Oligochaeta

Oligochaetes are a huge group of soft-bodied animals. They belong to a larger group called Annelida, which includes many types of worms. There are about 10,000 known species in this group. This means they make up about half of all Annelida. Most people know them as earthworms that live in the soil. However, many other types live in fresh water or the salty sea.

These worms move in a very special way. They use a process called peristalsis to push through the ground. First, the front part of the worm stretches out using circular muscles. Then, the part just behind it becomes short and fat. The worm uses tiny bristles called chaetae to grip the dirt. These bristles help the worm stay steady while it moves. They can take about seven to ten steps every minute.

Scientists have studied these worms for a long time. In 1930, a researcher named Stephenson had a theory about them. He thought their oldest ancestors came from a family called Lumbriculidae. We also have fossils that show worms lived long ago. One genus called Protoscolex was found in Kentucky, United States. Another was found in Herefordshire, England.

Oligochaetes live in almost every part of the world. You can find them on every continent except for deserts. They must stay in moist places to survive. Some giant species, like the Mekong worm, can grow very large. Most aquatic worms are small and slender. You can often see their organs through their clear skin. Some even live in the deep, dark parts of the ocean.

Inside their bodies, these worms have many interesting parts. They do not have gills to breathe. Instead, they breathe through their moist skin. They have a tube for a gut to digest food. Many eat dead matter, which is called detritus. Some use a muscular part called a pharynx to suck in food. Some worms even use their skin to sense light and touch.