Earthworm (Lumbricus sp.)
Although we can go long times without seeing one, earthworms are among the most common and widespread organisms. They are under our feet, ubiquitous in moist soils. Their abundance can be seen after a rain, when many of them come to the surface. Crawling above ground, some become stranded on sidewalks and streets.
Earthworms look like a cylinder pointed at both ends, but in fact they are bilaterally symmetrical and with much the same organ systems as we have. Above all, they are many-segmented, each segment much like the others in basic form and appendages. They have longitudinal muscles running along their bodies and circular muscles running around each segment, and by contracting and relaxing these muscles in turn, they burrow through the soil, crawl on the substrate, or even swim. Each segment has a set of tiny spines called chaetae that anchor parts of the body as it is pushed and pulled through the soil.
Earthworms are very efficient dirt-eaters. They take in bits of the soil through which they burrow and extract nutrients from the organic matter they digest. Their feces fertilize the soil even after they have extracted most nutrients, and their burrowing aerates the soil to the advantage of plant roots and the abundant soil fauna. They can also come to the surface and take in organic detritus such as pieces of dead leaves through their suckerlike mouth.
Earthworms are hermaphrodites, both sexes in the same body. The testes and ovaries and their ducts are separate, and their openings are on adjacent segments. But in hermaphroditic animals, even though both sexes are ready and able to reproduce, there has been selection for outbreeding, so two must come together and exchange gametes. The gonopores (reproductive openings) are on adjacent segments just in front of an obvious swollen area called the clitellum. Farther anterior, there are openings to spermathecae, sperm-storage sacs.
When mating time is at hand, two worms get together and appress their bodies, aligned in opposite directions. Mucus secreted by the clitellum binds them together. Sperm produced by one travels along a groove in the body surface to the openings of the spermathecae of the other. They then separate. In each worm, a further layer of mucus is produced around the clitellum and the body in front of it. The clitellum produces a proteinaceous cocoon that then travels forward, picks up the eggs deposited from the female gonopores and moves them forward to where they are fertilized by sperm extruded from the spermathecae.
The cocoon then continues to move forward and is shed from the anterior end of the worm. It closes and shelters the eggs as they develop. Development is direct, with juvenile worms hatching from the eggs, usually at a favored time such as spring.
Earthworms have many predators. Moles seek them out underground, and a foraging robin should strike terror into the heart of all the worms that contemplate emerging briefly from the soil into the light of day.