggers. Also called a night crawler, or Lumbricus terrestris, it is such a tunneler that during rainy season, its burrows serve as important channels for plant roots to penetrate and carry rain water deep into the soil. On days after rains, many end up stranded outside their burrows -- perhaps because they are waterlogged. The world seems to divide up between people who stop and bend over to return night crawlers stranded on sidewalks back to grass, and those who step over them. When exhumed by a gardener's shovel, it's easy to mistake a night crawler for its endogeic, or shallower, cousin, the "Southern worm," or Aporrectodea trapezoides.