Everybody knows earthworms are cool; from playing a role in many soil forming processes to serving as excellent fishin' bait, earthworms are some of nature's hardest working critters. Heck, Darwin himself wrote a hefty tome on the subject of the Humble Earthworm, showing in his own imitable style how absolutely amazing and utterly vital to the development of our life-sustaining soils these little fellows are.
Look! A cartoon from Punch about Darwin! What are the odds!?!
One of Darwin's observations in his 1881 volume "The formation of vegetable mould, through the action of worms, with observations on their habits" was that earthworms excreted little calcite balls, up to a millimeter in size, which actually made up an appreciable amount of mass in any given soil volume. Subsequent work has shown that your average worm (Lumbricus terrestris) can produce up to 2.2 mg of Calcite per day, and that this wormogenic calcite is indeed a commonplace feature in soils today.
Recognizing the ubiquity of worm-poop calcite in soils, a recent paper by Lee et al. (2008) in Geology interrogated the shit out of these little limestone pellets (get it!?!). Anyway, apparently no one had really taken the time to do a detailed bit o' mineralogy/petrology on these calcite granules. Lee et al. (2008) used a variety of analysis and imaging techniques, including backscatter electron diffraction, to really get into the nitty-gritty detail of these pellets (ha! another one!).
To get to the point, Lee et al. (2008) found that the calcite was initially precipitated as amorphous calcium carbonate in the guts of the worm (by the appropriately named calcifierous gland). This amorphous calcium carbonate phase gloms onto a quartz grain in the gut of the worm, where it undergoes eventual crystallization into calcite BEFORE being excreted. This crystallization process is mediated by a migrating fluid film, resulting in a stromatolite-esque coated grain (see below for a figure taken from the paper):
Apparently, the fine-scaled zonation of these calcite crystal is unique for biominerals; the authors point out that, given calcite growth rates inside the worm, the properties of the mediating fluid film must have fluctuated on the scale of tens of minutes in order to produce such zoning.
Why go through all that trouble? The authors suggest that crystallization may be a way the worm avoids remobilization and absorption of calcium carbonate. The solubility of the these different phases is dramatically different, and by forcing the amorphous stuff to crystallize into more stable calcite, the worm can ensure the stuff stays put. As for the larger question of "Why do worms secrete this stuff anyway?" is one that the authors can't answer. Apparently, no one knows why worms do this!
Of course, this probably isn't idle musing for cloistered academics. Lee et al. (2008) close their paper by pointing out that the amount of calcite produced by these worms is in such quantities that it probably plays a major role in the soil carbon cycle. As such, a clearer understanding of this process might have an impact on our understanding of and ability to model biogeochemical cycles.
I think Darwin would be pleased that the Noble Earthworm still has some important stories to tell us.
Lee, M.R., Hodson, M.E., and Langworthy, G., 2008, Earthworms produce granules of intricatley zoned calcite: Geology, v. 36, n. 12, p. 943-946.