Annelid

Annelids, also known as segmented worms, are fascinating animals that belong to the phylum Annelida. They include many familiar creatures such as ragworms, earthworms, and leeches. With over 22,000 different species, annelids live in many different places, from the ocean depths to freshwater streams and even in soil on land.

These worms are bilaterally symmetrical and have bodies made up of many similar segments. Each segment contains the same basic set of organs, and many annelids have special structures called parapodia that help them move. Their segmented bodies allow them to change shape and move in various ways, such as by rippling along or burrowing through sediment.

Annelids play very important roles in nature. Earthworms help keep soil healthy by aerating it and enriching it with nutrients. In the ocean, burrowing marine worms help create ecosystems by allowing water and oxygen to reach the sea floor. Some annelids are even used as food or bait by people, and scientists study them to learn about water quality. Though their soft bodies don't often fossilize, annelids have been around for hundreds of millions of years, with the oldest known fossils dating back to the early Cambrian period.

Classification and diversity

There are over 22,000 living species of annelids, which are also called segmented worms. These species come in many sizes, from very tiny to very large. Some, like the Australian giant Gippsland earthworm and Amynthas mekongianus, can grow up to 3 meters (9.8 ft) long. The largest annelid, Microchaetus rappi, can reach up to 6.7 meters (22 ft) in length.

Annelids are grouped into different categories based on their features. One group is called Polychaetes, which have special "hairs" on their bodies and usually live in the ocean. Another group is Clitellates, which includes earthworms and leech-shaped animals. These groups help scientists understand how these interesting worms live and grow.

Distinguishing features

Annelids, or segmented worms, have long bodies divided into segments. These segments are separated by small ring-like lines on the outside and by internal walls, though sometimes these walls are not complete. Most segments have similar sets of organs and share a common gut, circulatory system, and nervous system, making them rely on each other. Under their skin, they have strong muscles. They also have a closed blood system where the blood travels through blood vessels, and their bodies are covered by a tough but flexible outer layer made of a substance called collagen. This outer layer does not change or shed, unlike the outer layer of arthropods, which is made of a harder material and does change as the arthropods grow.

Description

Most annelids, also called segmented worms, have bodies made of many similar sections called segments. These segments have the same internal organs and outer parts, like tiny hairs called chaetae. The front part, called the prostomium, holds the brain and sense organs, while the back part, called the pygidium, has the anus. Between them is the peristomium, which may also have chaetae and parts like other segments.

Annelids grow by adding new segments one at a time from a special growth area near the pygidium. This way, the youngest segment is always just in front of the growth zone. Some annelids, like leeches, have a set number of segments, while others keep adding them throughout their lives. Their bodies are flexible and strong, with muscles that help them move in different ways, like crawling, swimming, or burrowing.

Ecological significance

Charles Darwin wrote a book called The Formation of Vegetable Mould Through the Action of Worms in 1881. This book showed how earthworms help keep soil healthy. Some earthworms dig tunnels, while others live on the surface, usually in damp leaf litter. The tunneling earthworms make the soil loose so air and water can get into it. Both types of earthworms help make soil by mixing organic material and minerals. They also help break down organic matter faster, making nutrients available for other living things. They also change minerals into forms that plants can use more easily. Earthworms are an important food source for birds, from small robins to large storks, and for mammals like shrews and badgers. In some places, protecting earthworms is important for protecting endangered birds.

Sometimes, earthworms can cause problems when they are not native to an area. In parts of North America that were covered by glaciers, almost all the native earthworms were lost. The earthworms now found there came from other places, mainly Europe and more recently Asia. These non-native earthworms can harm forests by eating the leaf litter, changing the soil, and reducing the variety of plant and animal life. One particular earthworm, Amynthas agrestis, is banned in some states like Wisconsin.

Earthworms do not move far on their own each year. However, people can help spread them when they carry dirt on fishing gear, tires, or shoes.

In the ocean, annelids make up a big part of the animals that live on the sea floor near coral reefs and in tidal zones. Those that dig tunnels help bring water and oxygen into the sand or mud on the sea floor. This supports the growth of tiny animals and bacteria.

Some leeches bite animals and take small amounts of blood. While this does not hurt the host much, some leeches can carry tiny organisms that make the host very sick. Also, some small tube-dwelling animals can spread parasites that cause a disease in fish called whirling disease.

Interaction with humans

Earthworms help make soil richer and healthier. Some people in Samoa enjoy eating a special kind of worm called the Palolo worm that comes out of the water to reproduce. Fishermen often use worms as bait because they catch more fish this way.

Scientists also study these worms to learn about water quality, like how clean rivers and oceans are.

In the past, some doctors used leeches to help with medical treatments, but we now know better ways. Today, we use leeches in very special surgeries to help heal cuts on fingers, toes, or ears. Their spit also has helpful substances that scientists study.

Some parts of ragworms have caught the attention of engineers because they are strong but very light. These parts are made of special proteins that stick well to certain metals.

Evolutionary history

See also: List of annelid families

Fossil record

Annelids, because they are soft-bodied, rarely become fossils. The fossils we do have are mostly jaws and mineralized tubes from certain species. Some fossils from a very long time ago, like Dickinsonia, look a little like annelids but are not clear enough to say for sure. One fossil called Cloudina, from about 549 to 542 million years ago, might be an annelid, but some think it belongs to a different group of animals. The oldest accepted annelid fossils, Canadia and Burgessochaeta, are from about 505 million years ago. Another possible early annelid, Myoscolex, was found but is not clearly an annelid. In 2008, scientists found Phragmochaeta, which they said was the oldest annelid known at the time. There is still debate about whether Wiwaxia was an annelid or a mollusc. Annelids began to diversify in the early Ordovician period, about 488 to 474 million years ago. By the end of the Carboniferous period, about 299 million years ago, fossils of many modern annelid groups had appeared.

Internal relationships

Annelids have traditionally been split into two main groups: polychaetes and clitellates. Clitellates include earthworms and leeches. For many years, it was hard to organize the many polychaete families into larger groups. In 1997, scientists tried to organize them based on body structures. They split polychaetes into groups like Scolecida, Canalipalpata, and Aciculata. Later studies using molecular data suggested a different organization. These studies showed that clitellates, pogonophorans, and echiurans were actually part of the polychaete family tree. They also suggested that sipunculans belonged to polychaetes and that leeches were a subgroup of oligochaetes. These findings changed how scientists classify annelids.

External relationships

Annelids belong to a group called protostomes, which also includes arthropods. Once, annelids and arthropods were grouped together because both have segmented bodies. But now, scientists think annelids are more closely related to molluscs, brachiopods, and other animals with lophophores or trochophore larvae, forming a group called Lophotrochozoa. Arthropods are now in a different group called Ecdysozoa. The Lophotrochozoa idea is supported by how the eggs of these animals develop, following a similar spiral pattern. Fossils suggest that annelids and lophophorates may be closely related, sharing features like chaetae and similar body structures.