The common earthworm plays a critical role in our planet's ecosystem by decomposing organic material into nutrient-rich soil. Its simple and efficient anatomy gives the earthworm several advantages in its natural environment, but its specific needs also restrict the earthworm's range and make it a delicate creature outside its primary underground habitat.
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The most visually obvious detail of an earthworm, and one of its most unique features is a segmented body. Each worm has more than 100 individual segments filled with muscular tissue that flex individually. These muscles provide the worm's primary locomotion, as portions of the worm expand to reach forward and contract to pull the rest of the worm's body along.
Covering each segment of the worm's skin is a less obvious but still critical feature; thousands of tiny hairs called setae. Though slimy to the touch, these bristly hairs work to anchor the worm to the ground, and in combination with the segmented muscles allow the worm to grasp one area of soil while reaching for another.
Earthworms have no lungs, but require oxygen to survive. They gather the air they need through diffusion; their porous skin allows oxygen and carbon dioxide to pass directly into their bodies. An earthworm must maintain moist skin for successful diffusion, which restricts their surface activities to wet weather or night, outside of direct sunlight.
The central digestive tube of an earthworm provides their only familiar features, including a mouth, intestinal tract and esophagus. Earthworms have no stomach, relying instead on a gizzard and the aid of stones to completely break down their food.
As hermaphrodites, earthworms have both male and female reproductive organs. These organs rest in the ringed band toward the earthworm's tail end, visible as an area of larger diameter than the rest of the body. After mating, earthworms leave behind small, egg-like cocoons from which their young eventually hatch.