Introduction

The Annelids, which means “little rings” in Latin, is commonly referred as segmented worms. Even though the fossil record for this phylum is unclear, but few forms existed already during the Cambrian period. Today, you can find these worms almost everywhere from the deepest sea floor to the soils in our backyards. There are over 15,000 of discovered species in the Annelid family. Scientists divided the segmented worms into three major groups:

Polychaeta: marine worms that are mostly found in wet and freshwater areas.
Oligochaeta: common earthworms and terrestrial worms;
Hirudinea: includes the parasitic blood sucking leeches.

Form and Body Structure

The body of members of the Phylum Annelida are round, wormlike animals that have long, segmented bodies. The have a large range in length going from under 1 millimeter to over 3 meters. The segments of an Annelid’s body are separated by internal walls called septa (singular: septum). Body segments of Annelid’s are virtually identical to one another however many are modified to perform special functions. For example, the first few segments in an Annelid’s body are designed to carry one or more pairs of eyes, several pair of antennae and other sense organs. The body of an Annelid can also be divided into 3 sections: the head, the trunk, and the pygidium.

All of the segmented worms are coelomic, which means that most of the organs are stored in fluid-filled body cavity. The coelom supports segmented worms' body structure and prodvides them with movement; this movement is called contraction. Moreover, the amount of fluid in the body cavity affects the muscle funciton of the worms. The segments of an Annelids' bodies are separated by intenal walls called septa. Body segments of these worms are identical to on another. They are invertibrate,which also mean they do not have back bones. The only time the worms are unale to move is when the fluid leaks out from coelom.

How Annelids fit and relate to our outside world

Annelids can be found in moist terrestrial and fresh water environments, such as well vegetated soil and ponds. The Oligochaete worms are usually found in mud and rotten leaves. These worms feed by passing soil through their body to create nutrients for bacteria and other worms. Polychaetes (bristle worms) are found in fresh water, and they live by extracting particles from the pond bottom. Leeches can be found in rivers and streams or on the surface of roads after rain. Even though leeches suck blood from their hosts, they can be used as medication to clot blood. The slow moving annelids are easily captured by their predators, such as birds and human.

Respiration System

Annelids have two kinds of respiration depending on different classes. If annelids live in sea or water like annelids in class Polychaeta, they have gills to breathe. Some annelids do not have lugs like earthworm (class Oligochaeta), and they use their outer skin to exchange gas. They get oxygen from the soil and release carbon dioxide from their skins. After they get oxygen, they exchange gas in their body through the circulatory system. So their skins need to keep wet to make gas exchange possibly. Respiration of annelids in class Hirudinea like leeches is similar to earthworm. They also breathe through their skins. However, leeches live in water not in soil. If some leeches cannot get enough oxygen in water, they will move to surface.

Reproduction System

Annelids have two types of reproductions:

asexual reproduction and sexual reproduction.

In asexual reproduction, some annelids use fission to produce quickly.. The backs of annelids will break and form a new alike worm. Usually, their break depends on their coats growing. Also, a few annelids reproduce by pudding. However, these annelids can reproduce themselves with using asexual methods because they have high organization, so they are able to reproduce by themselves.

Nevertheless, most annelids reproduce sexually because they are hermaphrodite, and they are unable to reproduce by themselves. Although these annelids like earthworm and leech can produce its own eggs and sperm, these eggs and sperm are hardly fertilized. So they need to pair a couple and connect each other to exchange their sperm. If the eggs are ready to be fertilized, the sperm will be discharged. After the eggs and sperm are fertilized together, the clitellum will produce a cocoon. Then the sexual reproduction is finished.
Fission: division of biological cells as a method of reproduction
Hermaphrodite: an animal that has both male and female sexual organs.
Clitellum: a part of worm's body wall that discharges a sticky substance during fertilization.

Circulatory System

Most of the segmented worms have two main pumping structure called dorsal and ventral blood vessels. The ventral blood vessel moving toward the posterior end provide intestine with blood. Hoever, different segmented worms have different circulatory system. Most of segmented worms have a red pigment that carries oxygen called hemoglobin. Nonetheless, other segmented worms have unpigmented blood, which green pigment carrying oxygen. Yet, circulatory system is what dominated all the segmented worms. It is a system that blood circulate only in the bllod vessels. The blood is not mixed with fluid in the body cavity. There are five ring vessels called hearts, which transport blood to the head and distributed it throughout the body.

Digestive System

When food is consumed by segmented worms, the food will first enter the mouth of the worms. The food will then pass through a muscular structure called pharynx; the structure used to such food into gastrovascular cavity. After, the esophagus expands into a thin-walled structure, the crop. Just posterior to the crop is the gizzard, which is a grinding sturcture with thick, muscular wall, and this also where the most of the digestion takes place. When food passes though the intestine, where further digestion and absorption of the nutrients occurs. The enzyme that produced in the intestine helps to break down food into small particles. Finally, the intestine ends at the anus, where the wastes exit.

Nervous System

Segmented worms have the most complex nervous system. They have nerve cord all around their bodies; these nerve cords controls the bodies' movements. We can easily find out that all the nerve cords are connected to a small brain called a "ganglionic mass", where all the commands from. Segmented worms have various sensory system such as eyes, taste buds, tentacles, and statocysts. In the pictures shown above, we can see the ventral nerve cord where the intestines of the worms are removed.

Photo Gallery

Live Pictures of Segmented Worms (Phylum Annelida) Earthworm Dissection Lab (Monday March 5th, 2007)

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