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Dorthea Garneau
Earthworms are invertebrates composed of many segments (Fig. 1). They don't have bones and move by contracting and relaxing the body segments in sequence. They also have little bristlelike organs that help them cling to slippery surfaces. Most earthworms have both male and female organs. Typically, however, they still need a partner to reproduce. When earthworms mate, they lay side by side in reverse position. At that moment they exchange sperm. The sperm of the partner is stored in little chambers called spermathecal apertures. These are positioned in front of the egg-producing organs. After mating, the swollen external gland, called clitellum, produces egg cases called cocoons. A cocoon case slides slowly forward, picking up eggs and sperm as it moves over the head of the earthworm. From 3 to 1,000 cocoons can be produced per year, depending on species and environmental conditions. Typically, an earthworm will produce 20 to 30 cocoons per year, with each cocoon containing 1 to 10 eggs. Peak cocoon production is in the spring or early summer. The eggs in the cocoons hatch when conditions are right. Under ideal conditions, it may take from 1 to 5 months for the eggs to hatch. It may then take from 3 to 12 months before these worms are sexually mature. Worms typically live only a few months because of the many environmental threats they face. They have been observed to live for 10 years in a protected environment. In a favorable environment previously without earthworms, earthworm populations increased 80-fold in 4 years after introduction.
Earthworms absorb and lose moisture through their skin. If soils are dry, earthworms may move to deeper soil layers, die, or revert to a hibernation condition called diapause. Earthworms in diapause are tied up in a knot in a little hole that is lined with a slimy substance to avoid moisture loss. Eggs in cocoons survive prolonged drought, allowing earthworm populations to survive drought periods. Because of dry conditions, earthworm populations in Pennsylvania are low in the summer. Earthworms can live under submerged conditions if the oxygen content of the water is high enough. In most cases, however, earthworms will die when exposed to excessive waterlogging. They move to the surface when the soil is saturated to avoid suffocation.
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