Introduction to Annelids

Candy-striped Worm

When people think of worms, we usually think about the small pink thin creatures in our gardens. These are soft-bodied earthworms, the most common land living segmented worm. However, earthworms are only are small faction within a very large group of animals that live almost everywhere on this planet. In fact, they are only one out of an approximately 9000 other species belonging to the phylum Annelida. So what is an annelid? An annelid refers to a creature with round wormlike characteristics and also has a long, segmented body. Its name originates from the greek word annelus, meaning little rings. Members of this phylum are referred to as such because of the ring-like appearance of these body segments.

Annelids are an important group of animals present in many habitats. They live just about everywhere on earth. Most can be found in wet environments, including marine and freshwater, while some live in moist soil underground. There are still many species within this phylum that we are unfamiliar with and many yet to be discovered. Segmented worms are usually small animals; tiny aquatic worms are less than half a millimetre long while they are some species that can grow up to as mush as 3 meters (the seep tube worm Lamellibrachia luymesi). Although most annelids are characteristically worm-like in appearance, they can range greatly in their colour, patterning, and other features.

There are three main classes with the Phylum Annelida:

Class Polychaeta: (poly=many, chaeta=bristles)
The largest group of annelids and the majority of which are marine. They are divided into equal segments, with paired paddlelike appendages (parapodia) tipped with bristles on each that they can use for swimming, burrowing, and creating feeding currents. Most Polychaetes live in coral reefs, sand, mud, pile of rocks, open water and even in self built tubes. They can range from being dull and unassuming to brightly coloured and luminescent.
Examples: (Sea Mouse)

(Sea Mouse Ventral View)

Class Oligochaeta (Oligo=few, Chaeta=bristles)

As the name implies, this class of annelids have fewer bristles than polychaetes. They usually have a few setae on each segment and more common on land and freshwater. They lack the parapodia of polychaetes and have clitellum for reproduction. Most are detritus feeders but there are some predatory species. The most well know examples of oligochaetes are the earthworms we see all the time

(i.e. earthworms)


Class Hirudinea

Members of this class are what we know as leeches. Mostly living in moist tropical, these include the bloodsucking variety we hear of all the time and some other predatory sepcies. Generally they are no more than 6 cm long but can some can grow up to about 30 cm. Most leehes live in freshwater and are external parasities that feeds on the blood on body fluids of its host. All leeches also have powerful suckers on both ends of their body, the posterior one used anchor itself and the anterior one that surrounds it's mouth attaches it to it's host. All leech species are carnivorous and are hermaphrodites like earthworms

(i.e. leeches)

Interesting Facts about Annelids

Annelids may just seem to be ordinary worms with little merit but in fact, they do some pretty remarkable things. Here are some interesting facts about segmented worms

1. Earthworms (Lumbricus terrestris) are some of the most important creatures in our enviroment. They dig through soil and help aerate it, while at the same time grinding and digesting incredible amounts of soil through their guts and returns many important nutrients back to the earth. In fact if all material that has moved through an earthworm was piled on the surface of the globe, it would accumalate to more than 30 miles, five times more than Mount Everest.

(a drawing of burrowing earthworms)

2. Leeches (Hirudo medicinalis) have been used for medicinal purposes for a very long time. Back then, it was believed that leeching would get rid of the "bad blood" that caused diseases. However when scientists began to understand the nature of diseases and that it was caused by micro-organisms, leeching fell into obscurity. Now, leeches are back in the medical spotlight. The chemicals they produce that stops blood from clotting and anesthetize the area the bite can be used to remove blood clots. Also the chemicals also harm bacteria by dissolving their protective coating. Guess leeches arn't so bad after all!

(medicinal leeches in a jar)

3. Ever wonder what those wierd piles of sand on the beach are? Well they are actually the castings of a polychaete called the sandworm /lugworm (Arenicola marina). They are similiar to earthworms as they dig through the sand and digests it. The indigestable sand takes about an hour to pass through its entire body and comes out at the other end. The worms then move backwards through the tunnel and their cylindrical castings are pused out of the burrow, which forms the shapes that we often see on the beach.

(lugworm castings)