Annelida

Sunday, March 4, 2007

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)

Thursday, March 1, 2007

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)

Tuesday, February 27, 2007

Comparing Other Phylums

Click to enlarge thumbnail in new window:

Monday, February 26, 2007

General Anatomy

Habitat
All annelids use their habitat primarily for food or shelter. Many species use the ground to burrow for safety, and many polychaetes are tube-dwellers, who burrow into tubes found in the muddy bottom of the ocean or beaches. Still others actually eat their habitats. Called non-selective deposit feeders, these annelids eat mud, sand, or soil. The organic matter is then digested; however, since the amount of nutrition gained by this type of feeding is quite small, non-selective deposit feeders must eat almost continuously.

Annelids are found world-wide. Nearly 2/3 of the known annelids falls under the Class Polychaetes and are typically marine organisms. These marine worms can be found anywhere from shallow beach waters to deep ocean trenches. The remaining 1/3 of the Class Polychaetes can be found in freshwater and very moist land. Another class of annelid worms is the Oligochaeta. Oligochaeta are typically found on land or in freshwater; these are the worms that people usually associate with “segmented worms”, such as earthworms. The last class, Hirundinea, is typically comprised of freshwater dwellers; however, there are a few land and marine species. Hirundinea includes animals such as leeches.

Vocabulary:
Annelid: round wormlike animal that has a long segmented body and belongs to the phylum annelida
Polychaetes: segmented worm usually marine characterized by paired paddle like appendages on its body segments and a bristly body
Oligochaeta: Annelid worm belonging to the class that contains common earthworm and related species that live in soil and in water
Leech: annelid worm that typically exists as an external parasite that drinks the blood and body fluids of its host
Longitudinal muscles: runs for the front of the worm to the rear. When these muscle contact they make the worm skinnier
Clitellum: it secretes a mucus ring into which egg and sperm are released.
Pharynx: muscular front end of the digestive tube which can be extend to catch food

Circulatory/Internal Transport:
The circulatory system varies among the different members of annelids. Most have blood with hemoglobin, a red pigment that carries oxygen, but some others have a green pigment that carries oxygen, or unpigmented blood. Most segmented worms (such as earthworms) have a closed circulatory system which means the blood circulates only within a system of blood vessels, rather than mixing with fluid in the body cavity. In each of the segments in the body of the annelid, there is a pair of smaller vessels called ring vessels. The ring vessels connect the dorsal and ventral vessels of the annelid and supply blood to some small internal organs and transports blood from the anterior to the posterior. There are 10 main ring vessels located on top of the gut that serve as hearts. In leeches, the closed circulatory system is reduced or absent and it may be replaced by coelomic canals. In other annelids, blood is helped moved around the body by muscle contractions when the annelid is moving itself.

The leech’s pharynx is used to suck blood.

Anatomy of an Earthworm

Feeding:
Feeding styles vary from eating organic material that settles on the surface of the muddy substrate (detritus feeding), to filtering plankton, sucking blood out of other animals (parasitic) and detritus from the water using feathery feeding tentacles (suspension feeding), to eating their neighbors (predation).

The filter feeder plume worm

Digestion:
Segmented worms have a gut that runs the length of the body, and is separated from the body wall by the coelom. Food enters the mouth and travels through the gut, where it is digested. An important feeding organ that has evolved in most annelids is the pharynx. The pharynx is the muscular front end of the digestive system. In carnivorous annelids, such as the sandworm, the pharynx has two or more sharp jaws which are useful for capturing prey. Herbivores also have this jaw used for tearing algae. In detritus feeders, such as earthworms, the pharynx acts as a pump to allow water and soil enter the earthworm. In parasites, such as leeches, the pharynx is used to suck blood out of its host. In the some filter feeder annelids, they use their long tube like burrows to catch passing algae in the ocean.

Excretion:
Annelids produce two types of wastes—solid and liquid. Solid wastes are removed from the gut through the anus and liquid wastes are removed by tube-shaped oranges called nephridia. These organs act very similar to kidneys and remove urea. A pair of nephirdia are present in each body segment and removes waste products from body fluids and excretes them.

Cites:
Biology textbook
http://www.tolweb.org/articles/?article_id=57
http://ebiomedia.com/prod/BOanelids.html
Leech:
http://biodidac.bio.uottawa.ca
Earthworm anatomy:
http://concise.britannica.com
Plume worm:
http://www.ryanphotographic.com

Monday, February 19, 2007

Respiration:
Annelids have no lungs, but many have gills for respiration. Respiration occurs by diffusion through the moist surface of the body. That's why earthworms die so quickly when their epidermis dries up. Most polychaetes also have gills to aid in respiration.

Response to the environment/Movement:
Annelids have various muscle groups and simple appendages. They use setae and parapodia for movement. Annelids such as the earthworm and the Lumbricus possess setae. A seta is a stiff hair, bristle, or bristle-like part of an organism. The setae are able to move in and out from the body wall. When the setae are withdrawn, the body extends, and the worm moves forward. Setae help earthworms attach to the surface and prevent backsliding during peristaltic motion. Parapodia (para = like; pod = foot) are mostly found in polychaetes and can extend and retract. Parapodia are paired, lateral appendages extending from the body segements.

Setae shown on the cross-section of a Lumbricus terrestris

Reproduction:
Annelids can reproduce asexually and sexually depending on the type of species.

Asexual Reproduction:
Asexual reproduction is by fragmentation, budding, or fission. Fission allows Annelids to reproduce quickly. The posterior part of the body breaks off and forms a new individual. For example, Lumbriculus and Aulophorus, are known to reproduce by the body breaking into such fragments. Many other species (such as most earthworms) cannot reproduce this way, though they have varying abilities to regrow amputated segments.

Lumbriculus variegatus

Sexual Reproduction:
Hermaphodites are common within Annelids, but others have seperate genders. Sexual reproduction allows a species to better adapt to its environment. Fertilized eggs of marine annelids usually develop into free-swimming larvae. Most polychaete worms have separate males and females and external fertilization. The earliest larval stage, which is lost in some groups, is a ciliated trochophore (type of larva with several bands of cilia), similar to those found in other phyla. The animal then begins to develop its segments, one after another, until it reaches its adult size.
Earthworms and other oligochaetes, as well as the leeches, are hermaphroditic and mate periodically throughout the year by copulation. Two worms lay their bodies together with their heads pointing opposite directions and sperm is transferred from the male pore to the other worm. Other methods of sperm transference have been observed in other genera and may involve internal spermathecae (sperm storing chambers) or spermatophores that are attached to the outside of the other worm's body. The embryonic worms develops in a fluid-filled "cocoon" secreted by the clitellum (a thickened glandular section of the body wall that secretes a viscid sac in which the eggs are deposited).