Earthworms are the most highly developed worms. They are divided into segments or parts. They are found in salt and fresh water as well as in the soil. Earthworms are helpful to man as bait for fishing and more importantly, because they loosen the soil for roots to grow.
There are 2700 species of earthworms. Earthworms have many enemies including animals that eat them and man who uses them.
A earthworm has no respiratory organ. It takes in oxygen directly through its skin and gives off carbon dioxide. Its skin is always moist.
The secret to earthworm travel lies in two things you can't see just by looking.
Though earthworms have no bones, their complex system of muscles enables them to not only wiggle like crazy but also to very quickly alternate between being stubby and thick, and long and slender
Earthworms possess tiny, practically invisible bristles, called setae (pronounced SEE-tee; singular form seta, pronounced SEE-tah), which usually are held inside their bodies. When the worms want to stay in their burrows, they jab their setae into the surrounding dirt, thus anchoring themselves in place. This comes in handy if a bird nabs a worm's head and tries to pull the worm from its burrow. The setae anchor the worm so well that it may break before coming out.
Here's how these two features enable earthworms to travel:
Let's say that a worm in its burrow wants to move forward. First, using its complex musculature, it makes itself long. Then it anchors the front of its body by sticking its front setae into the soil. Now it pulls its rear end forward, making itself short and thick. Once the rear end is in place, the front setae are withdrawn from the soil, but setae on the rear end are stuck out, anchoring the rear end. Now the front end is free to shoot forward in the burrow as the worm makes itself long and slender. Then the whole process is repeated.
Here are some visible and invisible adaptations to admire about earthworm physique:
The worm's streamlined body with no conspicuous appendages is an adaptation to living in narrow burrows underground In those narrow tunnels, antenna and fancy tails would just get in the way.
Earthworms don't have eyes, but they do have light-sensitive cells scattered in their outer skin. These cells don't enable earthworms to see images, or forms, but they do give their skin the capacity to detect light and changes in light intensity. The worm's skin cells are also sensitive to touch and chemicals
Earthworms have simple brains which specialize in directing body movement in response to light, and not much else. To show how simple the brain is, if an earthworm's brain is removed, changes in its general behavior are hardly noticeable
Earthworms, as the drawing above shows, possess five hearts. The calciferous glands shown between the hearts manage excess calcium in the worm's diet, a problem you may have if you eat a lot of dirt.
Earthworms have no lungs. Their "breathing" consists of oxygen from the air passively diffusing through the skin into the body. One problem with having such permeable skin is that earthworms dry out easily, but not as easily as mollusks
Each earthworm carries both male and female reproductive parts --they are, like snails and slugs, hermaphroditic
