Earthworms are simple creatures that pack a powerful environmental punch. They live in moist soil, digging tunnels that aerate the soil for plants and processing vegetation through their digestive systems to turn it into nutrients for the plant roots. Earthworms do all this with basic organ systems.
Earthworms don't have lungs like mammals do. Instead, they breathe by diffusing air directly through their skin. Oxygen passes in while carbon dioxide passes out. Their skin must stay moist for the gas diffusion to work properly, which is why they stick to damp soil and don't usually venture out into the hot sunshine.
An earthworm has a closed circulatory system that uses vessels to send blood through its body. There are five aortic arches throughout his body that serve as pumps. The dorsal vessels take blood from the back of his body to the front, and the ventral vessels take blood the other direction, from front to back. Smaller capillaries help the blood absorb nutrients and oxygen or carbon dioxide and carry them to the proper areas of his body.
For such a small creature, an earthworm has a powerful muscular system. Long muscles that run along his length help trigger a wave of movement from smaller muscles that circle the body in segments. The circular muscles squeeze inward while stretching out to the sides, going in waves down the earthworm's body. This pushes his body forward.
Digestion takes place in nearly a straight line down the earthworm's body. When he ingests food, he swallows it until it passes through the gizzard, which uses tiny stones to grind up the food and pass it onto the intestines. In the intestines, the nutritious pieces are absorbed and the waste passed to the excretory system.
Part of an earthworm's excretory system is directly connected to his digestive system, as the intestines pass waste through the anus and out of the worm. This poop is called castings, and it serves as a type of plant fertilizer. However, some of the waste is picked up by excretory cells and carried outward to the skin. This waste is secreted through the skin as mucus that helps keep the worm's skin moist.
Earthworms have nerve clusters called ganglia that are connected to a nerve cord that run down the worm's length. The ganglia transmit electrical signals from the nerve cord to various other systems, such as telling the muscles to contract or the gizzard to start grinding food.
An earthworms needs to mate with another worm to produce eggs, but they don't have to look for a specific gender. Most earthworms are hermaphrodites, which means they carry both male and female reproductive organs. To mate, the earthworms basically exchange sperm. Then, each earthworm lays eggs fertilized by the other's sperm.
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