Phylum Annelida(segmented worms)



Diagnostic characteristics:

Annelids have a segmented body resembling a series of fused rings. This segmentation is both internal and external, and each internal segment of the body is separated by a tissue called septa. Segmentation also means that the body is divided into repeated parts that can function on their own (CS 2). Hydrostatic pressure is maintained across segments and helps maintain body rigidity allowing muscle contractions to bend the body without collapsing the annelid. (22 VK) Much discussion surrounds the origins and homology of segmentation, which is a characteristic shared not only by Annelida but also by Arthropoda. Some scientists are declaring that Arthropoda are more closely related to Nematoda, suggesting that segmentation in Annelida is an apomorphy, or a derived trait. (5 AS) Segmentation increases efficiency during development as it involves creating an entire body out of identical modules. It also aids locomotion because each segment acts independently of the other segments, giving the body greater flexibility. (AR 15)

Annelids are coelomates. An annelid will have a true coelom- a body cavity lined with tissue derived completely from mesoderm. (2RM) There are 15,000 annelid species ,ranging in length from less than 1mm to 3m.Some annelids live in the sea, most freshwater habitats and damp soil. Another diagnostic characteristic of Annelids is that they have chaetae which are thin-walled cylinders that are held together by proteins. (EK)

The way annelida grow is significant because it can occur in two ways: either by enlargement of the segments during the juvenile stages or through the addition of new segments. New segments are produced by the first part of the pygidium, the body region at the end of the worm. In some species, these segments are produced throughout the animal’s life, but in many species production stops once a certain set number of segments is reached. (11 AW)

Annelids have hydrostatic skeletons.This means they consist of fluids under pressure in a closed body compartment.They control their form and movement by using muscles to change the shape of the fluid filled compartments.The coelomic fluid in annelids functions as the hydrostatic skeleton system.The coelomic activity is divided by septa between the segments of the worm,thus the animal can change the shape of each segment individually by using both circular and longitudinal muscles.The hydrostatic skeletons of most annelids enables them to move by peristalsis-a type of locomotion produced by rhythmic waves of muscle contractions passing from head to tail. Peristalsis is the same coordinated motion as is used in humans to force food down the digestive tract. (JE) annelida locomotion
Below is a diagram of the an annelid's motion towards the left.(JE)
external image 180px-Earthworm_movement_all.jpg

Acquiring and digesting food:

Some aquatic annelids swim in pursuit of food but most are bottom-dwellers that burrow in the sand and silt e.g earthworm. The digestive system,longitudinal blood vessels and nerve chords run the length of the animal.The digestive system has several specialized regions: the pharynx,the esophagus, the gizzard and intestine. Earthworms, for example, eat dirt, digesting the plant and animal matter in the soil and eliminating the rest. They have an esophagus for the food to go down, a crop which stores food, a gizzard which grinds the food, an intestine to remove nutrients from the food, and an anus to expel all unabsorbed material (1 MB).

Sensing the Environment
Annelids have many types of sense organs. Most noticeable, the annelid usually has light and heat sensitive cells on its front end (2T2). Concentrated anteriorly, a characteristic known as cephalization, an annelid's nervous system contains a ventral nerve chord with segmented ganglia, functional groups of nerve cell bodies, that give rise to cerebral ganglia (7 J. Stein).

Circulatory system:

The closed circulatory system consists of a network of vessels containing blood and carrying hemoglobin.The dorsal and ventral vessels are connected by segmented pairs of vessels. The dorsal vessel and five pairs of vessels that circle the esophagus are muscular and pump blood through the circulatory system. Segmental branches of the ventral vessel carry blood to the annelida's intestines and body wall. These branches then break up into capillary beds that exchanges nutrients and oxygen with the outside environment (10 JSun).


Tiny blood vessels are abundant in the skin, which functions as the respiratory organ. Some organisms in classes of annelids, such as polychaetas and clitellates, have gills associated with most segments. (2DC) Gas exchange occurs between the capillary beds on the body surface of the annelida and the environment (10 JSun). Annelids exchange respiratory gases between the environment and cells passively by diffusion through moist skin (CH).

Metabolic waste removal:

An illustration of metanephridia. (SW 14)
An illustration of metanephridia. (SW 14)

In each segment of the worm is a pair of excretory tubes called metanephridia with ciliated funnels called nephrostomes that remove wastes from the blood and coelomic fluids

The metanephrida lead to exterior pores through which metabolic wastes are discharged.
Aquatic annelids mainly secrete ammonia, while earthworms, which live in soil, excrete urea. (1 HL)


Annelida are hermaphrodites but they cross-fertilize. Annelida mate by aligning themselves in such a way that they exchange sperm and then separate.The received sperm cells are stored temporarily while a special organ the clitellum,secretes a mucous cocoon.The cocoon slides along the worm picking up the eggs and the stored sperm.The cocoon then slips off the worms head and resides in the soil while the embryo develops.Some worms can also reproduce by fragmentation followed by regeneration.

Although the majority of annelida are hermaphrodites, polychaetes (lug worms, bristleworms, fire worms..etc) have separate sexes. Trochopore, larva resembling plankton, hatch from eggs and develop into juvenile annelid. Reproduction is usually external; however, some polychaetes reproduce by budding (CH 12).


Phylum Annelida are divided in to three main groups:

Class oligochaeta:

This class of segmented worms include the earthworm and a variety of aquatic species.The earthworm extracts nutrients through the as it eats its way through the soil.Undigested material mixed with mucus secreted into the digestive tract is egested through the anus. There are about 3,250 known species of oligochaeta, which can be found in soil (e.g. earthworms) or in shallow or estuarial water (EG 2).

Class polychaeta:

Each segment of polychaeta has a pair of paddle-like structures called parapodia (''almost feet'').that function in locomotion many polychaeta. The parapodia are richly supplied with blood vessels and function as gills.Most polychaetes are marine.A few drift and swim among the plankton,some crawl on or burrow in the sea floor, and many live in tubes which they make by mixing mucus and broken shells. An example of tube dwellers is the brightly colored fanworm which traps microscopic food particles in feathery tentacles that extend from the opening of the tube. These have the highest physical capabilities for regeneration, but their ability is still generally restricted to the anterior end. (2 AN)

Class Hirudinea(leeches):

Majority of leeches inhabit freshwater, but some are land leeches that move through moist vegetation. Leeches are usually found in warm protected shallows that have little to no current and a slightly basic pH. Leeches also tend to avoid light and to hide under stones or within aquatic plants, while some species are active at night (2 KA). Many leeches feed on other invertebrates,and some are blood sucking parasites that feed on animals and humans.Leeches range in length from 1-30cm.Some parasitic species use blade like jaws to slit the skin of the host while others secrete enzymes that digest a hole through the skin.The host is usually oblivious to this attack because the leech releases an anesthetic
as well as an anticoagulant which allows them to feed easily. Leeches are occasionally used to help restore blood circulation after surgery, especially following tissue grafts (DPOD 2).
Review Questions:
1) What are the differences between the 3 main groups of Annelida? (2-SC)
2) What is the main respiratory organ in Annelida? (EK14)
3) What is a hydrostatic skeleton and how does it help the movement of annelids? (22 AL)

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